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Application of the Conservation of Energy

Worksheet Overview

QUESTION 1 of 10

A law in science is something that cannot be broken.  It will apply no matter where in the universe you are! 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. 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 sport the energy transfers that are taking place.

 


Image of a lightbulb

Look at this light bulb.

So 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 lightbulb to get hot, but they do 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 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 impossible – it is another one of those laws.

 

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

 

efficiency equation

 

So, let’s try an example:

 

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

 

Step 1 – highlight all of the numbers in the question.

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

 

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, always put the percentage sign after.

 

Finally, we need to discuss how to manage efficiency within a system. This will vary 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 energy loss from the heating effect of friction. You will now go further with your nicely oiled wheels - brilliant!

What is the law of the 'Conservation of Energy'?  (There is no need for punctuation)

Identify the wasted energy in this system. 

 

A car engine. 

Light

Heat

Kinetic

Sound

Gravitational

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.  (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 can you calculate the efficiency of the power station? 

 

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? 

 

Smart Phone

0%

25%

50%

75%

Describe the useful and wasted energy in an oven.  Explain why it is useful or wasted (3 marks). 

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 will effect. 

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 wasted energy every second. 

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

  • Question 1

What is the law of the 'Conservation of Energy'?  (There is no need for punctuation)

CORRECT ANSWER
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 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. 

CORRECT ANSWER
Light
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.
  • Question 3

Calculate the efficiency of the oven.

Energy in per second - 250 J 

Heat energy - 228 J

Light energy - 7 J

CORRECT ANSWER
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. If not, why don't you go back and take a look? Energy in = 250 J Useful energy = 228 J AND 7 J = 235 J That's right, there are two 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 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 can you calculate the efficiency of the power station? 

 

CORRECT ANSWER
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 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, 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? 

 

Smart Phone

CORRECT ANSWER
75%
EDDIE SAYS
Did you try and sit down to work it out? Or did you realize 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, 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? 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 marks). 

CORRECT ANSWER
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, Kinetic. Wasted include: Heat, 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 will effect. 

CORRECT ANSWER
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! 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. 

CORRECT ANSWER
400
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. 500 = 100% So, let's work out 1% 500/100 = 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 wasted energy every second. 

CORRECT ANSWER
9
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. Work out 1% 150/100 = 1.5 Multiply by the percentage to go 94% of the energy. 1.5 x 94 = 141 J This is useful, now we need to look at the wasted. Take this value from the total to get the wasted. 150 - 141 = 9 J 9 is our answer.
  • Question 10

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

CORRECT ANSWER
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.
Try it ---- OR ----

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