1. Conductors Metals and graphite are electrical conductors. Electrons are free to move in conductors. Electrons are negatively charged. The electrons carry the energy from the power supply to the appliance. A movement of electrons is called an electric current. Total charge = current x time Q = I x t Coulombs, C Amps, A seconds, s ( Basic electrostatic expts )

2. Calculation Calculate the current flowing if 500 C of charge flow around a circuit in 5 minutes. I = ? Q = 500 C, t = 5 mins = 300 s Q = I x t

3. Current Current is a measure of the number of charges that flow per second around a circuit. Units are Amps, A Instrument : ammeter, connected is series.

4. Potential Difference (Voltage) Voltage is a measure of the energy given to each charge in the circuit. The bigger the voltage the more energy is transferred. Units : Volts, V Instrument : Voltmeter connected in parallel Expts on series / parallel rules for current/ potential difference and useful circuits.

5. Series Circuits Components are connected in ‘line’ Current is the same AT ALL points

6. Series Circuits Sum of potential differences = Supply potential difference V s = V 1 + V 2 + V 3 Remember that potential difference is a measure of energy. This equation tells us that the total energy available from the supply equals the energy used to drive current through resistor 1 plus the energy used to drive current through resistor 2 plus the energy used to drive current through resistor 3

7. Parallel Circuits Components are arranged ‘above each other’ Total current drawn from supply equals sum of currents flowing through each component I t = I 1 + I 2 + I 3

8. Parallel Circuits Potential difference across each branch equals supply potential difference Investigation of ring circuit

9. Household wiring Appliances connected in parallel 230 V across each Independent switching Power circuit( plugs ) is wired as a RING circuit

10. Ring Circuit Current splits two ways Thinner cable can be used Less heating effect in each branch Easy to add extra sockets Neutral Live

11. Direct Current, D.C. Electrons flow in one direction only A battery produces direct current as it is always pushing electrons in the same direction. Voltage Time

12. Alternating Current, A.C. Alternating current the current flows back / forwards around the circuit. The frequency of the supply is the number of cycles per second. Mains frequency is 50 Hz I cycle = 1/50 s Time for 1 cycle ( period) = 1 / frequency ( T = 1 /f ) Time Voltage

13. Alternating Current, 2 The effective value of British mains is 230 V. This is called the quoted value and is less than the peak value. Peak value, measured on an oscilloscope Quoted value, measured using a meter.

14. To Measure Peak Voltage 1.Count the number of boxes from the trough to crest 2.Divide by 2 ( this is from the 0V line to the crest ) 3.Multiply the number of boxes by the Y gain setting Ex. 4 boxes from trough to crest Therefore from 0V line to crest = 2 boxes 2 x 5 = 10 V Y gain = 5V per box 0V Line

15. Resistance This is a measure of the opposition to flow of current Units are Ohms,Ω Symbol for resistor If resistance increases then current decreases ( provided supply pd is constant )

16. Ohm’s Law For a constant temperature pd ( V ) current ( A ) resistance ( Ω) Calculate the resistance of a 12 V car bulb that draws a current of 5 A. V = 12 V I = 5 A R = ? V = I x R

17. Ohm’s law 2

18. Heating conductors A the temperature increases the gradient and hence resistance increases. As temperature increases the particles vibrate more making it more difficult for the power supply to push charges around the circuit.

19. Series Resistors The total resistance is the sum of the individual resistances. R t = R 1 + R 2 + R 3

20. Parallel Resistors The effective resistance is always smaller than the smallest resistor in the network. Ex : Calculate the effective resistance of the circuit shown. R 1 = 10 Ω R 2 = 15 Ω Rp = ? Short circuits / open circuits

21. Potential Dividers These divide the voltage up. The resistors are in series therefore the same current flows through each. The bigger the resistance the bigger the share of the voltage.

22. Potential divider circuits Example: V S = V 1 + V 2 V S = 18 + 6 V S = 24 V V1V1 V2V2 R1R1 R2R2 = V1V1 6 12 4 = V1V1 x 6 12 4 = V1V1 = 18 V 4  12  6 V V1V1 VSVS 4  12  6 V V1V1 VSVS VSVS 4  12  V1V1 4  12  6 V V1V1 VSVS 4  12  V1V1 VSVS

23. Potential Dividers Calculate the voltage across the 20 Ω resistor. V 2 = ? R 2 = 20 Ω, R 1 = 30 Ω, V s = 10 V

24. Electronics Systems: All electronic systems can be simplified to the following block diagram, inputprocessoutput e.g. for a radio receiver, aerial tuner speaker decoder amplifier

25. Output Devices examples are, - speaker (electrical energy to sound energy) - buzzer (electrical energy to sound energy) - lamp (electrical energy to light energy) - motor (electrical energy to kinetic energy) - relay (causes other circuits to be switched) - solenoid (causes a straight movement) - seven segment display Output devices are energy changers.

26. Light emitting diode, LED This device changes electrical energy into light energy. It allows current only one way through it. Flow of electrons

27. LED 2 Unlike a lamp it produces very little heat energy as it does not contain a wire filament. It lights when the current is small (e.g. 10 mA). To prevent it being damaged by too large a current, it always has a resistor connected in series.

28. LED 3 example: An l.e.d. is designed to operate at 2.0 V and 10 mA. What size of resistor is needed when it is powered by a 9.0 volt battery? 9.0 V 2.0 V Pd across resistor = 9 -2 = 7V R = V/I = 7 / 0.01 = 700 Ω 10 mA = 0.01 A

29. Input Devices Two main kinds : energy changers such as a microphone or thermocouple Or the component changes the size of the input voltage as a physical property changes e.g. LDR or thermistor

30. Microphone energy change is, sound energy to electrical energy

31. Thermocouple energy change is, heat energy to electrical energy Used to measure high temperatures +-

32. Solar Cell energy change is, light energy to electrical energy

33. Thermistor The resistance of a thermistor usually decreases as the temperature increases.

34. Light Dependant Resistor The resistance of an LDR decreases as the light level increases.

35. LDR in potential divider circuit V 4.7 k  6.0 V Explain what happens as it gets darker: 1.Resistance of LDR goes up 2.Voltage across LDR goes up 3.Hence voltage across fixed resistor goes down

36. Thermistor in potential divider circuit V 2.4 k  9.0 V Explain what happens as it gets colder; 1.Resistance of thermistor goes up 2.Voltage across thermistor goes up 3.Voltage across fixed resistor goes down

37. Transistors Two types NPN and MOSFET Both act as voltage operated switches

38. MOSFET Metal Oxide Semiconductor Field Effect Transistor The MOSFET has a different construction from the NPN transistor and switches on at a higher voltage (about 2 volts). Gate Drain Source

39. NPN transistor This switches on when the voltage across the base emitter is 0.7 V Collector Emitter Base

40. Example of transistor switching 5.0 V As the temperature increases, the resistance of the thermistor decreases. As a result, the voltage across the thermistor decreases. This causes the voltage across the variable resistor to increase which switches on the MOSFET, causing the l.e.d. to light.

41. Electrical energy 1 When a current flows through a component there is an energy change. In a resistor E e E h In a loudspeaker E e E s

42. Electrical energy 2 Power is the rate at which energy is transferred I W is equivalent to 1 Js -1 Power can also be calculated from : Power = voltage x current P = V x I Joules, J Seconds, s Watts,W

43. Electrical energy 3 There are 2 other equations that can be used to calculate power : 1: Combine P = V x I and V= I x R Substitute for V = I x R into P = V x I P = I x R x I = I 2 x R 2 : Substitute for into P = V x I

44. Electrical energy 4 Four equations to calculate power : When energy ( joules ) and time ( seconds ) is known When current ( amps ) and voltage ( volts ) are known When current ( amps ) and resistance (ohms ) are known When Voltage ( volts ) and resistance (ohms ) are known

45. Electrical energy 5 Calculate the current flowing through a 230 V mains, 2 kW kettle. P = 2 KW = 2000 W V = 230 V I = ? P = V x I

46. Electrical energy 6 Calculate the voltage across a 4 Ω loudspeaker when it produces 20 W of sound power. P = 20 W R = 4 Ω V = ?

47. Electrical energy 7 Calculate the current flowing through a 2000 W, 20Ω resistor. P = 2000 W, R = 20 Ω, I = ? P = I 2 x R

48. Electromagnetism The term ‘electromagnetism’ comes from the fact that there is a magnetic field around a wire when there is an electric current in the wire. The magnetic field is stronger when, - the current is higher - the wire is longer The direction of the magnetic field reverses if the current flow reverses direction.

49. Electromagnetism When a wire is moved through a magnetic field a voltage is generated (induced) in the wire. This can be when the magnet is stationary and the wire is moved, or when the wire is stationary and the magnet is moved. The induced voltage is greater when, - the magnetic field is stronger - the movement is faster - the wire is wound into a coil