Electricity Magnetism

Chapter 16.1: Static Electricity Chapter 16.1: Introduction Learning Outcomes (a) state that there are positive and negative charges and that charge is measured in coulombs (b) state that unlike charges attract and like charges repel 5. The symbol for this is Q, used for both positive charges and negative charges. Obj113 6. 1. Electrostatics is the study of non – moving electrical charges, also known as static electricity. Negative charges are known as electrons and positive charges are known as protons. IMPORTANT: Protons do not move. Formula for charge: where I = electric current, Q = charge, t = Time. 2. Chapter 16.4: Electric Field Learning Outcomes: (c) describe an electric field as a region in which an electric charge experiences a force (d) draw the electric field of an isolated point charge and recall that the direction of the field lines gives the direction of the force acting on a positive test charge (e) draw the electric field pattern between 2 isolated point charges 3. 4. An object is neutral when it has an equal number of positive charge and negative charge. • An object will have a net positive charge when there are more protons through loss of electrons. • An object will have a net negative charge when there are more electrons than protons through gain of electrons. 1. Chapter 16.2: Laws of Electrostatics 1. In physics, the space surrounding an electric charge has a property called an electric field. This electric field exerts a force on other electrically charged objects.  An electric field is a region where an electric charge represents an electric force. Electric Fields exert forces on a positively charged particle, or in layman terms: Proton Electric fields do not touch each other due to repulsion of positive charges. The direction of the field is defined as the direction of the force on a small positive charge. When a positively – charged object and a negatively charged object is placed near each other, a positive charge (proton) will travel from a positive charge as it repels it towards a negative charge as it attracts it. The closer the field lines, the stronger the field. (The closer the proton is to the positive charge, the stronger the force exerted). Like Charges repel: 2 electrons will repel each other if they come close to each other. 2. Unlike Charges attract: An electron will be attracted to a proton (Note: Protons do not move!) 3. Note: A neutral charge will not attract or repel anything. 2. 3. Chapter 16.3: Electric Charge 1. Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. The SI unit for measuring electric charge is the coulomb (C). 4. 5. 2. 6. 3. As it is difficult to count the number of electrons as the number is too large, a coulomb is used for a big group of electrons (6.25 x 1018) electrons. As each electron has a negative charge of − 1.602×10− 19, when each electron with this amount of charge is added up, 6.25 x 1018 electrons are needed to form a coulomb of charge. Chapter 16.5: Insulators VS Conductors 1. Materials that do not allow electrons to flow freely inside them are called electrical insulators. It has electrons that are in fixed positions. • Therefore, the addition or removal of electrons in any part of the insulator does not result in the 4. Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. All Rights Reserved. No part of this publication may be reproduced without permission from the author. in both cases. Method 1: Charging by Friction • • By rubbing 2 materials together the materials will get different charges. • In electrical conductors. 3. • When electrons are gained or lost.7: Neutralizing charged insulators and conductors 2. 4. Remove the finger first before removing the rod. Protons do not move at all. Remove the rod. Chapter 16. 2. All Rights Reserved. Discharging a charged insulator: Heating a charged glass rod over a Bunsen flame can neutralize it as the intense heat causes the air surrounding the glass rod to be ionized which will neutralize the excess charges. Method 2: Charging by Induction Charging 2 conductors by induction 3. A positively charged insulator can be discharged by passing it quickly over a flame. Leaving the rod intact.electrons in the order part of the insulator to move. making the overall charge of the rod and the sphere neutral. This causes a positive charge to be induced on the left side and a negative charge to be induced on the right side of the sphere. which is providing a path for the excess electrons to flow away/into the conductor. 4. Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. it can be discharged. The type of charges present in the rod used will determine the type of charge in the conductor. 2. 3.6: Methods of Charging (f) show understanding that electrostatic charging by rubbing involves a transfer of electrons (g) describe experiments to show electrostatic charging by induction Charging 1 conductor by Induction 1. if not once the rod removes the sphere will be earthed again. No part of this publication may be reproduced without permission from the author. When an object is charged. The table below show some combinations the materials will produce specific charges. Place a negatively – charged rod near the sphere. Glass wool rubbed with silk Ebonite rod rubbed with fur Perspex rod rubbed with wool Rubber balloon rubbed with hair Polythene rod rubbed with wool Chapter 16. Note. Positive Glass Fur Perspex Rubber Wood Negative Silk Ebonite Wool Hair Polythene 1. The remaining charges will redistribute themselves evenly. the valence electrons are loosely bound and are delocalized. 2. Leaving the rod intact. 2. Place 2 conductors side by side. • Thus. other electrons will flow in automatically so that the electron will redistribute equally in the conductors. the charge is localized or confined to the region (charge occurs where you rub it) 2. 1. creating an induced positive charge on A and induced negative charge on B. which is the removal of excess charges. there are still some protons left in Sphere B. Electrons will flow away/towards the finger. Always remove the finger before removing the rod for the second case. This will cause the electrons in the metal spheres to be repelled to the far end of Sphere B. 1. All conductors can be discharged by earthing. causing the conductor to lose its charge and become neutral. Only the electrons can move. Bring a negatively charged rod near Sphere A. earth the conductor. For the first case. The charges now spread throughout the entire sphere. separate the 2 spheres. 3. . 3. 1. Discharging a charged conductor: A charged conductor can be discharged through earthing. A positively charged rod will create a negatively charged insulator. Does it matter whether you touch anywhere on the sphere? Ans: Nope. It depends on external influence. which will induce a negative charge on the part of the car facing the nozzle. remember to state the keyword “induced”. Chapter 17: Current Electricity Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. As the spray leaves the nozzle. 6. IMPORTANT: When describing in terms of electro statistics. Chapter 16. . Lightning: Lightning is due to the discharge of a large quantity of electric charge built up in the thunderstorms. Electrostatic Precipitator: Dust and air particles given a negative charge which is then attracted by positively charged plates. Chapter 16. 2. the particles become positively charged.8: Electroscope • • • An electroscope is used to test for charge and the sign of a charge (positive/negative) When it is uncharged it can be used to see if the material is charged or uncharged.g. electrons are induced to the top of the rod and thus the gold leaves become positively charged which will diverge. • • 4. they have the same charge and thus repel each other. Note: Repulsion (making 2 like charges repel) is the only test for the sign of a charge. No part of this publication may be reproduced without permission from the author. electrons flow in so as to neutralise the overall charge in both the rod and the sphere. it ionizes the air which allows a huge quantity of electric charge to be discharged to the nearest object on the ground. 1.8: Hazards/Applications of Electrostatistics Learning Outcomes: (h) describe examples where electrostatic charging may be a potential hazard (i) describe an example of the use of electrostatic charging e. When the charge becomes large enough. it is represented by the symbol: Thinking Questions: 1. Van der Graaff Generator: Used to produce a potential difference of up to 14 million volts. which allow it to spread out more evenly and attract the plant leaf. 3. Spray Painting: • • The car is neutral at first.4. The thunderclouds are charged by friction between the water molecules. Laser Printer: Uses static electricity to produce printouts. If something connected to a charged object earthed it. photocopier and laser printer 2. If the electroscope is charged then a like charge will cause the gold leaves to diverge. if the charged rod is positively charged. Crop Sprayers: As the droplets of pesticides leave the nozzle. what determines whether the electrons move in or out of the sphere? Ans: The charged rod. All Rights Reserved. If a charged rod is brought near to a positively – charged electroscope. For instance. • • 5. In case 2. The particles contain like charges which will repel will then spread out and Get attracted to the car body when sprayed as unlike charges attract. In layman terms. 4. the energy required to move electric charge through the load (bulb) will be contributed equally by each cell. SI unit: Joules/Coulomb (J/C) or Volt (V) It depends on the arrangements of dry cells. d. Q (charges) (C). All Rights Reserved. It is also the rate of change and has a SI unit of Ampere (A). b. Obj116 Obj115 2.m.f. the combined e.4: Resistance 1.m. A load where charges can do a useful job (light bulb. Equation: where V = Voltage (V) I = Current (A) Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. a. Obj117 2. An electric circuit can be represented by a simple circuit diagram (Refer to Appendix 1 for the table on Circuit symbols) An electric circuit is a complete/closed path where charge can flow from one terminal of an electric source to the other terminal. Definition: The electromotive force (e. Obj118 Chapter 17.2: Electromotive Force 3.m. T (time).f. Difference between conventional current and electron flow: Positive  Negative Negative  Positive 3. 4. it is the total amount of energy supplied to a circuit.3: Potential Difference 1. It is the ratio of the potential difference (V) across an electric component (resistor) to the current passing through it.f.) of an electrical energy source is defined as the work done by the source in driving a unit charge round a complete circuit. .m. Equation: whereis the e. Obj114 3. 1. If the arrangement is in parallel. Switches to open/close the circuit 7. 6. of the power supply. Resistance is the measure of how difficult is it for an electric current to pass through a material.f. 2. The voltmeter is used to measure the e. An open circuit is one where there is a break in the circuit to stop the current from flowing.Chapter 17.) between 2 points in an electric circuit is defined as the amount of energy converted to other forms of energy when 1 coulomb of positive charge passes between the 2 points. A short circuit is one where there is a wire which acts as a bypass for the current flowing through the lamp. A source of electromotive force (battery) b. 5.d. resistors) c. thus each cell only need to provide ½ the energy to move the charges through the circuit. Conventional Current Electron Flow 5. This equation determines the amount of charges passing any given point in 1 second. 8. No part of this publication may be reproduced without permission from the author. W is the amount of electrical energy converted from non – electrical forms (work done) and Q is the amount of charge. 2. Equation: where V = Potential Difference W = Electrical Energy converted to other forms Q = Amount of charge SI unit: Volt (V) Chapter 17. is increased as electric charges gain electrical energy from each cell when they pass through them. of a dry cell. Electric current is defined as the continuous movement of charges (current). Conductors to connect the components (copper wire) Potential Difference (p. If the arrangement is in series. It consists of 4 main components: a. Chapter 17.1: Electric Current 1. Equation: where I = current (A). SI unit is m. Nichrome wire is used as the heating coil in electric kettles as the high resistivity allows it to produce lots of thermal energy when a current flows through.E.Parallel: Chapter 17. Materials with high resistivity have their advantages. 5. if a graph is plot of current I against potential diff. Ohm’s Law states that the current passing through a metallic conductor is directly proportional to the potential difference across its ends. where the ratio V/I is not constant include filament lamp. All Rights Reserved.g.Series: Switch Open No current flow Current stops in open branch and continues to flow in other branches.5: Resistors 1. Sum of individual currents in each branch = Main current flowing in and out of the branches. provided that the physical conditions (e.Length (The longer. 6.g. a. the more resistance) d. . R = resistance. 4.Fixed resistors (have fixed values of resistance) b.Resistivity of the conductor (material) 2. thermistor and semiconductor diode. the higher the resistance) b. The arrangement of resistors affects the resistance. Potential difference is the same across each component in a parallel circuit. Obj121 Obj120 5. A = area. For ohmic conductors.Tungsten is used in light bulbs as it can convert electrical energy to light due to its high resistivity. In symbols. 3. Obj126 Obj122 Obj125 a.1: Types of Circuits Definition Series Circuit A series circuit has only 1 path through which electric charge can flow. No part of this publication may be reproduced without permission from the author. Components with smaller resistance get more current. 2. Same throughout the circuit as there is only 1 path for the current to flow. Obj123 b. V.Obj119 Obj124 R = Resistance () 4. Resistivity is how resistant the conductor is. Chapter 18: DC Circuits Chapter 18. They are mainly used to control the size of a circuit. Parallel Circuit A parallel circuit has more than 1 path for current to flow.6: Ohm’s Law 1. These conductors obey Ohm’s law and thus are ohmic conductors. temperature) are constant.2: Action/Use of Circuit Components Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. Ohm’s Law also means that the resistance of metallic conductors remains constant under steady physical conditions. Chapter 18.Rheostats (Variable resistors) (resistance can be varied). . Cross – Sectional Area (The thinner. Non – ohmic conductors. Factors affecting resistance of a material: a. Sum of Potential Difference across each component = Potential difference across whole circuit.Temperature (The higher the temperature. Combined Resistance is the sum of the total resistances. the more resistance) c. Combined resistance is lesser than the smallest individual resistance. l = length. Current Chapter 17. Resistance 3. it is a straight line. Voltage A resistor is a conductor in a circuit that has a known value of resistance. It can be calculated from the formula where p = Resistivity. 2 types of resistors include: a. b. Input Transducers: Thermistor a. Electric Power. Variable Potential Divider (Potentiometer): a. Electric Lighting: Electric light energy is used to light up our world. Obj127 Formula: . A thermistor is a device that has a resistance which changes with temperature. is defined as the rate of work done or energy converted per second. Loudspeakers. Learning Outcomes: (a) describe the use of the heating effect of electricity in appliances such as electric kettles. A potential divider is a circuit with resistors arranged in series. No part of this publication may be reproduced without permission from the author.) from the battery is shared between the 2 resistors c. Generally. It can also be rearranged: Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. Transducers are electrical or electronic devices that transform energy from 1 form to another b. 1. d. A variable resistor can be used to vary the amount of resistance the current has to pass through. washing machines. All Rights Reserved. Transducers that convert electrical energy  other forms of energy are called output transducers. d. LDRs. Thermal energy is generated when an electric current passes through the heating current (nichrome wire) which heats up the surroundings (e. E. most of the electrical energy is converted to thermal energy. Obj128 Formula: . It can be used to measure light intensity.g. lamps. Voltage. 4. The energy released when 1 coulomb of electric charge passes through a potential difference of 1 volt is given by. a. 2. As the amount of light shining on it increases. it gets extremely hot and generate light. SI unit: Volts Obj129 3. hair dryers and electric drills. c. However. a.1: Uses of Electricity 1.Physics Chapter 19: Practical Electricity Chapter 19. Transducers a. They convert electrical energy into rotational kinetic energy. If the resistance increases. between 2 points before/after resistor Zero Maximum Electric heating: Electric thermal energy is used to boil water. E. When electric current flows through it. it is one of the most useful inventions.g. Current Potential diff.g.m.2: Measuring Electrical Energy 1. b. In can be used in circuits for temperature control/measurement. Thus when placed in the dark it has a very high resistance. SI unit: Watt (W) / Joules/sec. water). the resistance of the LDR decreases. thermistors. Transducers that convert non – electrical energy  Electrical energy are input transducers. b. b. 3. ovens and heaters Chapter 19. more energy is required. it is also very thin (gives it a higher resistance). Electric Motors: They are used in household appliances like fans. Heating elements used in kettles/irons are usually made of nichrome wire due to its high resistivity and ability to withstand high temperatures. The total voltage (e. Filament Lamps: Filament is usually made of tungsten coil due to its high resistivity and melting point. cook food and keep ourselves warm. Variable Resistor Set to 0 Set to maximum reading can be used to Maximum Minimum • • Vary current when connected to 2 points Vary current and voltage when connected to 3 points (voltmeter included) 3. V. is defined as the potential difference between 2 points in the electric circuit. b. It is used to divide the total voltage into parts. b. e. Fluorescent Lamps: More efficient as they use less energy than filament lamps. It is a device that has a resistance which varies with the amount of light shining on it.f. P. However. Microphones. 2. voltmeters/ammeters. . LED. c. The larger resistor takes the larger proportion of the voltage. 2. Input Transducers: Light-Dependent Resistor (LDR) a. the resistanceof a thermistor decreases with increasing temperature. it must be handled carefully as the mercury vapour in it is toxic. It can also occur when too much current flows through a thin wire. The ELCB monitors the amount of current flowing from the live wire. b. The equation can also be rearranged to give: Obj136 Obj137 Obj138 Useful to comparing the power dissipated by resistors in series as the current is the same.3: Dangers of Electricity Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. Damp conditions: Many electrical accidents occur in damp conditions as wet skin can reduce the electrical resistance of the human body. . c. The cost of each kWh depends on the price of oil/natural gas. c. One common cause is poor insulation Obj139 5. c. 2 commonly used circuit breakers are the Miniature Circuit Breaker (MCB) and the Earth Leakage Circuit Breaker (ELCB). might occur when a fan motor overheats and melts. and f. exposing the conducting wires inside. If there is current leakage to the earth wire. and c. a. When too many electrical devices are connected to 1 wall socket. Overheating of cables. All Rights Reserved. the current that flows through may be excessive and can cause overheating of the cables. a. Take note of the units.4: Safe use of Electricity at Home 1. It can be reset by switching it on again. Obj130 Obj131 3. The current in the neutral wire should = current in live wire.d. 1 kWh is the amount of electrical energy used by the 1kW device in 1 hour. a. fusing the live and neutral wires. Earth wire. The exposed live wire can cause severe electric shock to the user if touched  Lead to severe injury or even death. e. 3 pin plugs. 2. This might cause the electrical insulation to crack and break. 4. Double insulation of certain appliances. it is detected and thus the ELCB trips. Energy consumption is calculated based on the number of kilowatt-hours. Safety features installed in our homes include: a. a. Overheating of cables: It occurs when an unusually large current flows through the conducting wires. Obj132 Obj133 Obj134 Obj135 4. The MCB prevents excessive flow through the circuit by tripping or breaking it. b. d. Damaged insulation. is the same. Circuit breakers are safety devices that can switch off the electrical supply in a circuit when there is an overflow of current. Useful to comparing the power dissipated by resistors in parallel as the p. power must be in kW and time must be in hours. Fuses. Electricity can be dangerous due to 3 reasons: a.1. Damp conditions. Learning Outcomes: (b) recall and apply the relationships P = VI and E = VIt to new situations or to solve related problems (c) calculate the cost of using electrical appliances where the energy unit is the kWh Chapter 19. Damaged Insulation: Electrical wires are usually insulated with rubber which might become worn with time and use. Power is the rate at which electrical energy is released. b. Learning Outcomes: (d) state the hazards of using electricity in the following situations: (i) damaged insulation (ii) overheating of cables (iii) damp conditions Chapter 19. 2. No part of this publication may be reproduced without permission from the author. causing more resistance which create more thermal energy  Fire b. Circuit breakers b. b. Correct placement of switches. Plastic. are materials like cobalt. it will come to rest in the North – South direction. 5. wood A material that retains its magnetism for a long time is known as a permanent magnet. c. b. 2. It can be used in a compass as well. fuses. They are found very near to the ends of the magnet. unlike poles attract. A fuse is a safety device included in an electrical circuit to prevent excessive current flow. When the magnetic material is magnetized. 3. E. Laws of Magnetism: Like poles repel. The electric cable is insulated from the internal components from the appliance. It must be connected to the live wire and not the neutral wire so that switching off disconnects the high voltage from the appliance. Magnetic Properties: a. Each of these groups is called a domain. a. These are the strongest parts of the magnet. Alignment of Dipoles: For a piece of magnet. A magnet can be identified through a repulsion test. millions of neighbouring atoms align themselves in groups each pointing in a certain direction. No part of this publication may be reproduced without permission from the author. The tiny poles at the end of the magnet are not parallel to one another due to the repulsion of like poles. Other materials not attracted by a magnet are non – magnetic materials.1: Magnets and Materials Chapter 20. It should be connected to the live wire so that the appliance will not become charged after melting due to overflow of current. North and South Poles: If a magnet is allowed to hang freely. The internal components are also insulated from the external casing. If excessive current flows through the appliance. e. a. b. or ferromagnetic materials. nickel or steel that is attracted by a magnet (magnetite – iron oxide) a. thus every atom have its magnetic field. All Rights Reserved. Switches are placed in a circuit to break or complete an electric circuit. the fuse blows and breaks the circuit. 5.in old electrical appliances. d. Plugs and sockets are used to connect a portable appliance to the mains supply. neutral and earth (h) describe the wiring in a mains plug (i) explain why switches.g. A thicker wire requires a greater current to melt it. b. Magnetic Poles: Every magnet have a North and South Pole. which is a mini magnet.2: Magnetic Induction Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. d. 6. a. Earthing thus prevent this from happening. 1. 4. and circuit breakers are wired into the live conductor Physics Chapter 20: Magnetism Chapter 20. Earthing: The Earth wire is a low – resistance wire which is usually connected to the metal casing of the appliance. a type of rock will point in the North – South direction when allowed to float freely / suspended. 3. Magnetism is caused by the orbiting electrons around each atom. a. b. Theory of Magnetism: In magnetic materials. b. Another type is the ceramic magnet made from ferrites (iron oxide compounds). It consists of a thin piece of wire which becomes hot and melts when the current flowing through it is greater than its rated value. 7. Fuses should have a current rating slightly higher than the current an electric appliance use under normal conditions. It is commonly found for appliances which use a 2 pin plug (no earth wire) b. a. . They are usually made of steel (alloy of iron and carbon) or other alloys. a. the domains are aligned in the same direction. c. The earth wire will divert the large current from the metal casing to the ground. the user could get an electric shock. b. 4. the more aligned its dipoles are. Double insulation is a safety feature in an electric appliance that can substitute the earth wire. If there is a fault (live wire not properly connected and touches metal casing). The end pointing to the Northern end of the Earth is the North – Seeking pole. c. d. a. Appliances with this feature usually have non – metallic casings like plastics. the stronger it is. and c. The cartridge fuse in the plug protects the appliance when there is an electric fault. Learning Outcomes: (e) explain the use of fuses and circuit breakers in electrical circuits and of fuse ratings (f) explain the need for earthing metal cases and for double insulation (g) state the meaning of the terms live. Magnetic materials. Lodestone. iv.6. only the part of the magnet out of the solenoid will stop rotating. Using 2 magnets will cause the magnetic field to be stronger. When a direct current is passed through it. 13. causing the magnetic domains to lose their alignment. 9. It is invisible. An alternating current is an electric current which varies its direction many times per second. ii. supply (alternating current) iii. a. The Earth behaves as if it has a large imaginary magnet within it.c. The north pole will attract the southern part of the domains and vice versa. making the stroking method more effective. The magnetic material will become a weak magnet.4: Magnetic Fields iii. i. Place a magnet inside a solenoid connected to an a. The stroking magnet is lifted high above the steel bar between successive strokes. All Rights Reserved. Heating: The atoms in the magnet will vibrate vigorously when heated. There are 3 ways to demagnetize magnets: a. Electrical Method using Alternating Current: i. The domains are all aligned and thus the magnet is magnetically saturated and cannot be made any stronger. The poles of the magnet can be determined using the Right hand grip rule. Thus bar magnets are stored in pairs using soft iron keepers to allow the poles of the atomic magnets to remain in closed loops. v. Magnetic Saturation: Occurs when a magnet reaches its maximum strength. Induced Magnetism occurs when a non – magnetized magnetic material is brought near to a magnet. 8. i. b. Electrical Method using Direct Current (Solenoid): 12. thus the poles at the end of the bar is opposite to the stroking pole used.3: Magnetization and Demagnetization 7. Disadvantage: Slow process and magnet produced is not very strong. a. Demagnetization of Magnets: Demagnetisation is the process of removing magnetism from a magnet by messing up the directions of the domains. experiences a magnetic force. vi. c. b. The pole which is induced is the opposite pole of the magnet inducing it. which is a cylindrical coil of insulated copper wires with lots of turns Chapter 20. iii. ii. thus it is easier to align the domains. Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. It is probably caused by electric currents circulating within the core of the Earth. . causing it to lose its magnetism. b. 11. It is the main reason why non – magnetized objects are attracted to magnets as the dipoles are aligned when the magnet induces it. There are 2 ways of making magnets. No part of this publication may be reproduced without permission from the author. a. ii. b. iv. Stroking Method: An unmagnetised steel bar is stroked several times with the same pole from one end to another in the same direction. 10. the unmagnetised steel bar will become magnetized after a while as a strong magnetic field magnetizes the steel bar. The steel object is placed inside a solenoid. placed within the influence of the field. A magnetic field line is the path taken by a free north pole of another magnet. A magnetic field is a region in which a magnetic object. Hammering: Hammering alters the alignment of the magnetic domains. The magnet is slowly withdrawn in the East – West direction with the alternating current still flowing within the solenoid. Thus. altering the domains. However it can be revealed by placing a piece of paper and sprinkling iron fillings around it to show the magnetic field pattern. the magnets will become weaker as the “free” poles near the ends of the magnet will repel each other. Chapter 20. Magnets in different sections will thus stop rotating at different positions. This is the most effective method. Storage of Magnets using Soft Iron Keepers: If magnets are stored side by side. Circles nearer to the wire are closer to one another as magnetic field is stronger in the region around the wire. Increasing the current. c. The Right Hand Rule can be used to determine the direction of the current flow. ii. Similarly to the electromagnet. a. Placing a soft iron core within the solenoid to concentrate the magnetic field lines. Magnetic field lines are directed outward from the North pole and toward the South Pole. By convection. b. a. however they have different magnetic properties. c. 14. A current – carrying conductor has a magnetic field set up around it. 3. Increasing the number of turns per unit length of the solenoid. d. They do not cross or intersect one another. Circuit breaker ii. Thus no magnetic field will travel through the air. or iii. The point between 2 similar poles where no magnetic field exists is known as a neutral point. Although both Iron and Steel are magnetic materials. All Rights Reserved. Magnetic Resonance Imaging (MRI) Chapter 20. 16. The strength of an electromagnet can be increased in several ways: i. the magnetic field will spread out and become weaker) Physics Chapter 21: Electromagnetism Chapter 21. Magnetic field lines form concentric circles around the wire. Increasing the magnitude of the current through the wire causes more magnetic field lines to form. Magnetic Shielding prevents surrounding magnetic fields from reaching sensitive areas of a piece of equipment which can be damaged by magnetic fields. d. b. c. b. the core will be magnetized. Inserting a soft iron core into solenoid (without it. c. Electric Bell iv. d. the stronger the magnetic field.5: Temporary and Permanent Magnets 15. b. An electromagnet consists of a solenoid covering a metal core. Increasing number of turns iii. Magnetic Relay iii. No part of this publication may be reproduced without permission from the author. Electromagnets can be used in the following devices: i. The North and South pole can be determined by using the Right hand grip rule. The magnetic field is stronger inside the solenoid due to the closer magnetic field lines. b. c. b. The resulting field pattern of the solenoid resembles that of a bar magnet. When the wood is replaced with soft iron. Increasing amount of current ii. Iron is a soft magnetic material. . as magnetic fields tend to travel through soft iron and concentrate there.a. It is achieved by surrounding the object with soft iron. the magnetic field of the solenoid can be made stronger by: i. a solenoid is formed. e. 2. A soft magnetic material magnetizes and demagnetizes very easily as compared to a hard magnetic material. e.1: Magnetic Field pattern around a straight wire 1. a. The parallel field lines also show that the magnetic field strength inside is almost uniform. whereas steel is a hard magnetic material. a dot represents the current coming towards whereas a cross represents the current flowing away. It will be demagnetized when the current is switched off. the magnetic fields travel through the iron and thus the paper clips fall. a. Reversing the current through the wire causes the direction of the magnetic field to reverse. Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. The closer they are. A soft magnetic material can be used as temporary magnets in electromagnets. When the current is switched on. a. By increasing the number of turns on a flat coil. Current in similar directions causes attraction.6: The d. b. Chapter 21.Chapter 21. Why? The plan view of the magnetic field shows that the magnetic field of the wires are opposing each other. a force is expected to act on each wire. Current in opposing directions causes repulsion. This force acts perpendicular to both the direction of the current and the direction of the magnetic field.c. All Rights Reserved. c. The magnetic field pattern of a current – carrying wire can be found by applying the right hand grip rule. A force thus acts on the wire from the stronger field to the weaker field. The sides of the wire AB and CD experience equal and opposite forces. 5. motor. This force is present as when a wire carrying current passes through a magnet. When a current carrying wire is placed in a magnetic field.3: Force on a moving charge in a magnetic field 6. one side of the wire have the magnetic field lines all acting in the same direction whereas at the other side the magnetic field lines of the current oppose those of the magnet. Fleming Left Hand Rule can be used to determine the direction of the force. making the combined field weaker. a turning effect will be observed on the coil. Chapter 21. here we shall examine the effect of charges in a magnetic field. d. a. b. b. When 2 current carrying wires are placed parallel to each other.c. the wire experience a force which is known as the motor effect. the coil will experience a clockwise movement about the axis PQ. This direction is reversed when the direction of the current or magnetic field is reversed. causing the wire to turn clockwise. c. motor Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. This effect is being used in the d. a. No part of this publication may be reproduced without permission from the author. Since a current – carrying wire experiences a force when it is placed in a magnetic field. . Chapter 21. a. and that current is due to the flow of electric charges.5: Force on a Current – Carrying Rectangular Coil in a Magnetic Field 8. Chapter 21.4: Force between 2 parallel current – carrying wires 7. a. If a stiff current – carrying wire coil is placed between the poles of a strong magnet. 9. If the direction of the current is in a clockwise direction.2: Force on Current – carrying Conductors 4. f. Number of turns in the solenoid. c. Magnetic Pole of Solenoid North Pole South Pole South Pole North Pole Direction of induced e. 2. a. This law explains then amount of e. Anticlockwise Clockwise Clockwise Anticlockwise 3. ii.c.10. induced in the presence of a changing magnetic field. a. Faraday’s Law of Electromagnetic Induction states that the magnitude of the e. Lenz Law states that the direction of the induced current in a closed circuit is always such that its magnetic effect opposes the motion or change producing it.m. produced is proportional to the rate of change of the magnetic lines of force linked with the circuit or the rate at which the magnetic field lines are cut. A soft iron core/cylinder can be inserted into the coil to concentrate the magnetic field. Permanent magnets c. depends on 3 factors: i. When the magnet is stationary in the solenoid. It consists of the following components: a. 5. b. This helps to ensure that the coil will always turn in the same direction. b. is maximum when the coil is parallel to the magnetic lines of force as the 2 sides are cutting the magnetic field at the greatest rate. The d. The purpose of the split ring commutator is to reverse the direction of the coil every half a revolution whenever the commutator changes contact from one carbon brush to the other. The direction of the induced current can be deduced using Flemming’s Right Hand Rule. Firstly. 12.m. (electromagnetic force) which will an induced current in a closed circuit. Rectangular coil connected in series to a battery and rheostat. 13.f.. The number of turns or the current in the coil can be increased. The turning effect (intertia) of the coil however carries it past that position which reverses the current direction of the wire and causing the forces to become opposite. d.f.f. Speed at which the magnet moves.f. Field (Forefinger) c. and iii. To increase the turning effect of the coil in the motor.f. c.m. the galvanometer needle deflected in one direction. b. .f. No part of this publication may be reproduced without permission from the author.m. 2 carbon brushes. b.m.m. Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010.2: Alternate Current Generator Physics Chapter 22: Electromagnetic Induction Chapter 22. a. This could be seen in Faraday’s Solenoid Experiment. b. a. Current (seCond Finger) The induced e. All Rights Reserved.1: Electromagnetic Induction 1. 11. The coil thus continues to move in an anticlockwise direction. When Faraday brought a magnet close to a solenoid. When Faraday withdrew the magnet. the galvanometer needle deflected in the opposite direction. It gives us the direction of the induced e.m. Direction of bar Magnet North pole inserted North pole withdrawn South pole inserted South pole withdrawn Chapter 22. the needle did not deflect. IMPT: Split ring commutator d. a downward force will act on the right side and an upward force on the left side till the coil reaches a vertical position. The magnitude of the induced e. Motion (Thumb) b. The current is then cut off for a moment as the split ring communtator is not in contact with the carbon brushes. motor is a device that converts electrical energy into mechanical energy. 4. a. Strength of the magnet. a. Electromagnetic Induction is the process whereby a varying magnetic field can produce an e. a. ) is used to show how voltage varies with time. Obj144 c. Leakages of magnetic field lines between the primary and secondary coils. No part of this publication may be reproduced without permission from the author. It consists of 3 parts: a.R. of turns on the coil. Using stronger permament magnets. Increasing no. c. One main problem in transmitting electricity is the loss of power due to Joule Heating in the cables. 14. or d. Heat loss due to resistance of coils. of an a.O. Chapter 22.R.f. The induced e. Diodes are used to convert an alternating current to a direct current. Chapter 22. Obj142 a. 15. Increasing the frequency of rotation of the coil. 9. Power loss as thermal energy: Chapter 22. Thisis because the 2 sides are moving parallel to the magnetic field are are not cutting any magnetic field lines. The induced e. The current flowing in the supply line can be found by using this formula: 7.99% or below efficient). The diode can convert a.m. Winding the coil on a soft – iron core to strengthen the magnetic field lines.C. In a step-down transformer. where V represents voltage and N represents No.C. Obj140 a. c.O. b. For a non – ideal transformer (99. 16. to D. b. as the power from the primary coil is completely transferred to the secondary coil.f. 13. 17.5: Cathode Ray Oscilloscope 10. through rectification. .4: Converting A. into d.c.: Electron Gun Grid Accelerator (Anode) X and Y plates Consists of a filament which release electrons when hot Controls brightness Attracts electrons and attracts it to the screen Controls where the electron lands. generator can be increased by: a. Laminated Core: Used to link the magnetic field from the primary coil to the secondary coil. For an ideal transformer (100% efficient). preventing the current to flow in the reverse direction.6. All Rights Reserved.c. the voltage of the primary coil is higher than the voltage of the secondary coil.m. is 0 when the coil is perpendicular to the magnetic field lines. Obj143 b. Parts of the C. of turns. . The cathode-ray oscilloscope (C. Likewise for step-up transformer. b. Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. Obj141 11. Secondary Coil. The causes of power loss include: i. There are more turns in the primary coil compared to the secondary coil. a. 12.c. This can be minimized by having thicker cables or reducing the current in the cables using a step-up transformer. and ii.3: Transformers 8. A transformer is a device that changes a high alternating voltage at low current to a low alternating voltage at high current and vice versa. The diode is a semiconductor device that allows a current to flow easily in 1 direction only. Primary Coil c. 2 other parameters to get a proper waveform: a. The X-shift and Y-shift knobs at the front panel are used to position the trace at the centre of the screen. Y – Gain: It amplifies the Y-deflection. b. Time Base: Controls the speed at which the electron bean shifts from left to right (done by altering the frequency of the time base) Appendix 1: Circuit Symbols Physics (Syllabus 5058): Measurement | Newtonian Mechanics | Thermal Physics | Waves | Electricity and Magnetism Copyright © 2010. All Rights Reserved.18. No part of this publication may be reproduced without permission from the author. .