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# Energy Transfers

## Conservation of Energy

The principle of the conservation of energy says that energy cannot be created or destroyed. It can only be transformed from one store of energy into another. Energy can be transformed by:

• Heating: Heating an object transfers energy to the object's internal store of energy (the sum of the energy in the kinetic and chemical stores of its particles).
• Waves: Light and other types of waves can transfer energy from one object to another.
• **Mechanical work: **Energy transferred from one object to another via a force is an example of mechanical work. When an object falls from a building, the weight does mechanical work to transfer energy from the gravitational potential store to the kinetic store. When you push a wheelchair, energy is transferred to the kinetic store of the wheelchair.
• **Electrical work: **An electrical current transfers energy from one object to another, such as a battery powering a torch.

## Examples of Energy Conversion

Energy is never created or destroyed, only converted from one store to another. Here are some examples of this process:

• An object accelerated by a constant force: Work is done by a force on an object. This work is converted to the object's kinetic store.
• An object projected upwards: The object's energy is initially in the kinetic store as it moves upwards. The object's energy is slowly transferred from the kinetic store to the gravitational potential store as it slows down and climbs higher. Once the object reaches its highest point, all of its energy is in the gravitational potential store. As the object falls again, energy transfers from the gravitational potential store to the kinetic store.
• A vehicle slowing down: To begin with, the vehicle's energy is in the kinetic store. The brakes do work slowing the car down. During this process, energy is dissipated (lost) through heat and sound.
• A moving object hitting an obstacle: The object's energy is in the kinetic store at the start because it is moving. When the object collides with the obstacle, energy is converted to: The kinetic store of the obstacle (making it move), The thermal store of the object and the obstacle (the particles in the object and the obstacle vibrate more). Some energy remains in the object's kinetic store as it moves away after the collision.
• Bringing water to a boil in an electric kettle: Energy transfers from the electrical store of the mains power supply to the thermal store of the water.