 # Understand the Conservation of Energy

In this worksheet, students will look at the law of conservation of energy and energy transfers. They will also apply these ideas to help them to work out efficiency. Key stage:  KS 4

Difficulty level:   ### QUESTION 1 of 10

A law in science is something that cannot be broken, no matter where you are in the universe it will always be true. We have several laws that you will know quite well, for example, 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, however, is the law of the ‘Conservation of Energy’. You will see this spoken about 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 we first tell people about this, it can blow their mind. They start to think about places where energy is made (like in a power station) and realise that they must change the way that they have been talking about energy. Energy is not made, it 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. Let's take this light bulb.

So what energy is going into it? Electrical.

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

When we compare the amount of energy, we want against the amount of energy we don’t, we come up with a number called efficiency. Efficiency is a simple measurement telling us how much of our energy is going into doing the job that we have designed for it. It can be presented as a number between 0 and 1 (where 0 is none of the energy is going into what we want it to and 1 is all of the energy is going into what we want it to) 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 it's impossible! It's another one of those laws.

Let's look at how you work out efficiency: Here's an example:

A light bulb takes in 100 Joules (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

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

What is the law of the 'Conservation of energy'?  Write it out in full.

Identify the wasted energy in this system.

A television playing a TV programme.

Electrical

Light

Heat

Sound

Chemical

Identify the wasted energy in this system.

A car engine.

Light

Heat

Kinetic

Sound

Gravitational

Name the wasted energy in this system.

An oven.

Calculate the efficiency of the oven.

Energy in per second - 250 J

Heat energy - 228 J

Light energy - 7 J

A power plant wastes a huge amount of energy mainly as heat and sound, but modern power stations are much more efficient than they have ever been in the past. 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.

Is it ever possible to get a system that will achieve an efficiency of 1?

Yes, of course!

No, of course not!

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%

The human body needs a minimum of 1,700 kJ of energy every day to remain alive. With this, it will keep a constant internal temperature of 17.5 oC, if this temperature goes up or down by more than 5 oC then the person can die. In order to maintain this temperature, the human body must sweat and shiver. How is energy wasted in humans?

Kinetic

Electrical

Sound

Light

Heat

Describe the difference between useful and wasted energy (2 marks).

• Question 1

What is the law of the 'Conservation of energy'?  Write it out in full.

Energy cannot be created of 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 we have seen mark schemes that will punish you if you get even a single word wrong. It really is that important!
• Question 2

Identify the wasted energy in this system.

A television playing a TV programme.

Heat
EDDIE SAYS
Remember, the word system? It means a machine (natural or human-made) that converts energy from one form to another. So all you are looking for is the wasted energies in this system. Guess what - 99.9% of the time energy is wasted as heat, so if you get stuck on a question like this, always put heat.
• Question 3

Identify the wasted energy in this system.

A car engine.

Light
Heat
Sound
EDDIE SAYS
So, this one is a little more complicated as there is 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 turn is wasted. If in doubt, always put heat :) (you'll get fed up of reading that by the end of these questions!)
• Question 4

Name the wasted energy in this system.

An oven.

Sound
EDDIE SAYS
Ah - in an oven you WANT to make heat, so you can't put heat as wasted energy here. Think about times you have used an oven, it converted energy into light so you could see into it; heat so that it cooked the food and SOUND so the cake had something nice to listen to whilst it cooked? Sound is wasted, surely!
• Question 5

Calculate the efficiency of the oven.

Energy in per second - 250 J

Heat energy - 228 J

Light energy - 7 J

0.94
EDDIE SAYS
Okay - so in this question, you need to find the useful energy in and out. I hope you read the explanation from the last question as it will help you here :) Energy in = 250 J Useful energy = 228 J AND 7 J = 235 J That's right, there are 2 sets of useful energies here so you need to do the calculation with both of them. Add them together. Then put then number into the calculation: 235/250 = efficiency efficiency = 0.94 If you get a number that is above 1 then you have done the calculation wrong. Try flipping the numbers around.
• Question 6

A power plant wastes a huge amount of energy mainly as heat and sound, but modern power stations are much more efficient than they have ever been in the past. 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.

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). But because everything was given to you in MJ you didn't actually need to convert anything into Joules. So - find the total energy in and the useful energy. Total energy in = 2000 MJ Useful energy = 2000 - 1050 (the wated energy) = 950 MJ Then put them into the equation and find out your answer: 950/2000 = 0.475 In real life, they are actually more efficient than this.
• Question 7

Is it ever possible to get a system that will achieve an efficiency of 1?

No, of course not!
EDDIE SAYS
We will NEVER be able to achieve an efficiency of 1 - if we did, then it would break a law known as the second law of thermodynamics. As you know by now, a law cannot be broken by the universe otherwise the nature of reality will implode in on itself! Okay, maybe not that, but it would mean that all the science we think we know will be wrong, literally, all of it!
• Question 8

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 and sit down 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 expect you to work anything out, but give a good guess based on the information you have been given. In this case, percent 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? Awesome! Everyone loves maths in their science exams!
• Question 9

The human body needs a minimum of 1,700 kJ of energy every day to remain alive. With this, it will keep a constant internal temperature of 17.5 oC, if this temperature goes up or down by more than 5 oC then the person can die. In order to maintain this temperature, the human body must sweat and shiver. How is energy wasted in humans?

Heat
EDDIE SAYS
Humans are just big radiators - we release about 80% of our daily intake of energy as heat. This is to make sure that we keep as close to the 37.5 oC that we need to be. There is some other wasted energy, but it is so small, we call it negligible (that means it is so small, it doesn't really make a difference so we ignore it).
• Question 10

Describe the difference between useful and wasted energy (2 marks).

EDDIE SAYS
Did you say system? it was a test to see if you could use that word in the correct context. Sometimes science is just about using fancy words (especially biology which is basically learning a new language). You need to be able to use them in the right place and at the right time. You needed to say that useful energy is energy that achieves what you want it to in a system and wasted is the opposite in a system. System, system, system! SYSTEM 