1. OCR Gateway 2016 Physics topic 3
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Electricity
OCR Gateway 2016 Physics topic 3
W Richards
The Weald School

2. P3.1 Static and Charge
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3. Static Electricity Introduction
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Here are two rods made of positive and negative charges (like everything is): + - - +
Q. Would these rods be attracted or repelled by each other or would they sit still and why?

4. Static Electricity + + - - + - + + + - - - + + - + - -
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Static electricity is when charge “builds up” on an object (insulators only) and then stays “static”. How the charge builds up depends on what materials are used: + + - - + - + + + - - - + + - + - -

5. 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?

6. Static Electricity + - - -
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Now that the rods are charged they will want to move towards or away from each other. Which direction will these pairs of rods move in? + - - -

7. Gold Leaf Electroscopes
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Consider a gold-leaf electroscope…
Now charge the top: - - - - - - - - -

8. Van de Graaf generators
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When a charge is neutralised by the movement of electrons either from the Earth or to the Earth we call this “earthing”

9. Van de Graaf generators and Electric Fields
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Here’s a charged VDG:
I can really feel the electric field here
I can’t feel it over here!
Clearly, the electric field gets weaker the further out you get.

10. 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” and it has the SAME value at any point in this circuit e-

11. Charge (Q)
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As we said, current is when electrons move around a circuit and carry energy with them. Each electron has a negative CHARGE. Charge is measured in Coulombs (C). For current to flow in a circuit there must be a potential difference across a closed circuit. Q T I
Charge = current x time
(in C) (in A) (in s)

12. Example questions Charge (C) Current (A) Time (s) 5 2 0.4 1 20 0.5 50
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Charge (C)
Current (A)
Time (s) 5 2
0.4 1 20
0.5 50
250 3 60
A circuit is switched on for 30s with a current of 3A. How much charge flowed?
During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for?
A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?

13. Example questions Charge (C) Current (A) Time (s) 10 5 2 0.4 1 20 0.5
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Charge (C)
Current (A)
Time (s) 10 5 2
0.4 1 20
0.5 40 50
0.2
250
180 3 60
A circuit is switched on for 30s with a current of 3A. How much charge flowed?
During electrolysis 6A was passed through some copper chloride and a charge of 1200C flowed. How long was the experiment on for?
A bed lamp is switched on for 10 minutes. It works on a current of 0.5A. How much charge flowed?
90C
200s
300C

14. P3.2 Simple Circuits
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15. 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

16. 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 ____, named after Andre Ampere.
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 Alessandro Volta.
Resistance is anything that resists an electric current. It is measured in _____, named after Georg Ohm.
Words: volts, amps, ohms, voltage, ammeter, voltmeter

17. 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

18. Circuit building practice
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19. What happens to Current in a series circuit?
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What happens to Current in a series circuit?
The current here will be…
If the current here is 2 amps… ?A ?A
And the current here will be… ?A
In other words, the current in a series circuit is THE SAME at any point

20. What happens to Current in a parallel circuit?
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What happens to Current in a parallel circuit?
A PARALLEL circuit is one where the current has a “choice of routes” ?A
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…
In other words, the current in a parallel circuit SPLITS UP down each “strand”

21. 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…

22. Some example questions…
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1A each

23. Voltage in a series circuit
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If the voltage across the battery is 4.5V… V
…and these bulbs are all identical… V V
…what will the voltage across each bulb be?
1.5V
In other words, voltage in a series circuit “SPLITS UP” over each component

24. Voltage in a series circuit
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If the voltage across the battery is 4.5V… V V
…what will the voltage across two bulbs be?
1.5V

25. Voltage in a parallel circuit
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If the voltage across the batteries is 4V…
What is the voltage here? 4V V
And here? 4V V
In other words, the voltage in a parallel circuit STAYS THE SAME across each “strand”

26. 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”

27. An example question: 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V
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28/02/2019 3A 6V A1 3A 6V A2 2A V1 1A A3 V2 V3 3V 3V

28. Another example question:
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10V A1 3A
1.2A A2 V1
6.7V A3
1.8A V2 V3 5V 5V

29. 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)

30. 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

31. An example question: Ammeter reads 2A A
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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

32. What is the resistance of these bulbs?
Example Questions
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12V 4V 2A 1A 2V 3A
What is the resistance of these bulbs?

33. Investigating Factors affecting Resistance
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Method:
Set up the circuit
Measure length, voltage and current
Calculate resistance
Draw a conclusion
Length of wire (m)
Current (A)
Voltage (V)
Resistance (Ω)

34. Understanding Resistance
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What happens inside a conducting material? The following model of a metal wire could help:
Electrons
Ions
At normal temperatures, with no current flowing, electrons hurtle around continuously. What happens if you connect a battery?

35. Using a battery to push a current
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Apply a voltage:
Electrons
Ions
Negative
Positive
This time we can see that the electrons are accelerated from negative to positive. This is an electric current and happens because the voltage “pushes” the electrons.

36. Resistors, bulbs and diodes
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37. I-V graphs for Resistors, Bulbs and Diodes
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Voltage on powerpack/V
Current/A
Voltage/V 12 10 8 6 4 2 -2 -4 -6 -8
-10
-12

38. I-V graphs for Resistors, Bulbs and Diodes
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Method:
Connect up the circuit as shown with a powerpack voltage of 2V
Measure the voltage across the bulb and the current through the bulb
Increase the voltage to 4V and repeat the measurements
Repeat up to 12V
Repeat with negative voltages from the powerpack

39. Current-voltage graphs
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3. Diode
1. Resistor
2. Bulb
A diode only lets current go in one _______ – it has very _____ resistance in the other direction
Current increases in proportion to _______, provided the temperature doesn’t change
As voltage increases the bulb gets ______ and _______ increases
Words – resistance, high, voltage, hotter, direction

40. Resistors in Series and Parallel
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12V 3A
In SERIES, the total resistance is given by the sum of the individual resistances:
RTotal = R1 + R2
What is the total resistance in this circuit?
In PARALLEL the total resistance is less than the resistance of the individual resistors as the current has more “routes” to flow through. What is the highest value the resistance of this circuit could be?

41. Equivalent Resistance
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What is the resistance of this resistor?
If all three resistors are the same, what is the total resistance?
“Equivalent resistance” is the idea of replacing all of the resistors in a circuit with one resistor of the same value. The above circuit becomes this: 9Ω

42. LDRs and Thermistors
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43. 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

44. Using Thermistors 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?

45. Using LDRs in circuits A
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Here’s a circuit that has been left in a room. Will the lamp come on when the room is light or dark? Explain your answer.

46. Energy transferred = charge x voltage
Energy and charge
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The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around:
Energy transferred = charge x voltage
(in J) (in C) (in V) W Q V

47. Example questions (1st set)
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In a radio circuit a voltage of 6V is applied and a charge of 100C flows. How much energy has been transferred?
In another circuit a voltage of 10V is applied and 200C of charge flows around the circuit. How much energy is transferred?
48J of electrical work is done by a 6V battery. How much charge flowed around the circuit?
Another battery transfers 20C of charge and does 100J of electrical work. What was the battery’s voltage?
600J
2000J 8C 5V

48. Example questions
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In a radio circuit a voltage of 12V is applied and a charge of 100C flows. How much energy has been transferred?
In the above circuit the radio drew a current of 0.5A. How long was it on for?
A motor operates at 6V and draws a current of 3A. The motor is used for 5 minutes. Calculate: a) the charge flowing through it, b) the energy supplied to it
A lamp is attached to a 12V circuit and a charge of 1200C flows through it. If the lamp is on for 10 minutes calculate a) the current, b) the energy supplied to the bulb.
1200J
200s
900C, 5400J
2A, 14,400J

49. Electrical Power
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The power of an electrical device is given by this equation: P I V
Power = voltage x current
in W in V in A

50. Which devices transfer the most power and why?
Example Questions
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Complete the following table:
Appliance
Power rating (W)
Voltage (V)
Current needed (A)
Toaster
230 4
Fire 8
Hoover
460 2
Hairdryer
1150
Computer
100
Stereo 80
Which devices transfer the most power and why?

51. Example Questions Complete the following table: Appliance
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Complete the following table:
Appliance
Power rating (W)
Voltage (V)
Current needed (A)
Toaster
920
230 4
Fire
1840 8
Hoover
460 2
Hairdryer
1150 5
Computer
100
0.43
Stereo 80
0.35

52. Measuring Power Power = voltage x current in W in V in A I / A V / V
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Power = voltage x current
in W in V in A
I / A
V / V
P / W V A 52

53. Another Equation for Power
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Here’s another way of calculating power – use this one if you’re given a value for resistance:
Power = I2R
A 10Ω resistor has 2A flowing through it. Calculate the power dissipated by the resistor.
A motor takes a current of 10A. If its resistance is 2.2MΩ calculate the power dissipated by the motor.
A 2KW heater has a resistance of 20Ω. Calculate the current through it.
40W
220MW
10A

54. In other words, 1 Watt = 1 Joule per second
Energy and Power
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The POWER RATING of an appliance is simply how much energy it uses every second.
In other words, 1 Watt = 1 Joule per second
Energy = Power x Time E T P
E = Energy (in joules)
P = Power (in watts)
T = Time (in seconds)

55. 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?
Callum 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?
Curtis’ brain needs energy supplied to it at a rate of 40W. How much energy does it need during a 50 minute physics lesson?
Fergal’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a 24 hour day?
60W
2KW
0.2MW
150J, 9KJ
120KJ
1.73MJ

56. Numerical quiz
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What is the resistance of a bulb with a voltage of 12V and a current of 2A through it?
An electric fire uses 1200C of charge over 2 minutes. What current did it draw?
What is the power rating of a kettle that runs on a voltage of 230V and draws a current of 8A?
In an electrolysis experiment a 2A current flowed for 5 minutes. How much charge flowed?
A 150 resistor is placed across a 3V battery. What current will it draw?
How much power will this resistor transfer? 6
10A
1.8KW
600C
0.02A
0.06W

57. Numerical quiz
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A thermistor has a resistance of 200 when 20V is applied across it. What is the current through the thermistor?
The same thermistor is put in a warm water bath. The resistance drops to 120. What is the current through it now?
A kettle runs on a power rating of 2.3kW. If it is plugged into a mains supply what current will it draw?
What fuse should this kettle have?
A 230V heater transfers 20C of charge. How much electrical energy has it converted?
What fuse should a 230V, 920W microwave have in its plug?
0.1A
0.17A
10A
13A
4.6KJ 5A

58. Random questions 4 1A 40C 5A 1500s, 8.7A, 13000C
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A battery has a voltage of 12V and it puts a current of 3A through a bulb. What is the bulb’s resistance?
Another bulb transfers 120C of charge in 2 minutes. What was the current through it?
A powerpack transfers 2,000J to a motor. If the motor ran on a voltage of 50V how much charge was transferred?
A hairdryer runs on a 50Hz power supply. If it has a power rating of 800W what fuse should it have?
An electric fire transfers 3MJ of energy. If it has a power rating of 2KW calculate how long it was on for, the current it ran on (assuming it was connected to the mains supply) and the amount of charge it transferred. 4 1A
40C 5A
1500s, 8.7A, 13000C