Core Physics Topic 5 - Energy Transfers and Efficiency

 

Energy Transfers

When we do a job of work, we transfer energy.  Some energy forms can be stored, while others cannot.  Look at the table.

 

Can be stored

Cannot be stored

Chemical

Heat

Gravitational potential

Light

Elastic

Sound

Kinetic

Electricity

Internal

 

 

Electricity cannot be stored as such.  A battery stores energy as chemical energy.  The chemical reaction produces electrons as it proceeds.  When the current is turned off, the reaction stops.  When the chemical reaction is complete, the battery goes flat.

 

The current produced by the chemical reaction can be used to light a bulb or run a motor.  We can trace an energy chain, which you will have done at Key Stage 3.  In this case:

 

chemical electrical kinetic

 

Question 1

Write down an energy chain for the following:

  • A runner going up a hill;

  • A pan on a gas ring;

  • A computer playing a video game.

Answer

 

 

The Law of Conservation of Energy

This law is an important rule of Physics:

 

Energy can neither be created nor destroyed. It is turned from one form to another.

 

What this means is that you cannot get something for nothing.  Nor can you lose it.  Every joule of energy has to be accounted for.  This is what this topic is about.

 

 

Useful Energy and Waste Heat

Think of a light bulb.  For every 100 J of energy contained in the electricity:

 

We can show this as an energy transfer diagram:

 

 

 

In this car, 100 kJ of energy comes from the petrol.  Of this:

 

Of that 40 kJ:

If you add up all the numbers, you will find that they add up to 100 kJ.

 

Of the 35 kJ to drive the wheels, some will be lost in friction.  Less than 35 % of the energy we put into the car actually ends up in moving the car along the road.  However none of the energy has been destroyed.  It has simply been turned into other forms of energy.

 

Question 2

How much energy is wasted?  What fraction is this? What happens to the energy?

Answer

 

So what happens to the energy that a car uses to go along the road?  It is used to:

So most the energy to make the car move is being turned into heat from friction. 

 

The Sankey diagram is a way of showing the way the energy is lost, and how much energy is lost.  It is quantitative, which means that it is drawn to scale.

 

 

The arrow pointing to the right shows the useful energy.  The arrows pointing downwards show where the energy is being lost.

 

Question 3

 Show that all the energy has been accounted for in this diagram.

Answer

 

And when the brakes are applied to slow down, the kinetic (movement) energy is turned into heat.  You can see the brakes of this racing car glowing red hot.

 

Source not known

 

All the of energy from the petrol we use on a journey is eventually turned into heat.  On the way we have diverted a little of that energy to do a useful job for us, namely to go somewhere!

 

 

Waste Heat

Whenever we have any machine to do a job of work, some energy is wasted.  This traction motor from an electric locomotive needs to be cooled by a fan, otherwise it gets hot.  The cold air comes in through the square pipe and leaves through vents at the far end of the motor.

 

Photo by Solaris2006, Wikimedia Commons

 

 

Any energy that is not useful is wasted.  It is possible to harvest some of the waste energy to make it useful, for example:

You can see the huge amount of waste heat (and smuts) coming from this plane as it takes off.

 

Photo US Navy, Wikimedia Commons

 

This waste heat is simply heating up the air. 

 

Low Grade Heat

There are ways of recovering waste heat.  Many devices have heat exchangers which use waste heat going out to warm up air or water going in.  In the end all energy ends up as low grade heat.  The further along the energy process, the lower the grade of heat.  This low grade heat is hard to extract energy from. 

 

Reducing the Waste

One way of reducing wasted energy from brakes is to use regenerative braking

 

This idea has been used with electric trains for many years.  When this locomotive goes down a hill, its motors act as generators and puts current into the wires.  This provides extra current for locomotives going up the hill in the opposite direction.

 

Source not known

 

You can't do that with a petrol or diesel engine, but there are now hybrid vehicles available.  The picture below shows the idea:

 

   

Graphic by Hammer51012, Wikimedia Commons                                                                                                                                          Photo Sudhanwa Dindorkar from Zurich, Wikimedia Commons

 

Question 4

How do you think this system works?

Answer

 

Efficiency

No energy converting process ever gives out as much energy as is put in.  You always have to put in more energy than you get out.   An efficient machine is one that uses as little energy as possible to do a particular job. Little energy is wasted.

 

We can measure the energy efficiency of a device using this simple equation:

 

Efficiency = useful energy output

              total energy input

 

Efficiency can be expressed as a fraction, a decimal, or a percentage.  Often the efficiency is given as a percentage:

 

Efficiency = useful energy output 100 %

total energy input

You cannot ever get more than 100 % efficient.

 

A diesel engine gives out 350 J of kinetic energy for every 1000 J of chemical energy put in. It is 35 % efficient. A petrol engine gives out 300 J of kinetic energy for each 1000 J put in. It is 30 % efficient.

 

The energy efficiency is always a fraction less than 1, which is multiplied by 100 to give a percentage.  (If you get 2/3 correct in your exam, you get 67 %.)

 

 

Examples of efficiency:

 

Device

Efficiency (%)

Steam engine

10

Car

35

Power station

45

Electric motor

70

Transformer

95

 

Question 5

An electric motor takes a power of 3000 W off the mains.  It is 70 % efficient.  What is the power that it can give out?

Answer

 

You never get devices that are 100 % efficient.  If your answer gives 100 % efficiency or more, you have done it wrong!  Watch out for this bear trap.

 

Worked Example

To do 1000 J of work, a motor is found to use 1500 J of electrical energy.  What is its percentage efficiency?

Energy Efficiency = energy got out 100 %

                               energy put in

 

Energy Efficiency = 1000 J 100 %

                             1500 J

 

                          = 0.67 100 % = 67 %

 

 

 

Question 6

An electric motor is 55 % efficient.  It uses 1000 J of electrical energy every second.  How much useful work can it do?  How much energy is lost?  What happens to this energy?

Answer

 

You can see that the efficiency is:

Efficiency = kinetic energy   = 300 J = 0.3 = 30 %

chemical energy    1000 J

 

Question 7

Do the interactive gap-fill exercise

Question

 

We can link efficiency to power as well.

 

Example

A crane uses 5000 W to lift a 200 kg mass 3 metres in 10 seconds. What is its efficiency?

Answer

1. Work out the potential energy the mass gains by being lifted:

Gravitational potential energy (J) = mass (kg) acceleration due to gravity (m/s2) vertical height (m)

= 200 kg 10 m/s2 3 m = 6000 J

2. Work out the power that is used to lift the load:

Power (W) = energy (J) time (s)

= 6000 J 10 s = 600 W

3. Now work out the efficiency:

       Efficiency = 600 W 100 % = 12 %

5000 W

 

No machine is ever 100 % efficient. If it were, it would be a perpetual motion machine. Some toys are sold as "perpetual motion machines" but actually have a small battery to power the machine (usually through a small electromagnet). If the battery runs out, the machine stops.

 

Electrical transformers are among the most efficient machines, about 95 %.

 

Question 8

Clever Trevor invents a bicycle that has a generator in the front wheel that can give out energy at a rate of 100 J every second.  On the back wheel there is a motor that uses energy at a rate of 100 J every second.  Trevor says that as soon as you have pedalled the bike up to speed, you don't have to pedal any more.  The bike will keep on going.  Do you think he's right?

Answer

 

 

Why should we worry about waste heat?

There are currently several pressing issues:

We need to think through ways in which we use energy, especially in transport:

Electric vehicles are quiet and non-polluting.  But we have to generate the electricity somewhere, and that does cause pollution.  We need to think about ways of generating electricity:

We have to think about the efficiency of the energy process.

 

Question 9

Answer the interactive multiple-choice questions on energy efficiency

Questions

 

 

The Wider Picture

It's essential to use less energy, as there are serious problems affecting the environment, such as global warming. Various small things can help:

If everyone did these, large amounts of energy could be saved.

 

We can make our homes use less energy by:

Each of these will cost us money, but we will get the money back in the savings we make. This is the payback time:

 

Payback time = cost of insulation saving made per year.

 

Loft insulation may cost 250 to install, and saves 50 a year on heating bills. The payback time is 250 50 = 5 years.

 

 

When we put energy saving measures into our homes, the argument about payback time is not the only thing we need to consider. Energy is used by the factories that make the products in the first place.

 

Also some draughts are needed, otherwise our homes would have a stale atmosphere and running gas appliances could be dangerous, because carbon monoxide could build up. Carbon monoxide is a highly toxic gas, which has no smell. Exposure to it can be rapidly fatal.

 

Energy saving bulbs might give out more light and less heat, but they are ugly and the light given out is like that in an office or classroom. Additionally they require much more energy to make, they are expensive, and contain toxic materials like heavy metals. Also an ordinary light bulb helps to heat the room, so less heat is needed from the central heating.

 

 

 

 

Summary

  • Energy never created nor destroyed; it is turned from one form to another.

  • When we do useful work, some energy is always wasted as heat.

  • This heat goes to warm up the surroundings.

  • Eventually all the energy ends up as low grade heat.

  • Low grade heat is hard to get further energy from.

  • The fraction of energy that does useful work is the efficiency.

  • We never get 100 % efficiency.