Physics: Energy

System

An object or group of objects

What happens if there is a change in a system?

When a system is in equilibrium, nothing changes and so nothing happens; when there is a change in a system, things happen, and when things happen energy is transferred

Units of energy

Joules (J)

Open system

Allows the exchange of energy and matter to or from its surroundings

Closed system

Can exchange energy but not matter to or from its surroundings

Isolated system

Does not allow the transfer of matter or energy to or from its surroundings

Energy stores

Energy is stored in objects, when a change happens within a system, energy is transferred between stores; there are different types of energy stores

Law of conservation of energy

Energy cannot be created or destroyed, it can only be transferred from one store to another

Kinetic energy store

Moving objects have energy in their kinetic store

Gravitational potential energy store

Objects gain energy in their gravitational potential store when they are lifted through a gravitational field

Elastic potential energy store

Objects have energy in their elastic potential store if they are stretched, squashed or bent

Magnetic energy store

Magnetic materials interacting with each other have energy in their magnetic store

Electrostatic energy store

Objects with charge (like electrons and protons) interacting with one another have energy in their electrostatic store

Chemical energy store

Chemical reactions transfer energy into or away from a substance's chemical store

Nuclear energy store

Atomic nuclei release energy from their nuclear store during nuclear reactions

Thermal energy store

All objects have energy in their thermal store, the hotter the object, the more energy it has in this store

Types of energy transfer

Mechanically, electrically, by heating, by radiation

Mechanical working

When a force acts on an object (e.g. pulling, pushing, stretching, squashing)

Electrical working

A charge moving through a potential difference (e.g. current)

Heating

Energy is transferred from a hotter object to a colder object

Radiation

Energy transferred by electromagnetic waves

Energy in kinetic energy store

The amount of energy an object has as a result of its mass and speed

What happens to an objects kinetic energy if it speeds up?

If an object speeds up, energy is transferred to its kinetic store

What happens to an objects kinetic energy if it slows down?

If an object slows down, energy is transferred away from its kinetic store

Kinetic energy formula

Eₖ = ½ × m × v²

Eₖ = kinetic energy in joules (J)
m = mass of the object in kilograms (kg)
v = speed of the object in metres per second (m/s)

Energy in gravitational potential energy store

The energy an object has due to its height in a gravitational field

What happens to an objects gravitational potential energy if it is lifted up?

If an object is lifted up, energy is transferred to its gravitational potential store

What happens to an objects gravitational potential energy if it falls?

If an object falls, energy will be transferred away from its gravitational potential store

Gravitational potential energy formula

Eₚ = m x g x h

Eₚ = gravitational potential energy, in joules (J)
m = mass, in kilograms (kg)
g = gravitational field strength in newtons per kilogram (N/kg)
h = height in metres (m)

Gravitational field strength on Earth

The gravitational field strength (g) on the Earth is approximately 9.8 N/kg

Compare the gravitational field strength of the Moon and the Earth

The gravitational field strength on the surface of the Moon is less than on the Earth; this means it would be easier to lift a mass on the Moon than on the Earth

Compare the gravitational field strength of the gas giants like Jupiter and Saturn and the Earth

The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn) is more than on the Earth; this means it would be harder to lift a mass on the gas giants than on the Earth

Energy in elastic potential energy store

The energy stored in an elastic object when work is done on the object

What happens to a springs elastic potential energy if it is stretched or compressed?

When a spring is stretched or compressed, work is done on the spring which results in energy being transferred to the elastic potential store of the spring

What happens to a springs elastic potential energy if it is released?

When the spring is released, energy is transferred away from its elastic potential store

Elastic potential energy formula

Eₑ = ½ × k × e²

Eₑ = elastic potential energy in joules (J)
k = spring constant in newtons per metre (N/m)
e = extension in metres (m)

Energy in thermal energy store

Energy can be transferred to or transferred from an object or system

Thermal energy formula

ΔE = mcΔθ

ΔE = change in energy, in joules (J)
m = mass, in kilograms (kg)
c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg °C)
Δθ = change in temperature, in degrees Celsius (°C)

Specific heat capacity

Amount of energy required to raise the temperature of 1 kg of a substance by 1 °C

Properties of substances with a low specific heat capacity

If a substance has a low specific heat capacity, it heats up and cools down quickly and takes less energy to change its temperature

Properties of substances with a high specific heat capacity

If a substance has a low specific heat capacity, it heats up and cools down slowly and takes more energy to change its temperature

What states are specific heat capacity used for?

Liquids and solids

Power

Rate of energy transfer or rate that work is done

Power formula

P = E/T
or
P = W/T

P = power in watts (W)
E = energy transferred in joules (J)
W = work done in joules (J)
t = time in seconds (s)

In what systems does the dissipation of energy occur most in?

Open systems

Dissipation of energy

Energy transfers which are not useful are described as being dissipated to the surroundings and considered wasted energy?

Useful energy

The energy that is transferred from store to store and used for an intended purpose

Wasted energy

The energy that is not useful for the intended purpose and is dissipated to the surroundings

Lubrication

Friction is a cause of energy dissipation, lubricating parts that rub together reduces energy dissipation

Insulation

Energy transferred by heating is often wanted so preventing this energy from dissipating will reduce the amount of energy needed to replace the wasting energy; insulation can surround the appliance to reduce energy dissipation

Thermal conduction

Energy is transferred by vibrating particles in a substance

Where do the vibrating particles transfer energy from in thermal conduction

Their kinetic store to the kinetic store of neighbouring particles

Direction of energy transfer

Hot to cold or high to low

Direction of energy transfer in thermal conduction

Hot to cold, so
the higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across the material

Which materials heat up faster, materials with a high or low thermal conductivity?

High

Examples of materials with a high thermal conductivity

Diamond
Aluminium
Graphite

Examples of materials with a low thermal conductivity

Air
Steel
Bronze

What is an insulator?

Substance that is a poor thermal conductor, (e.g. wood, plastic, wool)

Why are insulators used?

To reduce energy transfers, for example, to keep a house warm or to build a soundproof room

Effect of temperature difference across a material in thermal conduction

The greater the temperature difference, the more conduction

Effect of thickness of a material in thermal conduction

The thicker the material, the less energy will be transferred by conduction

Effect of thermal conductivity of the material

The higher the thermal conductivity, the more energy will be transferred by conduction

Efficiency

Measure of the amount of wasted energy in an energy transfer

If a system has a high efficiency...

Most of the energy transferred is useful

If a system has a low efficiency...

Most of the energy transferred is wasted

Efficiency formula

Efficiency = useful energy output / total energy input

Reasons for wasted energy and decreased efficiency in machines

Friction between moving parts, air resistance, electrical resistance, sound

Reducing friction in machines to increase efficiency

Friction can be reduced by adding bearings to prevent components from directly rubbing together or lubricating parts

Reducing electrical resistance in machines to increase efficiency

Electrical resistance can be reduced by using components with lower resistances or reducing the current

Reducing air resistance in machines to increase efficiency

Air resistance can be reduced by streamlining the shapes of moving objects

Reducing noise of machines to increase efficiency

Noise can be reduced by tightening loose parts to reduce vibration or lubricating parts

Energy resources

Large stores of energy that can be used to generate electricity and heat homes and businesses

Fossil fuels (energy resources)

Fossil fuels are combusted to heat water and produce steam to turn turbines to generate electricity

Nuclear (energy resources)

Nuclear fuels are reacted to heat water and produce steam to turn turbines to generate electricity

Bio-fuels (energy resources)

Plant matter, ethanol or methane, can be produced and used as fuel in place of fossil fuels

Wind (energy resources)

Wind turns turbines directly to generate electricity

Hydroelectric (energy resources)

Water is stored at a height, and when released, rushing water turns turbines directly to generate electricity

Tidal (energy resources)

The movement of water due to tides turn turbines directly to generate electricity

Geothermal (energy resources)

Hot rocks underground are used to heat water to produce steam to turn turbines which generate electricity

Solar (energy resources)

Solar cells use light to generate energy, solar panels use thermal radiation to heat water to produce warm water for household use

Water waves (energy resources)

Moving water due to waves turn turbines directly to generate electricity

Renewable energy resource

An energy source that is replenished at a faster rate than the rate at which it is being used

Examples of renewable energy resources

Solar energy
Wind
Bio-fuel
Hydroelectricity
Geothermal
Tidal

Examples of non-renewable energy resources

Fossil fuels (coal, oil, natural gas)
Nuclear fuel

Three main uses of energy resources

Electricity generation, transport, heating

Reliable energy resources

Energy resources that can produce energy at any time

Non-reliable energy resources

Energy resources that can only produce energy at some times

Advantages of fossil fuels (energy resources)

Reliable, can produce large amounts of energy at fairly short notice

Disadvantages of fossil fuels (energy resources)

Produces significant amounts of greenhouses gases and pollution

Advantages of nuclear (energy resources)

Reliable, produces no greenhouse gases or pollution, a large amount of energy is produced from a small amount of fuel

Disadvantages of nuclear (energy resources)

Produces dangerous radioactive waste that can take thousands of years to decay

Advantages of bio-fuels (energy resources)

The carbon dioxide produced while burning the fuel is balanced by thew carbon dioxide absorbed whilst producing it

Disadvantages of bio-fuels (energy resources)

Can take up a lot of land and consume resources that are needed for food production

Advantages of wind (energy resources)

Produce no greenhouse gases or pollution, land can still be used for farming

Disadvantages of wind (energy resources)

Not reliable, turbines can be noisy and ugly, not everywhere is suitable

Advantages of hydroelectric (energy resources)

Reliable, can produce large amounts of energy at short notice, produces no greenhouse gases or pollution

Disadvantages of hydroelectric (energy resources)

Can involve flooding large area which can destroy important wildlife habitats

Advantages of tidal (energy resources)

Tides are very predictable, a large amount of energy can be produced at regular intervals

Disadvantages of tidal (energy resources)

Very few suitable locations, can cause environmental harm to estuaries and disrupt shipping

Advantages of geothermal (energy resources)

Reliable, geothermal stations are usually small