Energy is neither created nor destroyed. It does not disappear when we use it – it changes from one form of energy to another.
Let’s look at energy in two situations.
A pendulum as energy transfer
A pendulum is a simple example of energy transfer. Beginning at position A, the pendulum fob is not moving. It has some energy because of its height (h) – called gravitational potential energy. When it is allowed to swing, that energy is gradually converted to energy of motion – kinetic energy. When the pendulum is at position B, all of its potential energy has been converted to kinetic energy and it is moving at its maximum speed. When the pendulum reaches position C, it has regained half of its potential energy and lost half of its kinetic energy. It continues trading speed for height (kinetic to potential) until it reaches position D, at which point it not moving and has regained almost all of its original potential energy.
The ‘almost’ part is important. If the pendulum could regain all of its original height it would swing forever, but there are small energy losses along the way. If we measure very carefully, we would find that the bending of the string where it is tied at the top would have become slightly warmer because of the fibres of the string rubbing together. Also, because of its speed in swinging through the air, the air would be moving, and some of the kinetic energy of the pendulum would have been transferred to the air. These small energy losses will eventually stop the pendulum, and it will come to rest at position B.
A peanut as energy transfer
A peanut is a much more complex example of energy transfer. If we look at how a peanut grows, a peanut plant begins life as a planted peanut. When soil conditions are right, the tiny embryo (nub at the end of a peanut) begins to grow using the stored energy in the rest of the seed to begin plant development. The plant eventually tunnels its way up and out of the soil into the light and forms leaves so that it can begin to collect energy from the Sun and absorb nutrients from the soil. The plant eventually matures, blooms and produces more peanuts. The energy from the Sun is translated into chemical energy and stored inside the seeds for the next generation of peanuts.
A peanut could also transfer its energy to a person who eats it. How much energy could we get from one peanut? The average peanut weighs about 2.5 grams and contains about 60 kilojoules (14 calories) of energy. How much energy is that? If the body was 100% efficient, it would be enough energy for a 65 kg person to climb to the top of a 25-storey building! It would also be enough energy (if you burned the peanut) to raise the temperature of 1 litre of water by 14°C. The peanut, if converted into electrical energy, would power a smartphone (screen on) for about 3 hours.
Whenever energy is converted from one form to another, it loses some energy along the way. When we eat a peanut, our body only converts about 25% of the energy in the peanut into usable energy for our body – the rest is lost along the way. If we tried to warm up water by burning a peanut, we would find that a portion of the energy would be wasted warming the air and the container. The energy content of food is found by carefully burning the food item in a device known as a calorimeter and measuring the amount of heat generated. Our automobiles are even less efficient than our bodies at converting energy. The average car is able to convert only about 20% of the energy in the fuel into useful motion.
When we switch on a light, we are converting electrical energy into light energy. Old-style incandescent light bulbs convert about 10% of the electrical energy into usable light energy, and the other 90–95% is lost to heat. Compact fluorescent bulbs are about twice as efficient, and the newer technology of LED bulbs are much more energy efficient, converting up to 40% of the electrical energy into visible light.