Physics: Energy

In physics, energy is the capacity to do work. It's what makes things happen – from a ball rolling down a hill to the Sun shining. The standard unit for energy is the Joule (J). The single most important rule in this topic is the Principle of Conservation of Energy. This principle states that energy cannot be created or destroyed, only transferred from one form to another. For example, when you switch on a lamp, electrical energy is not 'used up'; it is transferred into light energy and thermal energy (heat). The total amount of energy before and after the transfer remains exactly the same.

To make sense of energy transfers, we think of energy as being held in different 'stores'. When something happens, energy is transferred from one store to another along a 'pathway'. There are 8 main stores you need to know:

1. Kinetic: The energy of a moving object.
2. Gravitational Potential (GPE): Energy stored by an object due to its height in a gravitational field.
3. Chemical: Energy stored in the bonds between atoms, found in food, fuels, and batteries.
4. Elastic Potential: Energy stored when an object is stretched or compressed, like in a spring or rubber band.
5. Thermal: The internal energy of an object due to the motion of its particles – what we feel as 'heat'.
6. Nuclear: Energy stored in the nucleus of an atom.
7. Magnetic: Energy stored in the magnetic field between magnets.
8. Electrostatic: Energy stored between charged objects.

Energy moves from one store to another via one of four pathways:

• Mechanical Working: A force moving an object (e.g., pushing a box).
• Electrical Working: Charges moving due to a potential difference (e.g., a current in a circuit).
• Heating: Energy transferred due to a temperature difference.
• Radiation: Energy transferred as waves (e.g., light from the sun or sound from a speaker).

Kinetic energy is the energy of motion. Any object that is moving has kinetic energy. The amount of kinetic energy an object has depends on two things: its mass(m) and its velocity (v). A heavier object or a faster object will have more kinetic energy. The relationship is described by the formula: Kinetic Energy = ½ × mass × velocity². Notice that the velocity is squared, which means that doubling the velocity of an object quadruples its kinetic energy. This is why high-speed crashes are so much more destructive.

Gravitational Potential Energy (GPE) is the energy an object stores because of its position in a gravitational field. When you lift an object, you do work against gravity, and this work is stored as GPE. If you let go, this stored energy is converted into kinetic energy as the object falls. The amount of GPE an object has depends on its mass (m), the strength of the gravitational field it is in (g), and its vertical height(h) above a reference point. On Earth, g is approximately 9.8 N/kg (or often rounded to 10 N/kg in exams).

In physics, 'work' has a very specific meaning. Work is done whenever a force causes an object to move. The amount of work done is a measure of the energy that has been transferred. For example, when you push a box across the floor, you are doing work on the box, and you are transferring energy to it (as kinetic energy and thermal energy due to friction). The amount of work done is calculated by multiplying the force applied by the distance the object moves in the direction of the force. Since work is a measure of energy transfer, it is also measured in Joules (J).

Power is the rate at which energy is transferred, or the rate at which work is done. It's not just about how much energy is used, but how quickly it is used. For example, two kettles might use the same total energy to boil water, but the more powerful kettle will do it faster. The unit of power is the Watt (W), which is defined as one Joule of energy transferred per second (1 W = 1 J/s). A 100 W light bulb transfers 100 Joules of energy every second.

When energy is transferred, some of it is transferred to a useful energy store, but some is always transferred to less useful stores. This 'wasted' energy is said to be dissipated, which usually means it is transferred to the surroundings as thermal energy, making them warmer. For example, a light bulb is designed to produce light (useful energy), but it also gets hot (wasted thermal energy). Efficiency is a measure of how good a device is at transferring energy into the useful form. It is calculated as the ratio of the useful energy output to the total energy input. Efficiency can be given as a decimal (e.g., 0.8) or a percentage (e.g., 80%). No device is 100% efficient due to the dissipation of energy.