# Apply the Conservation of Energy

In this worksheet, students will recall the law of the conservation of energy as well as describe the ways of increasing efficiency within systems.

### QUESTION 1 of 10

A law in science is something that cannot be broken. We have several laws that you will know quite well, such as the law of gravity. We rely on these laws to make accurate predictions about the world around us, whether we are building houses, mobile phones or spaceships!

The most fundamental and important law is the law of the ‘Conservation of Energy’. You will see this referred to in physics, chemistry and biology as it is a key part of how all three sciences (and the world) works.

It is fairly simple, but because it is so simple you MUST remember it word for word! Here it is:

‘Energy cannot be created or destroyed; it can only be converted from one form to another’.

When people first come across this law, it can blow their mind. Think about places where energy is made (like in a power station). We can no longer say energy is made -  we can only say that energy is transferred.

Let’s look at an example of a system (something that converts energy from one form to another) and see if we can spot the energy transfers that are taking place.

Look at this light bulb.

What energy is going into it? Electrical.

The energy store that is depleting is the electrical store. This is because electrical energy is being converted into light energy and heat energy. We want the light energy, this is useful, but we also get some wasted energy as heat. We didn’t design a light bulb to get hot, but it does anyway. This is what we call wasted energy as it is not doing the job that we wanted it to do.

When we compare the amount of energy we want with the amount of energy we don’t want, we come up with a number that shows efficiency.  Efficiency is a simple measurement telling us how much of the energy is going into doing the job that we have designed it for. It can be presented as a number between 0 and 1 (where 0 is none of the energy going into what we want, and 1, where all of the energy is going into what we want) or as a percentage.

FUN FACT – it is impossible to get a system that has an efficacy of 100%. Not just difficult, not just that we have never managed to achieve it, but impossible – it is another one of those laws.

Let's look at how you work out efficiency then:

So, let’s try an example:

A light bulb takes in 100 J of energy every second. 40 J of that energy are given off as light. Calculate the efficiency of the light bulb.

Step 1  Highlight all of the numbers in the question.

A light bulb takes in 100 J of energy every second. 40 J of that energy are given off as light. Calculate the efficiency of the light bulb.

Step 2  Find out what each value is.

Energy in = 100 J

Useful energy out = 40 J

Step 3  Put the number into the equation.

Efficiency = 40 ÷ 100

Step 4  Press = and get your number (if it asks for a percentage, all you need to do is x 100 at the very end).

Efficiency  = 0.4 or 40%

There are no units for efficiency, but if it is a percentage, always put the percentage sign after it.

Finally, we need to look at how to manage efficiency within a system. This will vary from system to system, but you need to look for where there is energy loss and how you can manage that energy loss. A really common example is energy lost as friction in most systems. Let's say you skateboard, but your board isn't very fast or it stops too quickly. This is due to energy loss in the friction of the wheels and the bearings.  We can try to minimise this loss by oiling the wheels, which will reduce the loss of energy from the heating effect of friction. You will now go further with your nicely oiled wheels - brilliant!

Now it's time for some questions.

What is the law of the 'Conservation of Energy'?  Write the answer without any punctuation or it will be marked as incorrect!

Identify the wasted energy in this system.

A car engine.

Heat

Kinetic

Sound

Gravitational

Calculate the efficiency of the oven in this question.

Energy in per second =  250 J

Heat energy =  228 J

Light energy = 7 J

Write the answer as a decimal.

A power plant wastes a huge amount of energy, mainly as heat and sound.  (Modern power stations are much more efficient than they used to be, but they still waste energy!) In a single second, a power station can produce 2,000 MJ. If 1,050 MJ of energy is wasted as heat and sound, calculate the efficiency of the power station.

Write the answer as a decimal.

A mobile phone gets hot when it is being used. It converts 100 J of energy every second into light, sound and heat (as well as Netflix).

What is the percentage efficiency of the mobile phone if 50 J of energy is emitted as light, 25 J as sound and another 25 J as heat?

0%

25%

50%

75%

Describe the useful and wasted energy in an oven.

Explain why it is useful or wasted.

[3]

An electric motor is being developed for BMW cars. They want to reduce the rolling resistance of the motor as much as possible.  Rolling resistance is the resistance from friction between the parts of the motor. This will increase the efficiency of the motor.

Name one way they could reduce this energy loss and what store of energy this would affect.

[1]

A computer has an efficiency of 80% and input energy of 500 J per second.

Calculate the useful energy in the computer.

A television has an efficiency of 94% and an input energy of 150 J per second.

Calculate the amount of wasted energy every second.

Describe the difference between useful and wasted energy.

[2]

• Question 1

What is the law of the 'Conservation of Energy'?  Write the answer without any punctuation or it will be marked as incorrect!

Energy cannot be created or destroyed it can only be converted from one form to another
EDDIE SAYS
We told you that you'd have to remember it, right? Well, we're not joking! In an exam, you might be asked to quote this and you may not be given the mark if you get so much as one word wrong. It really is that important! Make sure you commit this to memory.
• Question 2

Identify the wasted energy in this system.

A car engine.

Heat
Sound
EDDIE SAYS
There can be more than one form of wasted energy. You need to think about what you want a car engine to achieve and what energies are not helping it to achieve that. Anything that is not helping your engine to turn is wasted energy! If in doubt, always put heat... heat is nearly always wasted in every system. In this case, sound is also wasted energy. The noise an engine makes is not helping the engine to turn, even if some people do like their engines to have a throaty roar!
• Question 3

Calculate the efficiency of the oven in this question.

Energy in per second =  250 J

Heat energy =  228 J

Light energy = 7 J

Write the answer as a decimal.

0.94
EDDIE SAYS
Okay, so in this question, you need to find the useful energy in and out. Energy in = 250 J Useful energy given out = 228 J and 7 J = 235 J That's right, there are two sets of useful energy here so you need to do the calculation with both of them. Add them together. Next, remember the formula: Efficiency = useful energy output ÷ total energy input Then put the number into the calculation: Efficiency = 235 ÷ 250 Efficiency = 0.94 If you get a number that is above 1, then remember this cannot be correct, as no system is 100% efficient. Try flipping the numbers around.
• Question 4

A power plant wastes a huge amount of energy, mainly as heat and sound.  (Modern power stations are much more efficient than they used to be, but they still waste energy!) In a single second, a power station can produce 2,000 MJ. If 1,050 MJ of energy is wasted as heat and sound, calculate the efficiency of the power station.

Write the answer as a decimal.

0.475
EDDIE SAYS
Did you see that it was talking about mega joules, not just joules? 1 MJ is 1,000,000 J (mega is scientist-speak for million). However, because everything was given to you in MJ, you didn't actually need to convert anything into joules. How do you find the total energy in and the useful energy? Total energy in = 2000 MJ Useful energy = 2000 - 1050 (the wasted energy) = 950 MJ Then put them into the equation and work out your answer: Efficiency = useful energy output ÷ total energy input 950 ÷ 2000 = 0.475 In real life, power stations are actually more efficient than this, but not by much!
• Question 5

A mobile phone gets hot when it is being used. It converts 100 J of energy every second into light, sound and heat (as well as Netflix).

What is the percentage efficiency of the mobile phone if 50 J of energy is emitted as light, 25 J as sound and another 25 J as heat?

75%
EDDIE SAYS
Did you try to work it out? Or did you realise that you didn't have to? In these types of questions where they give you a multiple choice for your answer, they don't always expect you to work anything out, but give a good guess based on the information you have been given. In this case, percentage means out of 100, and we have 100 J of energy going in. This means that if 1 J of energy is wasted, that would be the same as 1%. So if 25 J of energy is wasted, that means that 25% is wasted, right? So the efficiency is therefore 100 - 25 = 75%. Awesome! Everyone loves maths in their science exams!
• Question 6

Describe the useful and wasted energy in an oven.

Explain why it is useful or wasted.

[3]

EDDIE SAYS
You need to explain why each of the energy stores can be considered as useful or wasted in this answer, not just describe them. You know this because in the question it uses the word 'explain', which means you need to use science in your answers. Useful energies include heat, light and kinetic. Wasted include heat (again) and sound.
• Question 7

An electric motor is being developed for BMW cars. They want to reduce the rolling resistance of the motor as much as possible.  Rolling resistance is the resistance from friction between the parts of the motor. This will increase the efficiency of the motor.

Name one way they could reduce this energy loss and what store of energy this would affect.

[1]

EDDIE SAYS
Friction is a big factor in any system that has a moving part. It is the leading cause of energy loss in the UK (and everywhere else). Luckily, we have developed oils to help reduce the effects of friction and they appear to be working! This means there should be no more energy loss through heat produced by the rubbing of two or more materials.
• Question 8

A computer has an efficiency of 80% and input energy of 500 J per second.

Calculate the useful energy in the computer.

400
400 J
400 joules
EDDIE SAYS
Sometimes, you just need to be good at maths to be good at science. You will be expected to do percentages like this in the higher tier papers, so, let's have a look at how we would do it. Firstly, remember the formula and rearrange it so that 'energy output' is the subject of the equation. Useful energy output = efficiency x total energy input so, that is 80% x 500 1% of 500 = 5 5 J is 1%, so what is 80%? 5 x 80 = 400 J 400 J is our answer.
• Question 9

A television has an efficiency of 94% and an input energy of 150 J per second.

Calculate the amount of wasted energy every second.

9
9 J
9 joules
EDDIE SAYS
Again, a lovely problem that wouldn't look out of place in a maths paper! This time, it's looking for the leftovers from the same equation we did last time. Let's have a go together. Useful energy output = efficiency x total energy input This means 94% x 150 Work out 1% - it is 1.5 So, 94% is 1.5 x 94 = 141 J This is the total useful energy output. Now we need to look at the wasted energy. Take this value (of 141) from the total energy input (of 150) to get the wasted energy. 150 - 141 = 9 J 9 is our answer.
• Question 10

Describe the difference between useful and wasted energy.

[2]

EDDIE SAYS
There were various answers that would have been correct here but they needed to include the word 'system' to gain any marks. One set of possible answers would have been: Useful energy is energy that achieves the desired outcome for the system. Wasted energy is energy that is transferred that does not contribute to the aims of the system. How did you get on with this final question? Don't worry if you found it tricky - it will become easier to understand with practice.
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