How To Calculate Energy
Donald Sinclaire defines power and energy and shows the equations for calculating these terms. He also explains two possible units for energy, joules and kilowatt-hours.
Hi, I'm Donald Sinclaire. I'm a science teacher with Greater London Tutors, and today, we are going to be looking at a few topics in chemistry. This is how to do calculations involving energy.
Energy is a difficult concept to define, but one equation which gives a good definition of it to use in a basic term is the idea of work done. Energy, when measured in joules, can be expressed as the force required to move an object, or the force against which you do work, multiplied by the distance over which you move. So for example, if you were pushing a box, which offers a resistive force of twenty newtons, when you push that distance of two meters, then that simply gives you twenty times two equals forty joules.
Another concept which is closely related to energy is power, which is the rate of expenditure or energy or the rate of take up of energy. Power is defined as energy divided by the time over which the energy is used up or given out. So, for example, if we have a 2,000 watt heater and we allow it to run for two hours, to calculate the amount of power it uses up, we would rearrange this equation slightly to give us energy equals power times time and simply plug in the numbers.
Power here is 2,000 and time, which is two hours, we have to convert into seconds if we are going to use joules as our unit of energy. This gives us 2,000 times 7,200 which would give us a total of 14,400,000, in other words fourteen million, four-hundred-thousand joules. Now, when you're talking about numbers this big, this can get a little unwieldy.
The more common unit used to express energy used when you're using an appliance is the kilowatt-hour. This makes it sound a lot easier. Power is expressed in kilowatts, and time is expressed in hours.
So, the power of this heater now becomes two kilowatts (two thousand watts). And the time for which it's on is two hours. Two times two gives us four kilowatt-hours.
This is the unit you'll normally see on an electrical meter inside a house. .