# Understand Power

In this worksheet, students will learn the meaning of the term power, how to use it and develop knowledge of the power equation.

### QUESTION 1 of 10

What do we mean when we say something has a lot of power? It can mean a lot of things, can't it? Your teachers might have a lot of power to set you endless homework and make you work in silence, or someone who is lifting weights might have a lot of power. But this is physics and when we say power, we mean something specific. In this activity, we will be defining power and how we can use it as a concept to help us understand machines.

Power in physics is a really useful concept. It is also SUPER simple, so simple it is scary! Power is the amount of energy something converts per second. So, if you put 100 J of kinetic energy into walking every second, then you have a power of 100 watts. ‘Watt’ is the unit of power and we normally write it as W - 1 W is 1 joule per second. That’s it! That’s all you need to understand!

As you can imagine, it is really useful to know how much power something uses as it allows you to prepare a set amount of energy for that thing. If you know that a device uses 10 watts of power and you are going to use it for 10 seconds, you know you are going to need 100 J of energy to power it. And we can work this out by using this equation:

Power (W) = Energy transferred (J) ÷ time (s)

P = E ÷ t

Or this one!

Power (W) = Work done (J) ÷ time (s)

P = W ÷ t

So, let’s have a go at an example:

Question:  A device uses 15 J of energy over 20 s. Calculate the power.

Step 1   Highlight all of the numbers in the equation:

A device uses 15 J of energy over 20 s. Calculate the power.

Step 2   Write out the numbers next to their symbols:

P = ?
E = 15 J
t = 20 s

Step 3   Put the numbers into the equation:

P = E ÷ t
P = 15 ÷ 20

P  = 0.75 W

Let's try some questions on this!

Match the values to the units below.

## Column B

Energy (E)
Watts (W)
Power (P)
Joules (J)
Work Done (W)
Joules (J)
Time (t)
Seconds (s)

Write down the definition of a watt.

[1]

A robot does 1000 J of work when it is moving for 10 seconds.

Calculate the power of the robot.

A crane takes 120 s to lift a mass, using a total of 5,400 J of energy during the lift.

Calculate the power of the crane.

Calculate the power of a jet engine that is able to produce 30,000 J of energy in a period of 1 minute.

A toy car is pushed up a hill in order to calculate the energy its internal mechanisms can deliver. It is calculated that it used a total of 15 J of energy to get up the slope, by using the equation E = mgh. The toy was also timed to take 12 seconds before it stopped.

Calculate the power of the toy car.

An object with a mass of 1 kg is lifted 5 m into the air in a time of 5 seconds.

Equations to use:

E = mgh

P = E ÷ t

Gravitational field strength = 10 N/kg

Explain why the value you have just worked out is lower than the actual power of the toy car.

[2]

Describe how to work out power.

[4]

Which is the equation that correctly links power, energy and time?

P = E x t

P = E ÷ t

P = ½Et

• Question 1

Match the values to the units below.

## Column B

Energy (E)
Joules (J)
Power (P)
Watts (W)
Work Done (W)
Joules (J)
Time (t)
Seconds (s)
EDDIE SAYS
So, you might have had this marked wrong if you put the wrong joule with the wrong value - there are two of them, but they are the same, so don't worry! For this question, it is all about making sure that you have the correct values so you can spot them in a question! If you can find them in the question, then you can work with them to find the answer.
• Question 2

Write down the definition of a watt.

[1]

EDDIE SAYS
This is one of those definitions that you need to know, but it will also help you to work out the answer to maths questions. If you know the answer is in watts and you are given joules and a time, you know that if you divide the energy by the time then you will get your answer!
• Question 3

A robot does 1000 J of work when it is moving for 10 seconds.

Calculate the power of the robot.

EDDIE SAYS
Don't panic - this is a simple maths question and gives you a chance to use that equation. What are the values? P = ? W = 1000 J t = 10 s Put them into the equation: P = W ÷ t P = 1000 ÷ 10 P = 100 W It's not hard so long as you take it one step at a time.
• Question 4

A crane takes 120 s to lift a mass, using a total of 5,400 J of energy during the lift.

Calculate the power of the crane.

EDDIE SAYS
Another simple one - as long as you take it carefully. This time with the energy equation instead of the work equation, but it's basically the same thing! Find the values: P = ? E = 5,400 J t = 120 s Put them into the equation: P = E ÷ t P = 5,400 ÷ 120 P = 45 W
• Question 5

Calculate the power of a jet engine that is able to produce 30,000 J of energy in a period of 1 minute.

EDDIE SAYS
This one might be a little more complicated! You can see here that the time is not in seconds, but in minutes! So how are we going to do this? The first thing that we need to do is to convert the time from minutes into seconds. All you need to do for this is multiply it by 60 (the number of seconds in 1 minute). 60 x 1 = 60 s. Write down the numbers: P = ? E = 30,000 J t = 60 s Put the numbers into the equation: P = E ÷ t P = 30,000 ÷ 60 P = 500 W And there you have it!
• Question 6

A toy car is pushed up a hill in order to calculate the energy its internal mechanisms can deliver. It is calculated that it used a total of 15 J of energy to get up the slope, by using the equation E = mgh. The toy was also timed to take 12 seconds before it stopped.

Calculate the power of the toy car.

EDDIE SAYS
This question has a lot of words and formulae to try and distract you from the actual question. Always look at the last sentence first - this is going to tell you what the question is asking, then you can read through the rest knowing what to look out for. In this case, it's energy and time because this is a power question. So, put in the values: P = ? E = 15 J t = 12 s P = E ÷ t P = 15 ÷ 12 P = 1.25 W
• Question 7

An object with a mass of 1 kg is lifted 5 m into the air in a time of 5 seconds.

Equations to use:

E = mgh

P = E ÷ t

Gravitational field strength = 10 N/kg

EDDIE SAYS
What a lot there is to take in here! This is the type of question you might get in an exam - it will expect you to use more than one equation in working out the answer. Here we have given you the equations to use, you just need to find the correct way of solving them (the hardest part!) Hopefully, you realised that you can't work out the power until you have a value for the energy - so how do you work out the energy? You use the equation for energy that we have given you! E = mgh m = 1 kg g = 10 N/kg h = 5 m E = 1 x 10 x 5 E = 50 J Now you have this, you can put it into the equation for power: P = E ÷ t E = 50 t = 5 s P = 50 ÷ 5 P = 10 W Brilliant work!
• Question 8

Explain why the value you have just worked out is lower than the actual power of the toy car.

[2]

EDDIE SAYS
This is testing your knowledge of energy transfers! You need to understand that energy is lost whenever there is an energy change, and this case is not any different. We can't work out the energy loss using this model, so there is no way of knowing the real power of the toy car.
• Question 9

Describe how to work out power.

[4]

EDDIE SAYS
This involved you thinking about everything that you have done so far and putting it into a handy, helpful guide to yourself. This will help you remember how to work out power and all other equations that you need to know. 1 Write the equation. 2 Add the values to the equation. 3 Solve the equation. 4 Do not forget the units!
• Question 10

Which is the equation that correctly links power, energy and time?

P = E ÷ t
EDDIE SAYS
Did you get this one right? Remember that power = P, energy = E and time = t. So the correct equation is P = E ÷ t That's it for power for the time being. Well done for working through this activity - hopefully, you're feeling more confident with this equation now.