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# Electromagnetism

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### Electromagnetism

1. 1. ELECTROMAGNETISM 1. Define the following terms & phrases (charge, static electricity, charging by friction/contact/induction, conductor, insulator, uniform/non-uniform charge distribution, earthing and electrical discharge) 2. Describe the behaviour of like and unlike charges. 3. Name and give symbols for the following: DC power supply, cell, battery, switch, lamp, resistor, variable resistor, wires joined, wires crossing, ammeter, voltmeter & fuse 4. State the symbol and metric unit for: charge, current, voltage, resistance & power 5. Define the following terms and phrases: Ohm’s law, DC electricity, series, parallel, current rules and voltage rules. 6. Describe the differences and similarities in the way ammeters and voltmeters are used. 7. Draw and interpret DC circuit diagrams 8. Solve problems using V = IR, P = VI, P = E and Rtotal = R1 + R2 t 9. Describe the magnetic field patterns around permanent magnets, the earth, currents and coils. 10. State the symbol and unit for magnetic field. 11. Use the right hand grip rule to determine relative field (B) and current (I) directions. 12. Describe how the magnetic field due to a current in a straight wire varies with the size of current and the distance from the wire. 13. Solve problems using B = µ0I 2 πd Thursday, 16 September 2010
2. 2. Thursday, 16 September 2010
3. 3. LANGUAGE Thursday, 16 September 2010
4. 4. THE LANGUAGE OF ELECTRICITY Term Definition Word list Charge an electrical quantity based on an excess or deficiency of electrons static a form of electricity where charge does not flow continuously electricity Thursday, 16 September 2010
5. 5. NOTES Thursday, 16 September 2010
6. 6. ELECTRO STATICS Thursday, 16 September 2010
7. 7. CHARGING OBJECTS Based on atomic structure Electron (-ve charge) Neutron Proton (+ve charge) The process Empty space 1. Charge transfer - When two objects are in contact with each other, one object can transfer electrons to the other object. Protons are not transferred because they are “locked” into the nucleus. 2. Charge imbalance - When the two objects are moved away from each other the process of charge transfer is unable to be reversed. • Positively charged objects have had electrons removed • Negatively charged objects have gained electrons Oppositely charged objects attract each other. Those with like charges repel. Thursday, 16 September 2010
8. 8. CHARGE INTERACTION Oppositely charged objects Objects with the same attract each other charge repel each other + - + + - - + - + + - - Demo: The Van der Graaf Generator Thursday, 16 September 2010
9. 9. EXAMPLES Read p52 and 53 (Y10 Pathfinder) and then offer some examples to the class discussion: Static electricity around us The History of Electricity Generation Thursday, 16 September 2010
10. 10. LIGHTNING Warm air currents ascend. Ice crystals descend removing electrons from cloud particles in this zone. A zone of positively charged cloud particles results At ground level the air becomes ionized (by losing electrons) and these positively charged particles are attracted to the negatively charged base of the cloud to give rise to a lightning bolt (an upstrike) Thursday, 16 September 2010
11. 11. CLOUD TO GROUND Thursday, 16 September 2010
12. 12. CLOUD TO CLOUD Thursday, 16 September 2010
13. 13. BLUE JETS AND SPRITES Thursday, 16 September 2010
14. 14. CHARGING OBJECTS Methods of charging Induction - the object being charged is not in contact with the object doing the charging (usually a rod or a ruler). It involves charge transfer to or from the earth to generate the charge imbalance in the object being charged. A charge imbalance is a non-uniform charge distribution Eg. -- ++++ ---- The problem with moisture in the air: Moisture prevents objects from holding a + + charge because it transfers charge to or + + from the object resulting in a neutral This symbol object. represents a connection to the earth Contact - the object being charged is contacted by the other object and charge is transferred directly from one object to another. Friction - the object is rubbed by a material that has a greater or lesser affinity for electrons and a transfer takes place. It is more the contact than the friction that is necessary for the charging to take place. Thursday, 16 September 2010
15. 15. The importance of the material Conductors - are materials that allow charges to flow through them. They do not hold a static charge because any charge imbalance is easily conducted away. Insulators - are materials that doe not allow charges to flow through them. They will hold a static charge because the charge imbalance is not easily conducted away. Example - ESA: Ex 15A Q.1 ESA: Ex 15B Q.1, 2 & 3 Thursday, 16 September 2010
16. 16. CURRENT VOLTAGE Thursday, 16 September 2010
17. 17. IN TR O DU CT IO N Thursday, 16 September 2010
18. 18. WHAT IS ELECTRICITY? Thursday, 16 September 2010
19. 19. THE ELECTRICAL CIRCUIT - introducing the idea of the electron pump Thursday, 16 September 2010
20. 20. 1. What is an electric current? 2. What are the two requirements necessary for an electric current to exist? Power Supply + - A conducting path Thursday, 16 September 2010
21. 21. THE GRAVITY MODEL http://regentsprep.org/Regents/physics/phys03/bsimplcir/default.htm A B 1.Which part of the model represents the power supply? 2. Which part of the model represents the component? 3. What type of current is being modelled? Thursday, 16 September 2010
22. 22. THE WATER MODEL 1.Which part of the model represents the power supply? 2. Which part of the model represents the conducting path? 3. Which part of the model represents charge? Thursday, 16 September 2010
23. 23. THE BIKE MODEL C A B 1.Which part of the model represents the power supply? 2. Which part of the model represents the conducting path? 3. Which part of the model represents charge? Thursday, 16 September 2010
24. 24. THE BIKE MODEL one link THINK OF A LINK AS REPRESENTING A COULOMB OF CHARGE 1. In terms of this model, what do you think is meant by the term “current” ?? Thursday, 16 September 2010
25. 25. ELECTRIC CIRCUITS Requirements: Power Supply a power supply conducting path around which charge (electrons or ions) can flow. + - components (and sometimes meters) A component Current A conducting path is a flow of electrons through a circuit Two types: AC - Alternating current (electrons vibrate back and forth in wires) DC - Direct current (electrons flow in wires in one direction only) Conventional current - the direction in which positive charges would flow in wires if they could. Conventional current is from positive to negative in a circuit (see diagram above) http://regentsprep.org/Regents/ physics/phys03/bsimplcir/default.htm Thursday, 16 September 2010
26. 26. ELECTRIC CURRENT Current - is the rate of flow of electrical charge - it is the number of coulombs of electrical charge that passes a point in one second. A coulomb is 6.25 x 1018 charges - I = electric current (measured in amps, A) by an ammeter: Red V Black Connecting an ammeter + - I I A Red Black For charge to flow around an electrical circuit there is a need for a voltage source and a conducting path that is continuous and connects the positive to the negative terminal of the power supply. - + Example charge flows through the circuit as indicated by the arrows Thursday, 16 September 2010
27. 27. VOLTAGE Voltage • Voltage (V) is a measure of the energy lost or gained between two points in a circuit. • It is measured in the units volts , (V) where V = potential difference or voltage (Volts, V) V = ∆Ep q ∆Ep = change in potential energy that a charge experiences when it moves from one side to the other side of a component (Joule, J) q = the unit of charge (Coulomb, C) Unit of Voltage: Joule per Coulomb or Volt (JC-1) or (V) Example V Consider the voltage across a lamp: - + A B 1A If V = 6V then a coulomb of charge has 6J more electrical potential energy at point A than it does at point B Thursday, 16 September 2010
28. 28. Notes CIRCUIT SYMBOLS demo of circuit components -> + - V A • Two wires joined Two wires crossing Cell Lamp Battery (two cells in Switch series) Battery (several Diode cells) Voltmeter Ammeter Resistor Power supply variable resistor fuse (rheostat) Thursday, 16 September 2010
29. 29. RE SI & ST OH A N LA M’ CE W S Thursday, 16 September 2010
30. 30. IN TR O DU CT IO N Thursday, 16 September 2010
31. 31. RESISTANCE & ELECTROCUTION - 1 Thursday, 16 September 2010
32. 32. RESISTANCE & ELECTROCUTION - 2 Thursday, 16 September 2010
33. 33. FACTORS AFFECTING RESISTANCE Thursday, 16 September 2010
34. 34. FACTORS THAT AFFECT RESISTANCE Also, some materials conduct electricity better than others Eg. Copper is better than iron Thursday, 16 September 2010
35. 35. CONDUCTORS & INSULATORS In a conductor, electrons In an insulator, are free to flow electrons are fixed _______________ Label the materials that the _______________ arrows are pointing to Thursday, 16 September 2010
36. 36. THE VOLTAGE-CURRENT RATIO 1. Consider a lamp in an electrical circuit: 12V 2A 12V represents the energy difference across the lamp. This drives electrons through the lamp at the rate (or “speed”) of 2A. The voltage:current ratio is _____ 2. Consider a different lamp in an electrical circuit: 12V 1A This lamp has higher resistance because 12V across this lamp can only drive electrons through the lamp at a rate of 1A. The voltage:current ratio is _____ This example shows that the greater the voltage:current ratio then the greater the resistance is. Resistance is the voltage:current ratio Thursday, 16 September 2010
37. 37. RESISTANCE Definitions 1. Resistance, R is a measure of the “electrical friction” in a conductor. (the opposition to the flow of current) 2. It is the ratio of the voltage across a conductor to the current through it. Resistance = Voltage Current R=V Unit of resistance I is the ohm, Ω V I R Resistance is given by the slope or gradient of a voltage - current graph Example In an experiment, the voltage across a lamp is measured and recorded as the current is increased 1 A at a time. Calculate the resistance of the lamp. V (V) 24 20 16 12 8 4 0 1 2 3 4 5 6 I (A) Thursday, 16 September 2010
38. 38. WHATS HAPPENING TO THE RESISTANCE AS THE CURRENT INCREASES? 24 A conductor that retains a constant 1 V (V) 20 temperature as the current is increased: 16 12 8 4 0 1 2 3 4 5 6 I (A) A conductor that is allowed to heat 24 2 V (V) up as the current is increased 20 16 12 8 4 I (A) 0 1 2 3 4 5 6 A conductor that is cooled progressively 3 V (V) 24 20 as the current is increased 16 12 8 4 I (A) 0 1 2 3 4 5 6 Thursday, 16 September 2010
39. 39. MODELLING TEMPERATURE INCREASE IN A WIRE SHAKING Thursday, 16 September 2010
40. 40. Thursday, 16 September 2010
41. 41. LIMITATIONS OF OHM’S LAW 24 V (V) 20 When a temperature of a lamp increases its 16 resistance increases 12 8 4 I (A) 0 1 2 3 4 5 6 For most conductors, as the temperature increases the increased vibration of particles impedes the flow of electrons. Resistance in the conductor will therefore increase. The graph slopes upwards. V (V) 24 The resistance of a thermistor decreases as its 20 16 temperature decreases 12 8 4 I (A) 0 1 2 3 4 5 6 Thursday, 16 September 2010
42. 42. BASIC RESISTANCE PROBLEMS 1. What is the resistance of a bulb it a 240 V supply causes a current of 2 A to flow through it? 2. What current flows through a heating element of 40Ω resistance when the element is plugged into a 240 V supply? 3. If a current of 3 A is flowing in a resistor across which there is a voltage of 6 V, what is the resistance? 4. What current must be flowing through a lamp of 0.5Ω resistance if there is a voltage of 6V across it? 5. A current of 2 A flows through a 6Ω resistor. What is the voltage across it? 6. What voltage is needed it a current of 5A is to flow through a resistance of 3Ω? Thursday, 16 September 2010
43. 43. RESISTANCE CALCULATIONS Resistors which are connected end to end are in series with one another R1 R2 The total resistance of the series combination, Rs is the sum of the resistances R1 and R2. For two or more resistors in series: Rs = R1 + R2 + ........... Resistors which are connected side by side are in parallel with each other. R1 R2 The total resistance of the parallel combination, Rp is less than any individual resistor in the combination. For two or more resistors in parallel 1 1 1 + .... the total resistance,Rp is given by: RP = R1 + R2 Thursday, 16 September 2010
44. 44. Thursday, 16 September 2010
45. 45. SERIES & PARALLEL Thursday, 16 September 2010
46. 46. CIRCUITS: diagrams & assembly Draw the following circuit diagrams in the spaces provided AND when you have finished, assemble them: 1. + - 2. + - 3. A V + - Voltmeters are connected _________ components, ammeters are connected _____ a circuit Thursday, 16 September 2010
47. 47. CHARACTERISTICS OF SERIES & PARALLEL CIRCUITS Series Parallel + - + - + - + - components connected each component has its own 1._ 1._ connection with the power one other the other. supply 2._ Single pathway 2._ more than one pathway 3._ No junctions 3._ One or more junctions Thursday, 16 September 2010
48. 48. http://phet.colorado.edu/simulations/ CIRCUIT CONSTRUCTION sims.php? sim=Circuit_Construction_Kit_DC_Only 1. Enter the URL (above) into the address bar of your internet browser. 2. Use the simulation tools to construct each of the following 3 circuits (ensure that you use identical lamps and an the same power supply for each circuit). 3. Record the current in each circuit and explain your observation. 4. Repeat this exercise for the second set of 3 circuits. 1 2 3 + - + - + - A A A 3 + - 2 A 1 + - + - A A Thursday, 16 September 2010
49. 49. DRAW THE CIRCUITS AND SET THEM UP Thursday, 16 September 2010
50. 50. CIRCUIT RULES Thursday, 16 September 2010
51. 51. VOLTAGE & CURRENT IN SERIES CIRCUITS + - VT A1 A3 I1 I3 I2 A2 V1 V2 Current in series is constant I1 = I2 = I3 Voltage in series is shared VT = V1 + V2 Note Voltage is shared in proportion to the size of the resistance Thursday, 16 September 2010
52. 52. PARALLEL CIRCUITS + - Current in parallel is shared IT VT IT R1 IT = I1 + I2 I1 in other words “charge splits up as it enters a junction in a circuit” V1 R2 I2 Voltage in parallel is constant V2 VT = V1 = V2 Note Current is shared in an inverse proportion to the size of the resistance. For example: If R1 = 5 and R2 = 10 and IT = 3 “Double the resistance then halve the current” then I1 = 2 and I2 = 1 Thursday, 16 September 2010
53. 53. SIMPLE CIRCUIT CALCULATIONS Example 1 + 8V - A3 = ________ 3A A1 V1 V1 = ________ 2A A2 R1 V2 = ________ A3 R2 Rules used V2 ____________________________________________________________________ ____________________________________________________________________ Example 2 + 8V - 3V V1 V2 V1 = ________ R1 Rule used ___________________________________________________________ Thursday, 16 September 2010
54. 54. Example 3 + 8V - A4 A3 = ________ 3V 4A A1 V1 V3 A4 = ________ 3A A2 R1 V2 = ________ V3 = ________ A3 R2 Rules used V2 ______________________ ______________________ ______________________ Example 4 ______________________ ______________________ + 8V - A4 A1 = ________ A1 3A A4 = ________ A2 Rule used ____________________ 4A ____________________ A3 ____________________ Thursday, 16 September 2010
55. 55. ADVANCED CIRCUIT CALCULATIONS Examples + 9V - 1 A1 V1 A3 5Ω 10Ω A2 V2 V3 For the circuit represented by the circuit diagram above, what is the reading on: (a) V1 (b) A2 (c) V2 (d) V3 Thursday, 16 September 2010
56. 56. 2 + 15V - V1 5Ω A3 V2 A1 V3 R 10Ω A2 For the circuit represented by the circuit diagram above, what is the reading on: (a) V3 if V2 = 10 V (b) A1 (c) A2 (d) A3 (e) What is the value of resistor R? Thursday, 16 September 2010
57. 57. 3 + 12V - V1 1A 2Ω 4.8Ω V3 A1 V2 3Ω A2 For the circuit represented by the circuit diagram above, what is the reading on: (a) V1 (b) V2 (c) V3 (d) A2 Thursday, 16 September 2010
58. 58. RESISTORS IN SERIES + - R1 RT = R1 + R2 R2 Examples 1 2 25 Ω 30 Ω 50 Ω 100 Ω 100 Ω 100 Ω • • Total resistance = Total resistance = As we add resistors in series the resistance increases and therefore the current drawn decreases As we add resistors in parallel the resistance decreases and therefore the current drawn increases Thursday, 16 September 2010
59. 59. OT HE R ST UF F Thursday, 16 September 2010
60. 60. ESA: 13B Q.5 to 8 Thursday, 16 September 2010
61. 61. ESA: 13B Q.5 to 8 Thursday, 16 September 2010
62. 62. Thursday, 16 September 2010
63. 63. INTRODUCING “EMF” & “ELECTRICAL POTENTIAL Thursday, 16 September 2010
64. 64. METERS Thursday, 16 September 2010
65. 65. + 9V - One of these meters has IT a very high resistance The other meter has a A low resistance. Which is which? V Explain your answer Thursday, 16 September 2010
66. 66. METERS Ammeter + 9V - 1. connected in series with IT other components A 2. has low resistance so that it doesn’t slow the current that it is supposed to be measuring Voltmeter 1. connected in parallel with other V components 2. has high resistance so that it doesn’t allow much current to flow through it. This would reduce the current and voltage through the component. It is supposed to be measuring the voltage Thursday, 16 September 2010
67. 67. POWER & ENERGY Thursday, 16 September 2010
68. 68. POWER • Power is the rate at which electrical energy is transferred into other forms of energy. • It is the amount of work done per second E E = the amount of energy converted or work done (J) P = E t P t t = the time taken (s) • It can be shown that the electrical power supplied to a device is given by: P P = power (watts, W) (1W = 1 Js-1) P = VI V I V = Voltage (volts, V) I = Current (amps, A) P = Power (Watts, W) Thursday, 16 September 2010
69. 69. POWER & ENERGY Total energy used by a component/appliance can be calculated from the equation: E = P.t When the power value of the component/appliance is known and this value does not change over time. If power changes over time then this change can be graphed. P (W) 14 E = Area under the graph 12 10 8 E = 0.5 (9 + 13) = 4.4 J 6 10 4 2 0 2 4 6 8 10 t (s) Thursday, 16 September 2010
70. 70. TOTAL POWER USAGE in a parallel circuit Example The power usage of the 4 lamps in parallel shown in the circuit below can be calculated in two ways: (i) Use the total voltage (supply voltage) and the total current (current drawn from the supply) to calculate power. (ii) Add the power usage of each of the components in parallel. (i) lamps 1 & 2: I = P 12 V V = 6/12 = 0.5 A 12 V P1 6W Lamps 3 & 4: I = P V 12 V = 12/12 P2 6W =1A 12 V IT = I1 + I2 + I3 + I4 = 0.5 + 0.5 + 1 + 1 = 3A P3 12 W P = VTIT = 12 x 3 = 36 A 12 V P4 12 W (ii) PT = P1 + P2 + P3 + P4 = 12 + 12 + 6 + 6 = 36 W Thursday, 16 September 2010
71. 71. LAMP BRIGHTNESS IN CIRCUITS Three main points (i) The brightness of a lamp depends on its power output since for a lamp, power is the rate at which electrical energy is converted into light (and heat) (ii) In a circuit which has values of voltage and current, it is both the voltage and current that determine brightness. (iii) The lamp’s resistance will determine that voltage:current ratio that it possesses Example: The series circuit (below), shows 2 identical lamps. A third identical lamp is added to the circuit. Explain how the brightness of the lamps in the circuit changes 12 V + - Thursday, 16 September 2010
72. 72. EXERCISES Thursday, 16 September 2010
73. 73. POWERING THROUGH THE QUESTIONS TV Stereo Jug Heater Stove torch downlight Voltage (V) 240 240 240 6 Current (A) 0.3 0.02 8 0.05 Resistance (Ω) 250 450 Power (W) 1000 1200 1800 0.3 140 Running time 20 min 20 min 10 min 3h (s) Energy (J) 1000 125000 300000 Thursday, 16 September 2010
74. 74. “PARALLEL WITHIN SERIES” 0.5 A + 9V - A 9 V battery is connected in the circuit shown. 10Ω A current of 0.5 A is found to 0.1 A pass through the 10Ω resistor. 40Ω X (a) Calculate the voltage across the 10Ω resistor. (b) Show that the voltage across the parallel combination of resistors is 4.0 V. (c) If 0.1 A passes through the 40Ω resistor, determine the current through resistor X. (d) Show that the resistance of resistor X is 10Ω. (e) Determine the heat energy generated per second in the whole circuit. Thursday, 16 September 2010
75. 75. PARALLEL CIRCUIT IN ACTION A car has two tail lights and two brake lights connected as shown in the diagram: (a) Calculate the resistance of: (i) a tail light (ii) a brake light (b) Calculate the current supplied by the battery when both S1 and S2 are closed. (c) When the driver takes her foot off the brake S2 is opened state what happens to the size of the current from the battery and give a reason for your answer. Thursday, 16 September 2010
76. 76. Y11 Sci - W & W Thursday, 16 September 2010
77. 77. Y11 Sci - W & W Thursday, 16 September 2010
78. 78. answers “PARALLEL WITHIN SERIES” (a) 5V (b) V in series is shared. The combination of the 40Ω resistor and X are in series with the 10Ω. Therefore V of the combination is 9 - 5 = 4 (c) Current through X = 0.5 - 0.1 = 0.4A because current in parallel is shared. The total current flowing from the power supply is shared out between the 40Ω resistor and X. (d) I through X = 0.4A and V across X = 4V (since voltage in parallel is constant). R = V/I = 4/0.4 = 10Ω (e) “Heat energy per second” is the definition of power. For the whole circuit, I = 0.5A and V = 9V. P = VI = 9 x 0.5 = 4.5W PARALLEL CIRCUIT IN ACTION (a) The current through a tail light needs to be calculated first: P = VI => I = P/V = 6/12 = 0.5A For a tail light, R = V/I = 12/0.5 = 24 Ω For a brake light, P = VI => I = P/V = 12/12 = 1A R = V/I = 12/1 = 12 Ω (b) I total = 2 x 0.5 + 2 x 1 = 3 A (c) Less current flows through the battery because there is now more resistance in the circuit because of the reduction in the number of pathways available for charge to flow. Thursday, 16 September 2010
79. 79. MAGNETIC FIELDS Thursday, 16 September 2010
80. 80. THE MAGNETIC FIELD AROUND A BAR MAGNET AND THE EARTH’S MAGNETIC FIELD Thursday, 16 September 2010
81. 81. ---> Demo c bar magnet & major magnet WHAT IS MAGNETISM? • Magnetism is caused by moving electrons. (The smallest magnetic field is produced by the motion of 1 electron) • When electrons move in a common direction, a magnetic field is produced (sometimes called a magnetic force field) • A force will be exerted on an iron object placed in a magnetic field. • A magnetic field is a region in space where a magnetic force can be detected. The magnetic field around a bar magnet Charm compass S Magnetic field lines The compass needle is itself a tiny magnet (the N North pole of this magnet points towards the South end of the magnet) Strong magnetic field (high density of lines) Thursday, 16 September 2010
82. 82. THE EARTH’S MAGNETIC FIELD θ (angle of declination) = 11o Geographic North Magnetic South compass S N Earth’s axis • θ changes with time • Angle of Dip - is the angle that the field lines make with the ground. At the equator, the angle of dip is zero. Near the poles the angle of dip is close to 90 degrees. Thursday, 16 September 2010
83. 83. THE EARTH’S MAGNETIC FIELD IS ESSENTIAL FOR LIFE ON THE PLANET. Home to millions of species including humans, Earth is currently the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. The planet is expected to continue supporting life for at least another 500 million years. Thursday, 16 September 2010
84. 84. MAGNETIC DOMAINS Thursday, 16 September 2010
85. 85. MAGNETIC MAGIC --> MAGNETIC DOMAINS Thursday, 16 September 2010
86. 86. MAGNETIC THEORY Ferromagnetic materials (Iron, Cobalt and Nickel) can be permanently magnetised. Electrons spinning in atoms have magnetic fields around them. They set up tiny North and South poles. Such an arrangement for an electron is called a dipole moment. [Illustrate a mag. dipole and mention Exchange Coupling] N N e S S For most elements: magnetic fields cancel. Iron, Cobalt and Nickel: Electron structure is such that there is a resultant magnetic field produced by each atom. These atoms are sometimes called atomic magnets.) Thursday, 16 September 2010
87. 87. DOMAINS Regions in a metal where the orientation of the magnetic dipoles is the same are called domains. A domain Unmagnetised Iron Here, a large number of iron atoms => Domains are (magnetic dipoles) scrambled are aligned. Partially magnetised S N Fully magnetised => the orientation of the domains is the S N same Thursday, 16 September 2010
88. 88. BREAKING A MAGNET Thursday, 16 September 2010
89. 89. MAGNETISING AND DEMAGNETISING [Solenoid Demo] “Lining up” the domains - Magnetising • Stroke the object end to end with a permanent magnet , in the same direction, using the same pole of the magnet. • Hold the object inside an D.C or A.C solenoid (Domains line up in the direction of the magnetic field) “Scrambling” the domains - Demagnetising Heat or hammer the magnet (This disturbs the alignment of the domains) [CAN ALSO BE DEMONSTRATED WITH THE SOLENOID] “Domains are induced into alignment” - Picking up iron objects Thursday, 16 September 2010
90. 90. EXERCISES Thursday, 16 September 2010
91. 91. Thursday, 16 September 2010
92. 92. Thursday, 16 September 2010
93. 93. Thursday, 16 September 2010
94. 94. WIRES Thursday, 16 September 2010
95. 95. WIRES A circular magnetic field is formed around a straight current - carrying conductor: 3D View I View from above The direction of the magnetic field lines is given by the Right-hand Thumb Rule • ( “•” represents current directed out of the page) The right hand thumb rule: Thumb = direction of the electric current Curled fingers = direction of the circular magnetic field Thursday, 16 September 2010
96. 96. “View” from above” I • Magnetic field lines Thursday, 16 September 2010
97. 97. B = Magnetic field strength (in Tesla,T) I = Electric current in the wire (in Amps,A) B = µ0I 2 d π µo= the permittivity of free space (the ability of a material to support a magnetic field (TmA-1) d = Distance from the wire (in metres, m) Note 1. Reversing the direction of the current reverses the direction of the magnetic field. 2. Magnetic field strength (symbol, B) is measured in NA-1m-1 or Tesla,T. 3. As the current in the wire, I increases the strength of the magnetic field increases BαI i.e. B is proportional to I 4. As the distance,d from the wire increases the strength of the magnetic field decreases. B α 1/d i.e. B is inversely proportional to d Thursday, 16 September 2010
98. 98. Example A special meter able to measure the magnetic field strength at any given point in the vicinity of a wire is shown below (taking a reading). It measures the magnetic field strength as 8 x 10-4 T at a distance of 0.01m from the centre of the wire. The current through the wire is 5 A. Calculate the value of the constant µo. Exercises B I d µo 5 x 10-5 T 2A 20 mm 3.14 x 10-6 6 x 10-5 T 3A 0.159 m 2 x 10-5 TmA-1 7.2 x 10-5 T 3.11 A 2.2 cm 3.2 x 10-6 TmA-1 1.05 x 10-3 3A 20 mm 4.4 x 10-5 TmA-1 Thursday, 16 September 2010
99. 99. CURRENT CARRYING CONDUCTOR AND MAGNETIC FIELD -----> COIL Thursday, 16 September 2010
100. 100. COILS Thursday, 16 September 2010
101. 101. THE SOLENOID The magnetic field of a solenoid is similar in shape to that of a bar magnet: Draw the field lines If the current is known, the poles of the solenoid can be determined using the right hand thumb rule applied earlier to the straight wire: Complete this: Field lines are parallel in the core of the solenoid which --> the magnetic field in the core is uniform. The density of magnetic field lines is greatest in the core --> the magnetic field strength is greatest in the core. Thursday, 16 September 2010
102. 102. STRENGTH RULZ Factors affecting the strength of the magnetic field: 1. Increasing the current increases the magnetic field strength. 2. Increasing the number of turns of wire per given length of the electromagnet increases the magnetic field strength Predicting North and South poles: Thumb points to North pole of the solenoid from inside the coil Curled fingers indicate the direction of the current Thursday, 16 September 2010
103. 103. ELECTROMAGNETS A solenoid which contains an iron core is called an electromagnet. Adding an iron core increases the strength of the magnetic field because the iron core itself becomes magnetised and adds to the magnetic field of the solenoid. Uses of electromagnets 1. Electromagnets in relays are able to open and close electrical circuits (eg. starter motor circuit in a car). 2. Used in scrap yards to lift car bodies. 3. Create the ringing sound in electric bells. 4. Electromagnets in the recording heads of tape recorders are used to magnetise the audio tape during recording. Thursday, 16 September 2010
104. 104. THE COIL GUN Induced magnetism An unmagnetised object will have have its domains aligned and therefore develop a north and south pole. The object can be picked up by the magnet because opposite poles attract. Cross-section of coil Attraction to x x x x x x x x North of coil Attraction to South of coil N S N - - - - - - - - North end of coil South end The dipoles in the object change along the rod as the rod is drawn into the coil and it is this dipole change which pulls the rod into the coil Thursday, 16 September 2010
105. 105. ELECTROMAGNETS IN RELAYS Thursday, 16 September 2010
106. 106. Thursday, 16 September 2010
107. 107. Thursday, 16 September 2010
108. 108. PRACTICAL Thursday, 16 September 2010
109. 109. STATIC Thursday, 16 September 2010
110. 110. PLAYING AROUND WITH STATIC ELECTRICITY 1 plastic rod (charged by rubbing with a cloth) small pieces of torn paper Observation: Explanation: 2 Balloon rubbed against hair Removed from head and then brought back to hair Observation: Explanation: Thursday, 16 September 2010
111. 111. 3 Balloon rubbed against jersey Release Observation: Explanation: cotton (a) Each balloon charged separately by rubbing against 4 the sleeve of a jersey (b) Holding the balloons by the cotton, release them, allowing them to come close to each other. Observation: Explanation: Thursday, 16 September 2010
112. 112. 5 (a) Straw, charged at both (b) Straw, also charged using a ends (using a woollen woollen cloth held horizontally cloth) and brought close (c) Repeat (b) using a silk cloth. Observation: Explanation: Charged plastic rod is held near a thin stream of water 6 Observation: Explanation: http://phet.colorado.edu/new/simulations/sims.php? sim=Balloons_and_Static_Electricity Thursday, 16 September 2010
113. 113. Equipment CHARGING OBJECTS dry cloth/jersey Cap perspex rod Insulating material ebonite rod electroscope Body of electroscope Leaf Base Aim to charge an electroscope by both induction and by contact and to draw charge distribution diagrams Method 1. Follow the instructions below 2. Write observations as you perform each step 3. Complete the diagrams only after recording the observations (you may need some help with these) Part 1 - Charging by induction 1. Charge the rod by rubbing it with a dry cloth/jersey and hold the rod near the cap of the electroscope ++++ ---- Observation: + + + + If the rod was positively charged the charge distribution diagram would look like this: Thursday, 16 September 2010
114. 114. Complete the diagram to show how charges would distribute on the electroscope should the rod be negatively charged. ---- 2. With the rod in this position, earth the cap with your finger. Observation: ++++ ---- + + + + This symbol Complete the diagram to show how charge moves represents a when the cap of the electroscope is earthed connection to the earth Thursday, 16 September 2010
115. 115. Draw the charge distribution diagram (by adding to ++++ the existing diagram on the right) showing the ---- situation once this charge movement has finished. + + + + 3. Unearth the cap of the electroscope without removing the charged rod Observation: Draw the resultant charge distribution and the new position of the leaf on the diagram (right). 4. Remove the charged rod Observation: Finally, complete the diagram (right). Thursday, 16 September 2010
116. 116. Part 2 - Charging by contact Method 1. Follow the instructions below 2. Write observations as you perform each step 3. Complete the diagrams only after recording the observations (you may need some help with these) 1. A positively charged rod is held near the cap of the electroscope. ++++ 2. The rod makes contact with the cap. 3. The rod is removed. Thursday, 16 September 2010
117. 117. Lab 12 ALL CHARGED UP Equipment dry cloth/jersey Cap perspex rod Insulating material ebonite rod electroscope Body of electroscope Leaf Base Aim to charge an electroscope by both induction and by contact and to draw charge distribution diagrams Method 1. Follow the instructions below 2. Write observations as you perform each step 3. Complete the diagrams only after recording the observations (you may need some help with these) Part 1 - Charging by induction 1. Charge the rod by rubbing it with a dry cloth/jersey and hold the rod near the cap of the electroscope ++++ ---- Observation: The leaf of the electroscope springs up. + + + + If the rod was positively charged the charge distribution diagram would look like this: Thursday, 16 September 2010
118. 118. Complete the diagram to show how charges would distribute on the electroscope should the rod be negatively charged. Electrons at the cap are repelled by the negatively ---- ++++ charged rod. - - - - Electrons at the cap are held in position by the positively charged rod. The earth supplies electrons to the positively charged leaf and lower stem. 2. With the rod in this position, earth the cap with your finger. Observation: -- ++++ ---- Leaf of the electroscope drops + + + + This symbol Complete the diagram to show how charge moves represents a when the cap of the electroscope is earthed connection to the earth Thursday, 16 September 2010
119. 119. Draw the charge distribution diagram (by adding to ++++ the existing diagram on the right) showing the - - - - situation once this charge movement has finished. - The cap and leaf now have + - +- + + no overall charge. - Electrons on the cap are still held in position. 3. Unearth the cap of the electroscope without removing the ++++ charged rod - - - - Observation: Leaf of the electroscope remains in the “dropped” position. The charge distribution has not changed - + + - - + - + Draw the resultant charge distribution and the new position of the leaf on the diagram (right). Negative charge redistributes itself 4. Remove the charged rod around the metal parts of the electroscope leaving the stem and - - Observation: leaf with an overall negative charge The leaf of the electroscope springs up. - -+ + - - + + - - Finally, complete the diagram (right). Thursday, 16 September 2010
120. 120. Part 2 - Charging by contact Method 1. Follow the instructions below 2. Write observations as you perform each step 3. Complete the diagrams only after recording the observations (you may need some help with these) 1. A positively charged rod is held near the cap of the electroscope. ++++ - - - - Charge separation occurs. Positive repels positive at the stem/leaf + + + + 2. The rod makes contact with the cap. - Electrons migrate up + + into the rod + + 3. The rod is removed. + + + The electroscope is now left with + + an overall positive charge. + + Thursday, 16 September 2010
121. 121. SPARKS 12 Physics > resources > electricity > DC electricity > videos Thursday, 16 September 2010
122. 122. THE VAN DER GRAAF Label the picture of the Van der Graaf (left) using the labels in the box below ______________ Lower roller ______________ Belt - A piece of surgical tubing ______________ Output terminal - an aluminium or steel sphere Upper roller - A piece of nylon ______________ Motor ______________ Upper brush - A piece of fine metal wire ______________ Lower Brush ______________ • When the generator is turned on, the electric motor begins turning the belt. • The belt is made of rubber and the lower roller is covered in silicon tape. Silicon has a greater affinity for electrons than rubber and so it captures electrons from the belt. The belt in turn must capture electrons from the dome, leaving the dome positively charged. Reference: http://science.howstuffworks.com/vdg3.htm Thursday, 16 September 2010
123. 123. THE VAN DER GRAAF - OBSERVATIONS & EXPLANATIONS 1. Small dome held close to generator dome Drawn observation Explanation 2. Hair stands on end when contact is made with the generator dome Drawn observation Explanation 3. Aluminium foil plates flying of the top of the generator dome Drawn observation Explanation Thursday, 16 September 2010
124. 124. DC Thursday, 16 September 2010
125. 125. CIRCUIT RULES Thursday, 16 September 2010
126. 126. ADDING BULBS IN PARALLEL 1. Set up each of the following circuits, one after the other (making a mental note of the brightness of the lamps in the circuit. 2. For each circuit read the ammeter and record the current in the space provided. 1 2 3 8V 8V 8V + - + - + - A A A Current = ______ A Current = ______ A Current = ______ A Observation Explanation Thursday, 16 September 2010
127. 127. ADDING BULBS IN SERIES 1. Set up each of the following circuits, one after the other (making a mental note of the brightness of the lamps in the circuit. 2. For each circuit read the ammeter and record the current in the space provided. 1 2 3 + - + - + - A A A Current = ______ A Current = ______ A Current = ______ A Observation Explanation Thursday, 16 September 2010
128. 128. CURRENT IN THE SERIES CIRCUIT IS CONSTANT Aim to look for a pattern in the current through bulbs and resistors in a series circuit. 1. Use ONE ammeter in the three different places shown in the circuit diagram. 2. Without changing the setting on the power pack or the variable resistor write the current readings in the spaces provided (below): A1 + 8V - A1 = ______ A A3 A2 = ______ A A3 = ______ A A2 Equation Thursday, 16 September 2010
129. 129. CURRENT IN THE PARALLEL CIRCUIT IS SHARED Aim to look for a pattern in the current through bulbs and resistors in a parallel circuit. 1. Use ONE ammeter in the each of the four places shown in the circuit diagram. 2. Without changing the setting on the power pack record your results below: + 8V - Results A1 A4 A1 = ____ A A2 = ____ A A2 A3 = ____ A A4 = ____ A A3 Conclusion Current in a parallel circuit is ____________ . Equation relating the currents Thursday, 16 September 2010
130. 130. VOLTAGE IN THE SERIES CIRCUIT IS SHARED Aim to look for a pattern in the voltages across bulbs and resistors in a series circuit. 1. Set up the circuit (below) 2. Use ONE voltmeter in the three different places shown in the circuit diagram. 3. Without changing the setting on the power pack record your results below: + 8V - Results V1 V1 (power supply) = __ V V2 V4 A V2 (variable resistor) = __ V V3 V3 (bulb) = __ V V3 (ammeter) = __ V Conclusion The power supply voltage is _____________ between the components in the circuit Equation relating the voltages Thursday, 16 September 2010
131. 131. VOLTAGE IN THE PARALLEL CIRCUIT IS CONSTANT Aim to look for a pattern in the voltages across bulbs and resistors in a series circuit. 1. Set up the circuit (below) 2. Use ONE voltmeter in the three different places shown in the circuit diagram. 3. Without changing the setting on the power pack record your results below: + 8V - V1 Results V1 = ___ V V2 V2 = ___ V V3 = ___ V V3 Conclusion The voltage across components connected in parallel is __________ Equation relating the voltages Thursday, 16 September 2010
132. 132. VOLTAGES AND CURRENTS IN SERIES AND PARALLEL Aim to investigate voltage and current in a series circuit that has a parallel portion in it. 1. Set up the circuit (below) 2. Use ONE voltmeter in the four different places shown in the circuit diagram and ONE ammeter in the four different places shown. 3. Without changing the setting on the power pack record your results below: Results A1 + 8V - A1 = __A V1 A2 = __A V2 A4 V4 A3 = __A V3 A4 = __A A2 V1 = __V A3 V2 = __V V3 = __V V4 V4 = __V Thursday, 16 September 2010
133. 133. OHMIC CONDUCTORS Thursday, 16 September 2010
134. 134. Lab 14 OHM’S LAW Method 1. Set up the following circuit using iced water to cool the immersion + - coil. 2. Increase the voltage in regular increments through an A V appropriate range (widest possible range) ice immersion coil beaker water Results Voltage setting of Power pack 2 4 6 8 10 12 (V) Voltage, V (V) Current, I (A) Thursday, 16 September 2010
135. 135. Draw a graph of Voltage against Current on the grid provided Conclusion Notes • The iced water was used to keep the temperature of the coil constant . • If the iced water was forgotten and the coil was allowed to heat up then the graph would curve up. Repeat the experiment but this time replace the coil with a lamp (that will increase in temperature as the current through it increases) Thursday, 16 September 2010
136. 136. RESISTANCE Thursday, 16 September 2010
137. 137. RESISTANCE AND POWER in series and parallel Measurements Resistance Resistance calculated Calculated specified Voltage Current from Power output measurements R1 R2 R3 across drawn from Resistance calculated For any circuit, set power combination the power (formula provided for parallel resistors) of resistors supply supply voltage to 8V R1 & R2 in series R 1 & R2 & R3 in series R1 & R2 in parallel R 1 & R2 & R3 in parallel Thursday, 16 September 2010
138. 138. POWER Thursday, 16 September 2010
139. 139. CALCULATING POWER OUTPUT OF APPLIANCES WITH AN ELECTRONIC MONITOR Appliance UNDER CONSTRUCTION Thursday, 16 September 2010
140. 140. ELECTROMAG Thursday, 16 September 2010
141. 141. HANGING MAGNETS I 1. Cut out the net 2. Suspend the and fold it at the magnet in the dotted lines to cradle create a cradle S N + 3. Use a short length of cotton S N to suspend the 4. Repeat using a magnet from a second magnet. retort stand Thursday, 16 September 2010
142. 142. 1. Position your two magnets in the HANGING MAGNETS II orientations shown 2. For each orientation, record your observations N A B S N S N N S S Observation Observation C D S S N S N S N N Observation Observation Thursday, 16 September 2010
143. 143. STROKING MAGNETS A. Try magnetising an iron nail using a magnet S N B. Once you have finished, check for a magnetic field using a charm compass. Thursday, 16 September 2010
144. 144. PLOTTING MAGNETIC FIELDS Follow the instructions (below) and draw your observations A. Place one or more B. Use a pencil to mark the compasses around a bar North pole of each magnet magnet using a dot Charm compasses (moved around in a variety of positions around the magnet C. Connect the dots using a smooth curve D. Plot several field lines and mark the North and South poles of the magnet. E. Wrap your magnet in glad wrap and spring iron filings over it. Thursday, 16 September 2010