1. G482 Electricity, Waves & Photons
2.2 Resistance
G482 Electricity, Waves & Photons
2.2.5 Power
Ks5 OCR Physics H158/H558
Index
Mr Powell 2012

2. 2.2.5 Power (Prior Learning)
When we talk about Power what we mean is “the amount of energy delivered per second”
1 Joule / 1 Second = 1 Watt
It then makes sense that the Power used by a component can be found from the product of current through
and voltage across the component;
Power = Voltage x Current
P = V x I

3. Analogy
Another way of thinking about it is saying that the current carries the energy; 4V 1A 1J 1J 1J 1J 1J 1J 1J 1J C C C C C C C C C C C
= 1 Coulomb
of charge 1J
= 1 Joule of energy
As the Coulombs of Charge move they release their energy as heat and light (through the bulb)
= 1 Second of time

4. Analogy 2
If the voltage increases, more energy is delivered so the power increases; 5V 1A 1J 1J 1J 1J 1J 1J 1J 1J 1J 1J 1J 1J C C C C C C C C C C C
= 1 Coulomb
of charge 1J
= 1 Joule of energy
Power = 5V x 1A = 5J/s = 5W
= 1 Second of time

5. Analogy 3
If the current increases, more energy is delivered so the power increases; 4V 2A 1J C C C C C C C C C
= 1 Coulomb
of charge C C 1J 1J 1J 1J C 1J 1J 1J 1J
= 1 Joule of energy 1J
= 1 Second of time
Power = 4V x 2A = 8J/s = 8W

6. Flow of Charge
When an electrical appliance is on, electrons are pushed through the appliance by the potential difference
The potential difference causes a flow of charge particles (free electrons). The rate of flow of charge is the electric current through the appliance. 1A = 1C/s
The unit of charge, the coulomb (C).
The charge passing along a wire or through a component in a certain time depends on: the current, and the time.
We can calculate the charge using the equation:
Q=It

7. Energy and Potential Difference
E = VQ
Q=It
E = VIt
When a resistor is connected to a battery, electrons are made to pass through the resistor by the battery.
Each electron repeatedly collides with the vibrating atoms of the resistor, transferring energy to them.
The atoms of the resistor therefore gain kinetic energy and vibrate even more. The resistor becomes hotter.
When charge flows through a resistor, electrical energy is transformed into heat energy.
The energy transformed in a certain time in a resistor depends on: the amount of charge that passes through it, and the potential difference across the resistor.

8. 2.2.5 Power Assessable learning outcomes
describe power as the rate of energy transfer;
select and use power equations P = VI, P = I2R, P = V2/R
explain how a fuse works as a safety device (HSW 6a);
determine the correct fuse for an electrical device;
select and use the equation W = IVt;
define the kilowatt-hour (kW h) as a unit of energy;
calculate energy in kW h and the cost of this energy when solving problems (HSW 6a).
A Joulemeter (V, I and t) or a data-logger may be used to determine energy transfer.
A utilities statement can be used to illustrate the use of the kW h by electricity companies. (Homework)

9. a) describe power as the rate of energy transfer;

10. a) HSW - PC Energy Use Question
Here is an example of the energy usage of a home computer. Each internal component transfers a variable amount of energy. If you left this PC on for an hour it would transfer;
Energy = Power x Time
Energy = 154W x 1 hour
Energy = 154W x 3600s
E = 154J/s x 3600s
E = J
E = 554.4kJ
Current flow to the PC would then be…
P = VI or P/V = 0.67A

11. Resistance Heating...
Charge carriers transfer kinetic energy to positive ions through repeated collisions.
The pd across the material then provides an accelerating force to the charge carrier
which then collides with another positive ion.
V = I R
P = I V
P = V2 R
Energy per second transferred
to the component P:
P = I2 R
In the steady state this equals
the heat transfer to the surroundings

12. b) select and use power equations P = VI, P = I2R, P = V2/R
TASK: Try and rearrange the equations and substitute so you can get from one to another!
P = VI
P = Et
V = IR
P = I2R
P = V2/R
Q = It
E = Vit
E = VQ

13. c) explain how a fuse works as a safety device (HSW 6a);

14. Heater Circuit
Inside 3 Pin Plug
3 Pin Plug
/ Mains
Appliance

15. Facts Circuit Breakers
Facts Fuses
Fuses are thin pieces of wire which resist a large current
They melt when too much current flows
Circuit is broken
Have to be replaced
Facts Circuit Breakers
Circuit breakers are designed to flip over when too large a current flows
They break the circuit
Rely on magnetic forces

16. d) determine the correct fuse for an electrical device;
Domestic appliances are often fitted with a 3A, or a 5A or a 13 A fuse.
If you don’t know which one to use for an appliance, you can work it out from the
power rating of the appliance and its potential difference (voltage).

17. e) select and use the equation W = IVt; (SHC Practical)
The specific heat capacity of a substance is the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius.
E = mc
E is energy transferred in joules, J
m is mass in kilograms, kg
 is temperature change in degrees Celsius, °C
c is specific heat capacity in J / kg °C

18. 1 kg Metal blocks (Cu, Fe, Al)
Equipment
two digital multimeters (V & A)
power supply, 0 – 12 V DC
thermometer, 0 – 100C
12 V heater
Diagram
stopwatch
12V A V
Thermometer
Heater
electrical leads
Metal block
1 kg Metal blocks (Cu, Fe, Al)

19. Using a Multi-meter DC Volts Various Ranges DC mA various ranges
Resistance ranges in 
Normal Currents up to 10A
Volts or mA or 

20. Homework - Design a new experiment for water?
power supply, 0 – 12 V DC ?
thermometer, 0 – 100C
stopwatch
Diagram ?
two digital multimeters (V & A)
electrical leads

21. Results.... A B C D E F G Metal (1kg) Start Temp / C End Temp / C
Temp Rise / C
Current /A
PD / V
Time for rise / s
SHC
/ J /kg°C ?
21.5 34
12.5
2.62
9.1
420
801 Al Fe Cu
Quoted....
Aluminium (Al) is 902 J /kg°C
Iron (Fe) 450 J /kg°C
Copper (Cu) is 385 J /kg°C
Analysis Thoughts?
Are there any flaws with your experiment where electrical or thermal energy are lost and don’t raise the temperature of the block?
What could you do to make the experiment more reliable?
Can you think of a similar experiment to find the SHC of water?

22. f) define the kilowatt-hour (kW h) as a unit of energy;
The kilowatt hour, or kilowatt-hour, (symbol kW·h, kW h or kWh) is a unit of energy equal to 1000 watt hours or 3.6MJ.
For constant power, energy in watt hours is the product of power in watts and time in hours.
The kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.
The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h) of time.
Inversely, one watt is equal to 1 J/s.
One kilowatt hour is 3.6 megajoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.

23. When Maths starts to hurt!

24. g) calculate energy in kW h and the cost of this energy when solving problems (HSW 6a).
Poor Dr Frankenstein did not look at his electricity bill and check the cost of each unit of electricity.
1 Unit = 1KW hour of electricity.
They are shown on the bill here..

25. Plenary Question…. Answer….
Two resistors, A and B, have different resistances but otherwise have identical physical properties. E is a cell of negligible internal resistance.
When the resistors are connected in the circuit shown in figure 1, A reaches a higher temperature than B. When connected in the circuit shown in figure 2, B reaches a higher temperature than A. Explain these observations fully, stating which resistance is greater. (6 marks)
Answer….
power determines heat produced (1)
in series, current is same (1)
I2 RA must be > I2 RB (1)
in parallel, p.d. is same (1)
iSlice
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26. Can you connect the formulae...
P = V2/R
Q = It
E = Vit
E = VQ
P = VI
P = Et
V = IR
P = I2R

27. Connection
Connect your learning to the content of the lesson
Share the process by which the learning will actually take place
Explore the outcomes of the learning, emphasising why this will be beneficial for the learner
Demonstration
Use formative feedback – Assessment for Learning
Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex
Offer different ways for the students to demonstrate their understanding
Allow the students to “show off” their learning
Consolidation
Structure active reflection on the lesson content and the process of learning
Seek transfer between “subjects”
Review the learning from this lesson and preview the learning for the next
Promote ways in which the students will remember
A “news broadcast” approach to learning
Activation
Construct problem-solving challenges for the students
Use a multi-sensory approach – VAK
Promote a language of learning to enable the students to talk about their progress or obstacles to it
Learning as an active process, so the students aren’t passive receptors