# Physics

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## Energy Stores

All vehicles need fuel to move:

• Steam Trains use coal as fuel
• many cars now use diesel, petrol or even alcohol that has been produced from sugar.
• Some vehicles use chemical energy stored in a battery,

A clock needs energy to make the hands move, and this energy can be stored in a spring that you wind up with a key, in electrical battery, or in weight that are raised up.

If a spring is stretched or compressed, the spring will store elastic energy.

If a load is raised above the ground, it has gravitational potential energy. If the spring is released or the load moves back to the ground, the stored energy is transferred to a store of movement energy, which is called kinetic energy.

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## Energy stores and transfers

Energy stores

eg:

• Petrol in a ca stores chemical energy, which can be released when the fuel combines with oxygen
• A trampoline changes shape when a person lands on it. For a short amount of time a trampoline stores elastic energy.
• A moving object has a store of kinetic energy.
• The forces between particles in the nucleus of an atom are very large indeed. These force fields store nuclear energy which is released in radioactive decay, fission or fusion
• Two separated electric charges that are attracting or repelling, store electrostatic energy

Energy Transfer

eg:

• a flame transfers energy to the surroundings by heating
• a vibrating guitar string transfers energy to the room as sound. Asound wave carries very small amounts of energy from the vibrating string, setting air particles around it into vibrations. These vibrations are passed through the air as a longitudinal wave. When the wave reaches the ear it causes the eardrum to vibrate.

In some cases, this transfer can be done between just 2 stores, such as between kinetic energy and gravitational energy in a pendulum. In other cases, energy is transferred to more than one type of store.

eg: if you brake when you are riding your bicycle, work done by friction transfers energy to the brake pads, which heat up. A small amount of energy is also transferred to the surroundings by sound - the brakes produce a noise.

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## Energy stores and transfers

Energy:

• Energy can be stored in different ways
• There are certain ways that energy can be carried from one place to another - these are called energy transfers
• law of Conservation of energy
• Energy can neither be created nor destroyed, it can only be transferred from one form to another

Energy Stores:

• Kinetic energy (in a moving object)
• gravitational energy (due to the position of an object in a gravitational field)
• elastic (e.g. in a stretched or compressed spring)
• thermal (in a warm object)
• magnetic ( in 2 separate magnets that are attracted or repelling)
• kinetic (in a moving object)
• nuclear (released through radioactive decay, fission or fusion)

Energy transfers:

• 4 main energy pathways
• mechanically - when a force moves through a distance
• electrically - when a charge moves through a potential difference
• by radiation - because of a temperature difference
• by radiation - e.g. light, microwaves, sound
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## Efficiency and sanky diagrams

Energy in = energy out

only part of the energy will be transferred usefully to the type you want, the rest is wasted.

e.g.

Input energy 100

useful energy 20

wasted energy 80

100 - 20 = 80

efficiency tells us how much useful energy we get out compared to how much wasted energy we get out. The greater the amount of energy converted to the type of energy you want the more efficient it is.

Efficency = useful energy output / energy input 100 %

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## Sanky Diagrams 5 of 10

## Work Done and Energy Transfers

Work is being done when a force moves along the line of action of the force.When work is done energy is transferred

Work done = force x distance moved

force = 90N

Distance = 2m

W= F x S

W= 90 x 2

= 180 6 of 10

## Power

Power = energy / time

time = energy / power

energy = power x time

power: watts (w)

time: seconds (s)

energy: joules (j)

e

p  /   t

• Power is the rate at which an object transfers energy (the amount of energy transferred per second)
• The more powerful an appliance the faster it transfers energy
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## KE + GPE

KE stands for kinetic energy

n  Kinetic energy is the energy of motion

n  Any object in motion is using kinetic energy

KE = 0.5 mass x (speed)2

GPE stands for gravitational potential energy

n  Is a measure of how far an object can fall

GPE = mass x gravity x height = m x g x h

GPE = weight x height

GPE: J

Mass: kg

Gravitational field strength: N/KG

Height: m

M = GPE / G X H

GPE = m x g x h

H = GPE / g x m

G = GPE / m x h

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## CCR

When a substance is heated, its particles gain energy and vibrate more vigorously. The particles bump into nearby particles and make them vibrate more. This passes the thermal energy through the substance by conduction, from the hot end to cold end.

Conductions occur better in a solid because the particles are closer together, therefore energy can be transferred through it.

Convection happens when particles with a lot of thermal energy in a liquid or gas move and take the place of particles with less thermal energy.

Transfer in conduction

n  In conduction, vibration are passed between particles

n  Metals are good thermal conductors and make materials such as wood are poor thermal conductors (a good insulator)

Transfer of energy- convection

n  In convection, part of a fluid (or gas) that is warmer than the rest sets up a convection current

n  Hot air rises and cold air sinks

The transfer of energy – radiation

n  Radiation is the only way in which energy can be transferred through a vacuum

n  Infrared radiation can also pass through gases and some solid materials

n  Infrared radiation is absorbed and emitted easily by dull, dark, surfaces, and is absorbed and emitted poorly by light shiny surfaces

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## Home insulation

A poorly insulated house loses more energy and costs more heat. It also means that more pollution, particularly carbon dioxide, is created in order to heat a house.

There are three ways that thermal energy can be transferred from an object with a store of thermal energy to either the thermal energy store of the surroundings or the thermal energy store of another object. They are called convection, conduction, and radiation. Convection and conduction both use particles to transfer energy, so they do not work in a vacuum. Radiation does work in a vacuum. Convection, conduction and radiation should not be confused with the eight types of energy store:

1)  Nuclear

2)  Magnetic

3) Elastic

4) Thermal

5) Electrostatic

6) Electrical

7)Kinetic

8)Gravitational

however, radiation is one of the four types of energy transfers. Remember the other three energy transfers are electrical, mechanical and heating

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