SITEMAP   School Physics Notes: Energy transfers Section 0.3 Energy store conversions

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Energy: Part 0.3 Examples of types of energy store conversions in systems and closed systems

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0.3 Examples of types of energy store conversions in systems

Energy can be transferred between energy stores in four principal ways

1. By heating - the transfer of thermal energy from a hotter material/object to a cooler material/object.

You can only have a net heat flower from a higher temperature to a lower temperature region.

2. By radiation of a wave - sound wave vibrations transfer energy and electromagnetic radiation.

There seven types of EM radiation including microwaves, infrared and visible light.

Infrared radiation is referred to as thermal radiation - its a means of net transfer of thermal energy from a higher to a lower temperature material.

3. Flow of electrically - energy is transferred by electrical charge (electrons) moving around an electrical circuit down a potential difference i.e. moving from a higher to a lower potential energy, releasing energy in the process e.g. in the form of heat or light.

See also Usefulness of electricity and energy transfer for lots of examples starting with electricity

4. Mechanically - anything  that is moved by a force acting on it involves a mechanical transfer of energy - anything rotating, pushed, squashed or pulled etc.

Reminder of the law of conservation of energy

No matter the nature of an energy store or an energy store transfer, the following law applies ...

Energy can be (i )stored, (ii) changed from one form to another and (iii) dissipated, but the total energy of a closed system is constant and you cannot create or destroy energy

There is no net change in total energy, no matter what energy transfers take place.

Dissipated usually means wasted energy like heat spreading out to increase the thermal energy store of the surroundings.

In the 19th century scientist began to realise that mechanical work generated heat, and so concluded that there was a connection between them e.g. heat is generated by friction when a mechanical device does work.

Work done in joules = force in newtons x distance through which force acts,

which means the same amount of work transfers the same amount of energy, but not all usefully.

Electrical heating elements transfer energy from an electrical energy store to the thermal energy store of the material being heated e.g. oil in a heater, water in a kettle.

Moving objects have kinetic energy. If you want to slow a moving object down you must reduce its kinetic energy store e.g. car brakes by operating a force. In doing so work must be done and heat from friction is released to increase the thermal energy store of the surroundings.

Whenever you get an increase in temperature of a system, energy must be transferred from one energy store to another.

A system is the physical components involved with the energy transfer conversion. e.g. your hand and winding up a clock, a car engine, your muscles being used to lift a weight, a photocell, an electrical appliance, objects colliding.

A closed system is a collection of objects you treat on their own and energy can be transferred between different energy stores of the system, but not between energy stores outside of the system - in other words no energy leaves or enters the defined closed system.

e.g. a hot drink standing on the table - the system includes the surroundings too, because thermal energy is readily transferred to the surrounding air - its an 'open' system..

However, if the hot drink is in a well insulated thermos flask, then this can be considered a closed system.

When a system changes it involves an energy transfer. Energy might be transferred into or out of a system between the different components of a system or between different types of energy stores.

You can transfer energy mechanically when a force is applied to do work e.g. winding up a clock, dragging a heavy box across the floor.

You can transfer energy electrically when work is done by moving charge e.g. an electrical heater, electric motor.

When you raise the temperature of a material (e.g. from a flame, electrical supply, electromagnetic radiation) you increase the thermal energy store of that material.

When your vocal chords vibrate you are transferring kinetic energy into sound energy

Know and understand that energy can be transferred usefully from one form to another, or stored, or dissipated, but energy cannot be created or destroyed.

This is the law of conservation of energy.

Another way of expressing this law to say energy is never lost but transferred between different energy stores often involving different objects or materials.

However, energy is only useful if it can be converted from one form to another.

Be careful in using the term energy loss!

The phrase 'energy loss' is used in the context of energy transfer when not all the energy is transferred into a useful form e.g. some energy from a car fuel is 'lost' in the friction of moving parts, i.e. some chemical energy ends up as heat or sound rather than kinetic energy to move the car.

Examples - from a suitable energy source ==> useful form of energy (plus waste in most cases)

Energy is usually transferred by radiation, electricity flow, heating or doing work in a mechanical sense.

When a gun fires chemical energy is converted into heat energy, sound energy, light energy and mainly kinetic energy. When the bullet embeds itself into some material the kinetic energy of movement is converted into some sound energy, but mainly heat energy. Some of the energy is wasted but most of the chemical energy stored in cartridge is converted to the useful kinetic energy of the bullet.

Photovoltaic solar panels convert light energy into electrical energy. If this electrical energy is used to charge up a battery then you increase chemical energy store of the battery.

We use a large number of electrical devices in the home eg

TV converts electrical energy into useful light and sound, but some waste heat

A charged mobile phone battery converts chemical energy into electrical energy, which in turn is converted into useful light and sound energy.

In the charging process you are increasing the energy store of the mobile phone battery.

Wind turbines convert kinetic energy into electrical energy

The kinetic energy store of the wind is decreased, whereas that of the turbine blades is increased, initially an energy transfer involving the same type of energy store.

The start of making a cup of tea is a system. You use an electrical heating element in the kettle base to boil water by transferring energy to water via the conversion of electrical energy to thermal energy. The water increases in temperature and therefore its thermal energy store is increased. The water and heating element constitute a system.

Whenever an electrical current flows in a circuit, work is done against the resistance of the wire. This is all to do with the behaviour of the electrons moving due to an electrical potential difference.

It doesn't seem the same as say, pushing a lever to operate the machine, but you are applying a force to move the lever against a mechanical resistance. In fact the work done or energy transferred is equal to the force applied x the distance through which the force operates (see calculations further down the page).

When the car of a 'big dipper' fun ride is raised to the top of a loop using an electric motor you are converting electrical energy mechanically into the car's kinetic energy store.

As the car increases in height its gravitational potential energy store (GPE) increases.

When the car rolls down the other side its GPE store decreases and its kinetic energy store increases.

Keywords, phrases and learning objectives on energy

Be able to describe, analyse and explain examples types of energy store conversions involving closed systems, heating by thermal energy transfer, thermal energy radiation transfer,  electrical current energy transfer and mechanical work done energy transfer.

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