Energy — It Changes Form, but the Total Stays the Same

1.Kinetic energy and potential energy

Energy is the ability to do work. Its unit is the J (joule). The energy a moving object has is called kinetic energy, and the energy an object has because it is up high is called (gravitational) potential energy.

Kinetic energy is proportional to the square of speed. So when speed doubles, kinetic energy quadruples. That is why the faster a car goes, the more sharply its braking distance grows and the worse the damage in a crash. Potential energy is proportional to mass and height.

2.Conservation and conversion of energy

Energy is neither created nor destroyed; it only changes from one form to another, and the total amount stays constant. This is the law of conservation of energy. No phenomenon that violates this law has ever been found.

A roller coaster has the most potential energy at the top, and as it comes down, that energy turns into kinetic energy. Without air resistance and friction, the sum of the two (mechanical energy) would stay constant. In reality, some of it turns into heat and sound, which is why a roller coaster's second hill is built lower than the first.

Every device in daily life is an energy converter. A microwave oven turns electrical energy into electromagnetic waves, which shake the water molecules in food and produce heat. When you charge a phone, electrical energy is stored as chemical energy in the battery, and when you use the phone, it turns back into electrical energy and becomes light and sound. People move their muscles with the chemical energy in food.

3.Work and power

In science, work means applying a force to move an object in the direction of that force. The amount of work is found by multiplying the force by the distance moved, and its unit is the J, the same as for energy. If you stand still holding a heavy bag, your arms get tired, but since the bag didn't move, the work done in the scientific sense is 0.

Power is the amount of work done in 1 second — in other words, how fast work is done. Its unit is the W (watt), and 1 W means doing 1 J of work per second. Whether you walk or run up the same stairs, you do the same work, but running takes less time, so your power is greater.

4.Energy use in kWh — the unit on your electric bill

The power rating (W) printed on an appliance is the same idea as power: it tells you how much electrical energy the appliance uses per second. The amount of energy actually used is found by multiplying the power rating by the time it runs.

The J is too small a unit for everyday use, so electricity is billed in kWh (kilowatt-hours). 1 kWh is the energy used by running 1000 W for 1 hour: 1000 W × 3600 s = 3,600,000 J. The usage on your bill is exactly this kWh figure.

5.Heat and efficiency

Heat is energy that moves from a hotter place to a colder one. Temperature is a measure of how vigorously the particles that make up a substance are moving, while heat is the amount of energy that flows because of a temperature difference — so the two are different concepts.

When energy is converted, some of it almost always scatters as heat, and scattered heat is hard to gather up and use again. So the fraction of the energy put in that comes out in the form you want is called efficiency. An incandescent bulb gives off most of its electricity as heat and turns only a little into light, while an LED light produces the same brightness with far less power. When you charge a phone and the charger and phone get warm, that is lost energy coming out as heat.

Refrigerators and air conditioners are devices that move heat from a cold place to a hot one. Heat doesn't flow that way on its own, so they spend electrical energy to circulate a refrigerant. Warm air blows out from the back of a refrigerator or from an outdoor unit because the heat removed from inside comes out together with the energy the machine used.