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  Energy and Power

Everybody is familiar with the concept of doing work. Whether it is manual labour or an industrial machine. Less familiar perhaps is the concept of energy as the thing that makes work possible. But that is probably because it hasn't been explained very well.
 
Anybody doing physical work soon realises that you need food to keep working and drivers realise that they need to put fuel in their cars to keep them moving. Food and fuel are sources of chemical energy that animals and cars can use to do useful work. But what happens to the energy when the work is done? Does It disappear into thin air?
 
For the answer we must look to something called thermodynamics. This may sound very complicated and only useful to scientists, but it has some very simple principles that are very useful when you a trying to decide how something works.Thermodynamics has a number of laws. These laws are the result of a lot of careful measurements and thinking about the results.

The most important two laws for most practical purposes are ;
 
1. Energy cannot be created or destroyed. It can only be converted from one form of energy (like heat) , into another form of energy (like motion)
2. When energy is converted it is never a perfect conversion. Some part of the energy will be changed into another less useful form of energy such as heat.
 
The most important effect of the second law is that it makes all energy conversion processes irreversible because you can never get back the bit of energy that is 'lost'.

Everyday experience shows us that when we do physical work we get hot because some of the chemical energy in our food gets turned into heat instead of muscle power. In the same way our car engine gets hot when we drive the car. All that heat is wasted and just warms up the air around us without doing anything useful. So useful work happens when energy is converted from one form into another.

To actually calculate what this means in the real world we need to define some units of measurement for all these things. Energy is about a capacity to do useful work. We instinctively realise this but probably don't think about it too much. We know that the energy in a gallon (4.5 litres) of petrol will let our car drive 30 miles. We know that the chemical energy in a phone battery will power our phone for a day. But how is energy measured? How many phone batteries equals one gallon of petrol? Or put another way, how many phone batteries would we need to move our car 30 miles?
 
The answer is that energy is measured in Joules. A gallon of petrol contains the equivalent of 250 million Joules of energy. A phone battery contains up to  100,000 Joules of energy. So you would need the equivalent of 2,500 phone batteries to move a car 30 miles. If you imagine a pile of 2,500 phone batteries compared to a gallon of petrol (about the size of a football) you can start to understand why it is so difficult to make an electric car with a range that even gets close to the range of a car powered by petrol.
 
So what is a Joule? To define a Joule in everyday terms we need to define some other units. We can see from the comments above that energy can be used to do work. Reversing this we can define energy in terms of doing work. So lifting up a weight can be a definition of energy if the weight is defined and the distance it is lifted is defined as well. At this point we need to introduce another unit that may not be familiar to you and that is the Newton. The Newton is a unit of force or weight in the international system of weights and measures. A kilogram weighs about 9.81 Newtons and the strange ratio is because that is the effect of gravity at the surface of the earth. Click this link for a more detailed explanation - Definition of a Kilogram.
 
The important point for us is that a Joule is defined as the energy needed to move a force of 1 Newton through a distance of 1 metre. A small apple weighs about 1 Newton (100 grams force) and if we lift it to a height of 1 metre from the floor it will take 1 Joule of energy to do that. If we reverse the process and let the apple fall then it starts with a potential energy of 1 Joule. When it falls that energy is used up in overcoming the air resistance and when it hits the floor the remainder of the 1 Joule of energy will be used to deform (squash) the apple and make a noise. Click on this link for a more detailed explanation of a Joule - Definition of a Joule
 
A common suggestion for getting useful power is to use big weights such as old boats that are lifted up by the tide. When they fall again we can recover the energy by driving a generator. We explain how much energy you can extract and you can read this by clicking here.
 

We also put energy into things when we accelerate them and this leads to another definition of the Joule. A Joule is the amount of energy needed to accelerate a 1 kilogram mass to a speed of 1 metre per second in one second. The kilogram mass now has kinetic energy or the energy of motion.

The kinetic energy in Joules is the mass multiplied by the square of the speed divided by two. So a mass of 1 kilogram going at 5 metres per second has a kinetic energy of 1 x 5 x 5 / 2 or 12.5 Joules.This relationship is important when considering things like wind turbines because we need to know how much energy there is in the wind.
 
It so happens that the average wind speed in the UK is about 5 metres per second. It also happens that a cubic metre ( a box 1 metre high, 1 metre wide and 1 metre long) contains a mass of just over 1 kilogram of air. So applying our kinetic energy equation our cubic metre of air moving at an average wind speed has a kinetic energy of about 12.5 Joules.We now need to see how useful that is.
 
We need to know how much power we can generate from the 12.5 Joules of energy. Power is defined as the rate of transforming energy, so if we convert one Joule in one second it corresponds to a power of 1 Watt. The watt is something that everyone has heard of for things like an incandescent light bulb. We know that a 100 Watt light bulb is very bright and a 40 Watt bulb is much dimmer.

Our cubic metre of air contains 12.5 Joules of energy which means that in one second the air could generate 12.5 Watts of power. Not enough to light a small light bulb. A family car engine generates about 100,000 watts of power at full power. The phone battery mentioned above generates about 4 watts of power so we would need 25,000 phone batteries to produce the same amount of power. Electric vehicles are possible because for most of the time a car is only using a fraction of its power capacity.
 
A small wind generator that passed 1 cubic metre of air per second would be about 1 metre in diameter. So the type of wind generator that you see on house roofs is probably only generating enough power to light a small light bulb. This is why wind turbines have to be very big to be of any use. To find out more about wind turbines click here.
 
So for any machine we need to know how much power is it using in watts. For each second that it is working we know that it is using the same number of Joules of energy and therefore there must be at least that number of Joules of energy in the input fuel. Because of the second law of thermodynamics we know that not all the energy in the fuel (or other energy source, like the wind) is converted into useful work and the ratio of the output energy divided by the input energy is the efficiency of the machine.
 
Most energy conversion processes are quite inefficient. For example in an internal combustion engine only about one third of the energy in the fuel gets turned into useful work. One third is lost as heat in the cooling system and one third is lost in the hot gases in the exhaust.
 
This has been a long explanation but we hope that it has been useful in understanding some of the very fundamental issues about the way things work. If you would like to find out more about thermodynamics you can read more by clicking here.
 
 Definition of a Joule >>>Definition of a Kilogram >>>