# Understanding Electrical Power

In this worksheet, students will understand the term power and be able to apply it to electrical circuits. They will also be able to use the power equation.

Key stage:  KS 4

Difficulty level:

### QUESTION 1 of 10

Before we start, you should have an understanding of the terms Power and Watts to do this activity. If you don’t (or you’d like a recap) then head over to the activity called ‘Understanding Power’.

Have you ever thought about how people work out how much you need to pay for electricity? You didn’t even know you have to pay off electricity, did you… Electricity costs money because it is a form of power and people have to spend money burning fuel to make the electricity. The more power you use the more you have to pay for that electricity. But how do you work out how much power one house is using? Luckily there are some equations we can look at 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 as current is a measurement of how many electrons are flowing through the wire and voltage is a measurement of how much energy each one is carrying.

INSERT IMAGE OF EQUATION HERE

P = power (Watts (W))
I = current (Amps(A))
V = potential difference (Volts (V))

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

INSERT IMAGE IF EQUATION HERE

P = power (Watts (W))
I = current (Amps (A))
R = resistance (Olms (Ω))

Let’s take a look at an example.

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

Step 1 – highlight all of the number 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 = 3 x 400

P = 1,200 W

Let’s try some questions on that!

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

## Column B

Power (P)
Watt (W)
Voltage (V)
Ohms (Ω)
Current (I)
Volts (V)
Resistence (R)
Ampere (A)

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.

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.

A circuit with a power of 3 W was on for 30 s. Calculate the energy is 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 a lightning strike. Give your answer in GW (Giga Watts).

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 lightbulb needs a current of 5 A and has a resistance of 4 Ω. Calculate the power of the lightbulb and include the unit in your answer.

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)
Watt (W)
Voltage (V)
Volts (V)
Current (I)
Ampere (A)
Resistence (R)
Ohms (Ω)
EDDIE SAYS
If you know the symbols and their units, then you will be able to pick them out in the question. If you can pick thme out int he question, 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! (That's going to be physics' new catchphrase!)
• 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. Then 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.

15
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 = 3 x 5 Step 3 - Do the maths: P = 15 W DONE! Simple, right?
• 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 - P = ? I = 2 A V = 4 V Step 2 - P = 2 x 4 Step 3 - P = 8 W 8 is less than the 20 needed, so we cannot run the circuit on this setup.
• 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.

EDDIE SAYS
If we think about the equation P = IV, we know we want the answer (P) to be 20. These 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 number you wanted, 40 and 0.5, 2 and 10, any! 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 is used by the circuit.

90
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 jouls 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 Amazing answer to an amazing question. We told you questions from other topics might come up!
• 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 a lightning strike. Give your answer in GW (Giga Watts).

30,000
30000
EDDIE SAYS
The hard part of this question os working out how many 0's to take off the answer when you are making it into Giga Watts! 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 3 0's 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
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
• Question 9

Use P = I2R to answer this question.

A lightbulb needs a current of 5 A and has a resistance of 4 Ω. Calculate the power of the lightbulb and include the unit in your answer.

100W
100 W
EDDIE SAYS
Did you get the unit? Did you make sure that it was a capital? It's important when looking at units to make sure that you get you capitals correct! 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
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 DONE! yay!