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# P5 Electric Circuits

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### P5 Electric Circuits

1. 1. 05/06/09 OCR Additional Science Electric Circuits
2. 2. Static Electricity 05/06/09 Static electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used: + + - - - + + + - - + + + - - - + - + -
3. 3. Static Electricity 05/06/09 + + + - - - - - - - - -
4. 4. Circuit Symbols 05/06/09 Battery Cell Fuse Voltmeter Ammeter Switch Bulb V A
5. 5. Electric Current 05/06/09 Electric current is a flow of negatively charged particles (i.e. electrons). By definition, current is “the rate of flow of charge” Note that electrons go from negative to positive - + e - e -
6. 6. More basic ideas… 05/06/09 If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit
7. 7. Current in a series circuit 05/06/09 In other words, the current in a series circuit is THE SAME at any point If the current here is 2 amps… The current here will be… The current here will be… And the current here will be…
8. 8. Current in a parallel circuit 05/06/09 A PARALLEL circuit is one where the current has a “choice of routes” Here comes the current… And the rest will go down here… Half of the current will go down here (assuming the bulbs are the same)…
9. 9. Current in a parallel circuit 05/06/09 If the current here is 6 amps The current here will be… The current here will be… The current here will be… And the current here will be…
10. 10. Some example questions… 05/06/09 3A 6A
11. 11. Voltage in a series circuit 05/06/09 2V V V V If the voltage across the battery is 6V… … and these bulbs are all identical… … what will the voltage across each bulb be?
12. 12. Voltage in a series circuit 05/06/09 4V V V If the voltage across the battery is 6V… … what will the voltage across two bulbs be?
13. 13. Voltage in a parallel circuit 05/06/09 4V 4V If the voltage across the batteries is 4V… What is the voltage here? And here? V V
14. 14. Summary 05/06/09 In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand”
15. 15. An example question: 05/06/09 V 1 V 2 6V 3A A 1 A 2 V 3 A 3
16. 16. Another example question: 05/06/09 V 1 V 2 10V 3A A 1 A 2 V 3 A 3
17. 17. Resistance 05/06/09 The resistance of a component can be calculated using Ohm’s Law: Georg Simon Ohm 1789-1854 Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. Resistance = Voltage (in V) (in  ) Current (in A) V R I
18. 18. An example question: 05/06/09 <ul><li>What is the resistance across this bulb? </li></ul><ul><li>Assuming all the bulbs are the same what is the total resistance in this circuit? </li></ul>V A Voltmeter reads 10V Ammeter reads 2A
19. 19. More examples… 05/06/09 12V 6V What is the resistance of these bulbs? 3A 3A 4V 2A 1A 2V
20. 20. Resistance 05/06/09 Resistance is anything that opposes an electric current. <ul><li>What is the resistance of the following: </li></ul><ul><li>A bulb with a voltage of 3V and a current of 1A. </li></ul><ul><li>A resistor with a voltage of 12V and a current of 3A </li></ul><ul><li>A diode with a voltage of 240V and a current of 40A </li></ul><ul><li>A thermistor with a current of 0.5A and a voltage of 10V </li></ul>Resistance (Ohms,  ) = Potential Difference (volts, V) Current (amps, A)
21. 21. Electromagnetic induction 05/06/09 <ul><li>The direction of the induced current is reversed if… </li></ul><ul><li>The magnet is moved in the opposite direction </li></ul><ul><li>The other pole is inserted first </li></ul><ul><li>The size of the induced current can be increased by: </li></ul><ul><li>Increasing the speed of movement </li></ul><ul><li>Increasing the magnet strength </li></ul><ul><li>Increasing the number of turns on the coil </li></ul>
22. 22. Generators (dynamos) 05/06/09 <ul><li>Induced current can be increased in 4 ways: </li></ul><ul><li>Increasing the speed of movement </li></ul><ul><li>Increasing the magnetic field strength </li></ul><ul><li>Increasing the number of turns on the coil </li></ul><ul><li>Increasing the area of the coil </li></ul>
23. 23. Power and fuses 05/06/09 Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: Power = voltage x current in W in V in A Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A). P I V
24. 24. Power and fuses 05/06/09 Copy and complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Fuse needed (3, 5 or 13A) Toaster 960 240 Fire 2000 240 Hairdryer 300 240 Hoover 1000 240 Computer 100 240 Stereo 80 240
25. 25. Energy and Power 05/06/09 The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second E = Energy (in joules) P = Power (in watts) T = Time (in seconds) E T P
26. 26. Some example questions 05/06/09 <ul><li>What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? </li></ul><ul><li>What is the power of an electric fire that transfers 10,000J of energy in 5 seconds? </li></ul><ul><li>Farhun runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating? </li></ul><ul><li>How much energy does a 150W light bulb transfer in a) one second, b) one minute? </li></ul><ul><li>Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson? </li></ul><ul><li>Damien’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day? </li></ul>
27. 27. The Cost of Electricity 05/06/09 Electricity is measured in units called “kilowatt hours” (kWh). For example… A 3kW fire left on for 1 hour uses 3kWh of energy A 1kW toaster left on for 2 hours uses 2kWh A 0.5kW hoover left on for 4 hours uses __kWh A 200W TV left on for 5 hours uses __kWh A 2kW kettle left on for 15 minutes uses __kWh
28. 28. The Cost of Electricity 05/06/09 To work out how much a device costs we do the following: Cost of electricity = Power (kW) x time (h) x cost per kWh (p) For example, if electricity costs 8p per unit calculate the cost of the following… <ul><li>A 2kW fire left on for 3 hours </li></ul><ul><li>A 0.2kW TV left on for 5 hours </li></ul><ul><li>A 0.1kW light bulb left on for 10 hours </li></ul><ul><li>A 0.5kW hoover left on for 1 hour </li></ul>48p 8p 8p 4p
29. 29. Efficiency 05/06/09 Efficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it. For example, consider a TV: Electrical Energy (200J) Light (80J) Sound (40J) Heat (?) Efficiency = Useful energy out Energy in x100%
30. 30. Some examples of efficiency… 05/06/09 <ul><li>5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it? </li></ul><ul><li>A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go? </li></ul><ul><li>A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it? </li></ul>
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