Energy Transfers

We speak a lot about energy, but probably don't stop to consider what exactly energy is stop energy, quite simply is what makes everything happen. The principle of or "the law of conservation of" energy states that it can neither be created or destroyed, so if energy is "used" this can only occur by energy being moved around in different systems. We will define the word "system shortly" but suffice it to say that energy can only be "shuffled around" from one place to another in a systems "energy store". When energy is transferred to an object it is therefore stored in one or more of the objects energy stores. An object may have one or more energy stores and the important 8 for you to remember in GCSE physics will follow now in the short table:

Kinetic Energy	Anything that moves has energy in its kinetic energy store.
Thermal Energy	Any object. The hotter it is, the more energy it has in the thermal energy store. These may also be called internal energy stores.
Chemical Energy	Anything that can release energy by a chemical reaction, for example foods or fuels.
Gravitational Potential Energy	Anything that has a mass and his inside a gravitational field.
Elastic Potential Energy	Anything that is stretched or compressed for example springs.
Electrostatic Energy	Anything with electric charge it is interacting with another electric charge for example 2 charges that attract or repel each other.
Magnetic Energy	Anything magnetic that is interacting with another magnet example 2 magnets which are attracting or repelling each other.
Nuclear Energy	Atomic nuclei have energy in this store that can be released in nuclear reactions.

Earlier we spoke about the word "system" and this is just a scientific sounding word for a single object or a group of objects working together. A simple system might be a beaker of water being heated up on a Bunsen burner or 2 objects in collision with each other when we are for example looking into momentum (see later). Generally we consider a system to be "open" or "closed". In a closed system neither matter nor energy can enter or leave so the total net change in the energy of such a system is always zero. In an open system matter and energy can leave but will probably do so by being transferred to the environmental energy stores. For example a beaker of water on the Bunsen burner will lose heat energy to the thermal energy stores of the surrounding, as will the Bunsen burner flame itself. This is clearly an open system where energy has been transferred, or "dissipated" but it cannot be created or destroyed.

If energy is neither created nor destroyed can only be transferred between energy stores, 8 of which are shown in the table above. There are 4 ways in which energy can be transferred between stores.

1. Mechanically, for example by an object moving due to a force acting upon it, pushing pulling stretching or squashing.

2. Electrically, for example a charge (current) moving through a potential difference in other words charges moving around an electrical circuit.

3. Heating, energy transfers from a hotter object to colder object (for example when you wrap your hands around a warm cup of tea or coffee) or when you boil a kettle or heat a pan of water on your gas cooker.

4. Radiation, energy is transferred for example by light/sound waves a perfect example of this is the energy from the sun reaching Earth by light.

When energy is moved around (or transferred) "work" is done.

The "Law of Conservation of Energy" states that energy can be transferred usefully, stored, or dissipated but can never be created or destroyed. We will be putting this statement to good use when we start looking at calculations involving energy transfers.

Let us take a look at the example of a cyclist, rolling downhill. At some point the cyclist will need to slow down, or stop. When the cyclist decides that he or she wants to stop he or she will squeeze the brake levers and this will cause the brake blocks to grip the wheel rims, or for the brake pads to grip the disc of the disc brake if the cycle has one.

Our cyclist will possess a certain amount of kinetic energy based on the combined mass of him/her and the cycle and most of this kinetic energy will be transferred to the thermal energy store of the disc pads/disc brake or brake blocks/wheel rims, in other words the braking system of the cycle. Remember that no energy is lost, the brake discs/pads will get hot and the brakes might squeal, in the first case because of friction which is responsible for the brake disc pads/brake blocks gripping and in the 2nd case the blocks might give out some sound energy (cycle brakes are notoriously noisy, at least the cycles I've always owned have been!). My example of "squealing brakes" is a good way of explaining that not all energy transfers are useful, squealing brakes do nothing to slow you down so this sound energy is useless for the purposes for which the brakes are intended. This is known as "dissipation" of energy and is sometimes regarded as 'wasted' energy.

Q. Describe the main energy transfer between stores including the way in which the energy transfer takes place in the following scenario:

(a) An arrow is released from a bow.

(b) A portable gas fire is used to heat a small room.

(a) Energy is transferred mechanically from the elastic potential energy store of the bow to the kinetic energy store of the arrow.

(b) Energy is transferred by heating from the chemical energy store of the gas in the fire to the thermal energy stores of the surroundings.

(c) Energy is transferred electrically from the chemical energy stored of the battery inside the fan to the kinetic energy store of the motor and the blades rotating on the fan.