1. Electricity and Circuits

2. Resistance
Resistance is friction that electricity experiences while flowing thru something.
Resistance (or the lack of resistance) is used in circuits to control the flow of current.
Symbol :

3. Short Circuit

4. Short circuit Very low resistance of conductors.
This example circuit connects the poles of the voltage source together using a simple piece of wire.
High current !
DANGER : High conversion of energy into heat ! Burns, fire ! (don’t mess with it!).

5. Building fire (example of short circuit consequence)‏

6. Short circuit (conclusion)‏
In a short circuit the magnitude of the current can be very large.
The release of energy is dramatic, usually in the form of heat : melts the metal, burns insulators, can start a fire, and injuries (burns) to the man.

7. Simple Circuit

8. Simple circuit
Normally circuits are constructed to make practical use of the released energy in a manner as safe as possible.
One typical and practical use of a circuit is for the production of electric lighting.

9. Light Bulb

10. Light bulb (incandescent lighting)‏

11. RESISTANCE
Electrons encounter opposition when flowing thry the filament, due to friction.
This phenomenon is known as RESISTANCE.
Resistance depends on material, cross section, length.

12. Simple electric light circuit
Bulb

13. Inside the light bulb Conductors have low resistance.
Insulators have high resistance.
Resistance limits the amount of current flowing thru the circuit for a given voltage of the power supply.
In the light bulb, a relatively high amount of energy is dissipated due to the resistance of the filament, and manifests as heat – so high that light is emited (filament glows).

14. Open circuit

15. Open Circuit
Breaker (switch)‏

17. Closed Circuit Current flows through the switch.
Continuity is restablished.
Light bulb glows.

18. KEYWORDS CIRCUIT RESISTANCE LOAD BREAKER SWITCH INCANDESCENSE CURRENT
VOLTAGE
SHORT CIRCUIT
CONTINUITY

19. Static Electricity + + - - + - + + + - - - + + - + - -
08/12/2017
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: + + - - + - + + + - - - + + - + - -

20. Static Electricity
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21. Circuit Symbols Switch Bulb A V Ammeter Voltmeter Cell Fuse Battery
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Switch
Bulb A V
Ammeter
Voltmeter
Cell
Fuse
Battery

22. By definition, current is “the rate of flow of charge”
Electric Current
08/12/2017
Electric current is a flow of negatively charged particles (i.e. electrons).
Note that electrons go from negative to positive - + e-
By definition, current is “the rate of flow of charge” e-

23. More basic ideas…
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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

24. Current in a series circuit
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If the current here is 2 amps…
The current here will be…
The current here will be…
And the current here will be…
In other words, the current in a series circuit is THE SAME at any point

25. Current in a parallel circuit
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A PARALLEL circuit is one where the current has a “choice of routes”
Here comes the current…
Half of the current will go down here (assuming the bulbs are the same)…
And the rest will go down here…

26. Current in a parallel circuit
08/12/2017
If the current here is 6 amps
And the current here will be…
The current here will be…
The current here will be…
The current here will be…

27. Some example questions…
08/12/2017 3A 6A

28. Voltage in a series circuit
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If the voltage across the battery is 6V… V
…and these bulbs are all identical… V V
…what will the voltage across each bulb be? 2V

29. Voltage in a series circuit
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If the voltage across the battery is 6V… V V
…what will the voltage across two bulbs be? 4V

30. Voltage in a parallel circuit
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If the voltage across the batteries is 4V…
What is the voltage here? V 4V
And here? V 4V

31. Summary Current is THE SAME at any point
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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”

32. An example question:
08/12/2017 6V A3 3A A1 V1 A2 V2 V3

33. Another example question:
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10V A3 3A A1 V1 A2 V2 V3

34. Resistance V R I Resistance = Voltage (in V) (in ) Current (in A)
Georg Simon Ohm
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Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me.
The resistance of a component can be calculated using Ohm’s Law: V R I
Resistance = Voltage (in V)
(in ) Current (in A)

35. An example question: Ammeter reads 2A A V
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Ammeter reads 2A V A
What is the resistance across this bulb?
Assuming all the bulbs are the same what is the total resistance in this circuit?
Voltmeter reads 10V

36. What is the resistance of these bulbs?
More examples…
08/12/2017 3A 6V
12V 4V 2A 1A 2V 3A
What is the resistance of these bulbs?

37. Resistance Resistance is anything that opposes an electric current.
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Resistance is anything that opposes an electric current.
Resistance (Ohms, ) = Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
A bulb with a voltage of 3V and a current of 1A.
A resistor with a voltage of 12V and a current of 3A
A diode with a voltage of 240V and a current of 40A
A thermistor with a current of 0.5A and a voltage of 10V

38. Power and fuses
08/12/2017
Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: P I V
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).

39. Power and fuses Copy and complete the following table: Appliance
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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
Hairdryer
300
Hoover
1000
Computer
100
Stereo 80

40. In other words, 1 Watt = 1 Joule per second
Energy and Power
08/12/2017
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 T P
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)

41. Some example questions
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What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds?
What is the power of an electric fire that transfers 10,000J of energy in 5 seconds?
Farhun runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating?
How much energy does a 150W light bulb transfer in a) one second, b) one minute?
Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson?
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?

42. The Cost of Electricity
08/12/2017
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

43. The Cost of Electricity
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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…
A 2kW fire left on for 3 hours
A 0.2kW TV left on for 5 hours
A 0.1kW light bulb left on for 10 hours
A 0.5kW hoover left on for 1 hour
48p 8p 8p 4p

44. Efficiency
08/12/2017
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:
Light (80J)
Electrical
Energy (200J)
Sound (40J)
Heat (?)
Efficiency = Useful energy out
Energy in
x100%

45. Some examples of efficiency…
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5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it?
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?
A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it?