# Understand Electrical Power

In this worksheet, students will learn the meaning of the term power and apply this to electrical circuit problems using the power equation.

Key stage:  KS 4

Difficulty level:

### QUESTION 1 of 10

Have you ever thought about the way people work out how much you need to pay for electricity? Or perhaps you didn’t even know you have to pay for electricity?!  Electricity costs money because it is a form of power and money is spent burning fuel to make the electricity. The more power you use, the more you have to pay for that electricity. But how can we work out how much power one house is using? Luckily, there are some equations we can use to work out power, and that is what we are going to be doing in this activity.

First of all, there is a power equation that uses current and voltage. This should hopefully make sense since current is a measurement of how many electrons are flowing through a wire, and voltage is a measurement of how much energy each one is carrying.

P = IV

P = power (watts (W))
I = current (amps (A))
V = potential difference (volts (V))

The second equation is a little tougher – but it comes from putting the first equation and V = IR together. It looks like this:

P = I²R

P = power (watts (W))
I = current (amps (A))
R = resistance (ohms (Ω))

Let’s take a look at an example:

Question: A kettle uses 400 V with a current of 3 A. Calculate the power of the kettle.

Step 1   Highlight all of the numbers in the equation:

A kettle uses 400 V with a current of 3 A. Calculate the power of the kettle.

Step 2   Write out the numbers next to their symbols:

P = ?
I = 3 A
V = 400 V

Step 3   Put the numbers into the equation:

P = IV
P = 3 x 400

Step 4    Do the maths and write your answer:

P = 1,200 W

Don’t forget your units!

Let’s try some questions on this!

First of all, let's have a go at matching all of the symbols to their units.

## Column B

Power (P)
Volts (V)
Voltage (V)
Watts (W)
Current (I)
Amperes (A)
Resistance (R)
Ohms (Ω)

What is power the measurement of?

Strength

Energy per second

Speed

Voltage per second

Use the equation P = IV for this question.

Sandra connects up a circuit with a current of 3 A and a potential difference of 5 V.

Calculate the power of the circuit.

Strength

Energy per second

Speed

Voltage per second

Use the equation P = IV for this question.

James is working on a robot that needs a minimum of 20 W to work. He makes a circuit that has a current of 2 A and a voltage of 4 V.

Will this circuit be enough to power the robot?

Yes

No

James wants to build a circuit that will run his robot, but he doesn't know what current and voltage to use.

Suggest a current and a voltage for him to use in his circuit in order to get his power to 20 W.

[1]

A circuit with a power of 3 W was on for 30 s.

Calculate the energy that was used by the circuit.

Use the equation P = IV for this question.

A lightning strike has a voltage of 1,000,000,000 V and a current of 30,000 A.

Calculate the power of the lightning strike.

Use the equation P = I2R for this equation.

A phone needs a current of 3 A and has a total resistance of 20 Ω.

Calculate the power of the phone.

Use P = I2R to answer this question.

A light bulb needs a current of 5 A and has a resistance of 4 Ω.

Calculate the power of the light bulb.

A computer uses a current of 2 A and has a voltage of 40 V.

Calculate the power of the computer.

• Question 1

First of all, let's have a go at matching all of the symbols to their units.

## Column B

Power (P)
Watts (W)
Voltage (V)
Volts (V)
Current (I)
Amperes (A)
Resistance (R)
Ohms (Ω)
EDDIE SAYS
How did you get on with this first question? If you know the symbols and their units, then you will be able to pick them out in a question. If you can do that, then it makes it that little bit easier to answer the question because the first step is basically done for you! Life is just a whole lot more simple when you know your symbols!
• Question 2

What is power the measurement of?

Energy per second
EDDIE SAYS
When we talk about power in any topic (apart from maybe history), we are talking about energy. The more energy you have, the more stuff you will be able to do, and the more stuff you are able to do, the better! Power means you can do a lot of stuff - power means energy!
• Question 3

Use the equation P = IV for this question.

Sandra connects up a circuit with a current of 3 A and a potential difference of 5 V.

Calculate the power of the circuit.

EDDIE SAYS
This is a nice simple one to ease you into the use of the equation. Let's go through it step by step together to get to the answer. Step 1 Identify the numbers: P = ? I = 3 A V = 5 V Step 2 Put them into the equation: P = IV P = 3 x 5 Step 3 Do the maths: P = 15 W Don't forget the unit!
• Question 4

Use the equation P = IV for this question.

James is working on a robot that needs a minimum of 20 W to work. He makes a circuit that has a current of 2 A and a voltage of 4 V.

Will this circuit be enough to power the robot?

No
EDDIE SAYS
This question is a lot more simple than it looks! We've told you what equation to use, you just need to apply that equation in the correct way. Let's have a look at what we would have done to answer this question. Step 1 Identify the numbers: P = ? I = 2 A V = 4 V Step 2 Put them into the equation: P = 2 x 4 Step 3 Do the maths: P = 8 W 8 W is less than the 20 W needed, so he cannot run the circuit on this setup. He will need to increase either the current or the voltage, or even both, if he wants to make his robot work.
• Question 5

James wants to build a circuit that will run his robot, but he doesn't know what current and voltage to use.

Suggest a current and a voltage for him to use in his circuit in order to get his power to 20 W.

[1]

EDDIE SAYS
If we think about the equation P = IV, we know we want the answer P to be 20. This means that all we need to do is to work out a way of making 20 by multiplying two numbers together. You could have had any numbers you wanted, 40 and 0.5, 2 and 10, any so long as they multiplied together to make 20. But you would not have got the marks unless you also labelled these numbers correctly! This means that your answer should have looked something like this: Voltage = 4 V and Current = 5 A
• Question 6

A circuit with a power of 3 W was on for 30 s.

Calculate the energy that was used by the circuit.

EDDIE SAYS
What is the definition of a watt? A joule per second! This means that if something has a power of 3 W, it is using 3 joules of energy every second. So if this device was on for 30 seconds, we need to multiply the power by the time that it was on for: 3 x 30. 3 x 30 = 90 J Don't worry if you found this one a bit challenging - it is referring to a different topic which you might not have revised yet.
• Question 7

Use the equation P = IV for this question.

A lightning strike has a voltage of 1,000,000,000 V and a current of 30,000 A.

Calculate the power of the lightning strike.

EDDIE SAYS
The hard part of this question is working out how many zeros to take off the answer when you are making it into gigawatts! Let's take a look at how we did this whole question from start to finish. P = IV P = ? I = 30,000 A V = 1,000,000,000 V P = 30,000 x 1,000,000,000 P = 30,000,000,000,000 W Now - giga is the third one (kilo, mega, giga) and each one is worth three zeros, so: P = 30,000,000,000 kW P = 30,000,000 MW P = 30,000 GW Boom!
• Question 8

Use the equation P = I2R for this equation.

A phone needs a current of 3 A and has a total resistance of 20 Ω.

Calculate the power of the phone.

180 W
180
EDDIE SAYS
This is a different equation, so you need to do stuff a little differently to get to the answer. Let's go through some of this together so you know how to do it: Step 1 Find the numbers: P = ? I = 3 A R = 20 Ω Step 2 Put them into the equation: P = I2R P = 32 x 20 Step 3 Do the squared first: 32 = 3 x 3 = 9 Step 4 Do the maths: P = 9 x 20 P = 180 W How did you get on with it?
• Question 9

Use P = I2R to answer this question.

A light bulb needs a current of 5 A and has a resistance of 4 Ω.

Calculate the power of the light bulb.

100 W
100
EDDIE SAYS
Did you remember what to do this time? Let's have a go at the equation: P = ? I = 5 A R = 4 Ω P = 52 x 4 P = 25 x 4 P = 100 W
• Question 10

A computer uses a current of 2 A and has a voltage of 40 V.

Calculate the power of the computer.

80 W
80
EDDIE SAYS
What? We didn't tell you what equation to use? Must mean we're confident in your ability to answer the question without help, or we just wanted to make things harder for you! You should have used P = IV. P = ? I = 2 A V = 40 V P = 2 x 40 P = 80 W Brilliant! Well done if you got this final question - it means you're gaining confidence with your ability to choose and use the appropriate equation.
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