1. P5 Electric Circuits OCR 21st Century Georg Simon Ohm 1789-1854
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P5 Electric Circuits
OCR 21st Century
Georg Simon Ohm

2. P5.1 Electric Current – a Flow of What?
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P5.1 Electric Current – a Flow of What?

3. An introduction – click here
Static Electricity
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An introduction – click here

4. Static Electricity + + - - + - + + + - - - + + - + - -
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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: + + - - + - + + + - - - + + - + - -

5. Static Electricity
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6. Short Static Experiments
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Try the following quick static electricity experiments:
Rubbing a balloon on your jumper and “sticking” it to the wall
Charging a plastic rod by rubbing it with a cloth and then holding it near the water from a smooth-running tap
Charging a plastic rod and trying to pick up small pieces of paper (or someone else’s hair!) with it
Rubbing a balloon on someone else’s head – you might want to ask their permission first…
Can you explain what you saw in each of these experiments?

7. Gold Leaf Electroscopes
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Consider a gold-leaf electroscope…
Now charge the top: - - - - - - - - -
This effect was seen because charges can easily move through conductors

8. By definition, current is “the rate of flow of charge”
Electric Current
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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-

9. Words: volts, amps, ohms, voltage, ammeter, voltmeter
Basic ideas…
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Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.
Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.
Resistance is anything that resists an electric current. It is measured in _____.
Words: volts, amps, ohms, voltage, ammeter, voltmeter

10. Understanding Current
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When a voltage is applied it basically causes the electrons in a conductor to move towards the positive end of the battery:
Electrons
Ions
Negative
Positive
The main difference between conductors and insulators is that insulators have less of these free electrons.

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P5.2 Current and Energy

12. Circuit Symbols Variable resistor Diode Switch Bulb A V Ammeter
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Variable resistor
Diode
Switch
Bulb A V
Ammeter
Voltmeter
LDR
Resistor
Cell
Fuse
Thermistor
Battery

13. 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

14. Electric Current revisited
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Work is done by the battery to move the electrons around the circuit.
Note that we usually assume the resistance of these leads is very small. - + e-
When the electons arrive at the bulb energy is transferred to it. e-

15. Electrical Power revision
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Power is defined as “the rate of transferring energy” and is measured in units called “Watts” (W). P I V
The amount of power being transferred in an electrical device is given by:
Power = voltage x current
in W in V in A
How much power is transferred by a 230V fire that runs on a current of 10A?
An electric motor has a power rating of 24W. If it runs on a 12V battery what current does it draw?
An average light bulb in a home has a power rating of 60W and works on 230V. What current does it draw?

16. Understanding Resistance
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Recall our previous model of electric current but this time we’ll use it to explain resistance:
Electrons
Ions
Negative
Positive
Notice that the ions were vibrating and getting in the way of the electrons – this is resistance. This effect causes the metal to heat up.

17. Using this heating effect
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This heating effect can have its advantages and its disadvantages. For example, consider an old-fashioned light bulb:
This heating effect causes the filament to emit light…
…but it also causes a lot of energy to be wasted to the environment

18. 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)

19. 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

20. What is the resistance of these bulbs?
More examples…
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12V 4V 2A 1A 2V 3A
What is the resistance of these bulbs?

21. 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 3 4 6
20

22. LDRs and Thermistors
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23. Two simple components:
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1) Light dependant resistor – resistance DECREASES when light intensity INCREASES
2) Thermistor – resistance DECREASES when temperature INCREASES
Resistance
Amount of light
Resistance
Temperature

24. Using Thermistors and LDRs in circuits
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What will happen to the resistance of the thermistor when it gets hotter?
How will this affect the brightness of the bulb and the reading on the ammeter?
How will this affect the readings on the ammeter and voltmeter?

25. Resistors in Series and Parallel
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Consider the total resistance of these combinations:
10Ω
10Ω
The total resistance of this combination is 20Ω as the battery has to move charges through both resistors
The total resistance of this combination is only 5Ω as the charges have more paths to move through

26. Current-voltage graph for a Resistor
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Notice that a current-voltage graph for a resistor of fixed value shows that current increases in proportion to voltage.
Resistor

27. P5.3 How do Parallel and Series Circuits work?
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P5.3 How do Parallel and Series Circuits work?

28. 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

29. 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…

30. Current in a parallel circuit
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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…

31. Some example questions…
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1A each

32. Voltage and Work done What does “voltage” mean?
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What does “voltage” mean?
The voltage (or potential difference) between two points is a measure of the work done per “bit” of charge moving between these points.

33. 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

34. Voltage in a series circuit
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Notice that the voltages add up to the voltage across the battery – this is because the work done on each unit of charge by the battery must equal the work done by it to the bulbs. V V

35. What if the Resistances are different?
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Q. What would each of these voltmeters read?
60V V1 V2
10Ω
20Ω
Total resistance = 30Ω
Therefore current = 2A
Therefore V1 = 2x10 = 20V and and V2 = 2x20 = 40V
The voltage across the bigger resistance is higher as more work is done by the battery to get the current through it.

36. 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

37. 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

38. 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”

39. An example question: 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V
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40. Another example question:
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10V A1 3A
1.2A A2 V1
6.7V A3
1.8A V2 V3 5V 5V

41. P5.4 How is Mains Electricity Produced?
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P5.4 How is Mains Electricity Produced?

42. Electromagnetic Induction
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The direction of the induced current is reversed if…
The wire is moved in the opposite direction
The field is reversed
The size of the induced current can be increased by:
Increasing the speed of movement
Increasing the magnet strength

43. Electromagnetic induction
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Electromagnetic induction
The direction of the induced current is reversed if…
The magnet is moved in the opposite direction
The other pole is inserted first
The size of the induced current can be increased by:
Increasing the speed of movement
Increasing the magnet strength
Increasing the number of turns on the coil

44. Transformers Time Time Time Current through primary Magnetic field
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Current through primary
Time
Magnetic field
Time
Voltage induced in secondary
Time

45. Words – alternating, magnetic field, induce, step up, voltage
Transformers
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Transformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.
Words – alternating, magnetic field, induce, step up, voltage
We can work out how much a transformer will step up or step down a voltage:
Voltage across primary (Vp)
No. of turns on secondary (Ns)
Voltage across secondary (Vs)
No. of turns on primary (Np)

46. Some transformer questions
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Primary voltage Vp
Secondary voltage Vs
No. of turns on primary Np
No. of turns on secondary Ns
Step up or step down?
12V
24V
100 ?
400V
200V 20
25,000V
50,000V
1,000
23V
230V
150

47. Some example questions
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Primary voltage Vp
Secondary voltage Vs
No. of turns on primary Np
No. of turns on secondary Ns
Step up or step down? 6V
24V
100 ?
400,000V
200V
1,000
25,000V
20,000 20
230V
150
1,500
A transformer increases voltage from 10V to 30V. What is the ratio of the number of turns on the primary coil to the number of turns on the secondary coil?
A step-down transformer has twice as many turns on the primary coil than on the secondary coil. What will be the output (secondary) voltage if the input voltage is 50V?

48. AC Generators
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Slip rings and brushes
Magnetic Field

49. Generators (dynamos)
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The coil already has an iron core, but the induced current could still be increased by:
Increasing the speed of movement
Increasing the magnetic field strength
Increasing the number of turns on the coil

50. Large-scale production of Electricity
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A generator at Drax power station in England

51. AC Generator N S N S Induced current can be increased in 4 ways:
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Current
Induced current can be increased in 4 ways:
Increasing the speed of movement
Increasing the magnetic field strength
Increasing the number of turns on the coil
Increasing the area of the coil
Time

52. DC and AC
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DC stands for “Direct Current” – the current only flows in one direction and a common example is a battery:
Time
AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz). In the UK mains electricity is 230V AC, not DC, as AC is easier to generate and transmit over long distances.
1/50th s
230V T V

53. P5.5 How do Electric Motors work?
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P5.5 How do Electric Motors work?

54. The Motor Effect N S What will happen to this wire?
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What will happen to this wire?
How can you make it move faster?
How can you make it move in a different direction?

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Electric Motors

56. Using Motors
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The following devices are just some examples of devices that use electric motors: