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    Energy stores and transfers

    Revision of Energy stores and transfers for GCSE Physics/Combined Science.

    Energy stores

    There are 8 energy stores where energy can be ‘kept’:

    chemical store (in  a chemical reaction e.g. fuel + oxygen)
    kinetic store (in a moving object)
    gravitational store (due to the position of an object in a gravitational field)
    elastic store (e.g. in a stretched or compressed spring)
    thermal store (in a warm object)
    magnetic store (in two separated magnets that are attracting, or repelling)
    electrostatic store (in two separated electric charges that are attracting, or repelling)
    nuclear store (released through radioactive decay, fission or fusion.

    Key definition – what is a system?

    A system is an object or group of objects.

    Energy stores and transfers
    Energy can transfer between stores when a system changes

    Examples

    An object projected upwards: (e.g. ball thrown upwards):

    Kinetic store of ball → Gravitational potential store of ball

    A moving object hitting an obstacle: (e.g. car hitting a traffic cone):

    Kinetic store of moving object → Kinetic store of obstacle

    An object accelerated by a constant force: (e.g. skydiver accelerated by their

    weight):

    Gravitational potential store of skydiver → Kinetic store of skydiver

     A vehicle slowing down (e.g. car applying brakes):

    Kinetic store of car → Thermal store of brake pads.

    Bringing water to boil in an electrical kettle:

    Thermal store of element → Thermal store of water in kettle

                             

    Energy transfers

    There are four pathways along which energy is transferred from one store to another:

    – Heating

    – Electrical

    – Radiation (including light, all electromagnetic waves and sound).

    – Mechanical

    Quick Check 1

    Example – energy stores and transfers in a roller coater ride

    People on a roller coater

    As a roller coaster climbs to it’s highest point, it transfers energy from the electrical supply to a gravitational store. There is a motor doing mechanical work on the roller coaster, applying a force to make it move up. As it goes over the highest point and starts to move downwards, energy is rapidly shifted from this gravitational store to a kinetic store. The force of gravity is doing mechanical work on the roller coaster, pulling it down the slope.

    Energy calculations

    You need to be able to calculate the energy transferred, using the following equations:

    In heating

    (ΔE = mcΔθ)

    Change in thermal energy = mass x specific heat capacity x change in temperature

    Mechanical work done by forces

    (W = Fd)

    Work done = Force x distance

    Electrical work done when a current flows

    (W = IVt)

    Work done = Current x potential difference x time

    Quick Check 2

    Kinetic Energy

    The equation for kinetic energy:

    EK      ½ x m x v2

    Kinetic energy = ½  x mass x velocity2

    EK = Kinetic energy measured in joules (J)

    m = mass measured in kilograms (kg)

    v = velocity measured in metres per second (m/s)

    Quick Check 3

    Elastic Potential Energy

    The equation for elastic potential energy is:

    Ee= 0.5 x k x e2

    Elastic potential energy = 0.5 x spring constant x extension2

    Ee = Elastic potential energy (J) k = Spring constant (N/m) e = extension (m)

    Quick Check 4

    Thermal Energy

    The equation for change in thermal energy is:

    ΔE = m x c x Δθ

    Change in thermal energy = mass x specific heat capacity x change in temperature

    ΔE = Change in thermal energy (J); m = Mass (kg); c = Specific heat capacity (J/kgoC)

    Δθ = Change in temperature (oC)

    The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.

    Quick Check 5

    Power

    Power is defined as the rate at which energy is transferred or the rate at which work is done.

    The equation for power is:

    P = E ÷ t

    Power = Energy transferred ÷ time

    P = Power (W); E = Energy transferred (J); t = time (s);

    The equation for power can also be written:

    P = W ÷ t

    Power = Work done ÷ time

    P = Power (W) W = Work done (J); t = time (s)

    Watt is the unit of Power!? The question that answers itself.

    An energy transfer of 1 joule per second is equal to a power of 1 watt.

    Quick Check 6

    Conservation of Energy

    Energy can be transferred usefully, stored or dissipated, but energy cannot be

    created or destroyed.

    Sometimes energy is dissipated, so that it is stored in less useful ways. This energy

    is often described as being ‘wasted’.

    Because energy cannot be lost: Total energy = useful energy + wasted energy

    Unwanted energy transfers can be reduced by a range of methods, for example

    through lubrication in car engines and the use of thermal insulation in houses.

    The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material. The rate of cooling of a building is affected by the thickness and thermal conductivity of its walls.

    Quick Check 7

    Efficiency

    Efficiency is a measure of how much of the available energy is transferred to useful stores.

    The energy efficiency for any energy transfer can be calculated using the equation:

    efficiency = useful output energy transfer ÷ total input energy transfer

    Efficiency may also be calculated using the equation:

    efficiency = useful power output ÷ total power input

    Quick Check 8

    Energy resources

    A renewable energy resource is one that is being (or can be) replenished as it is used.

    The uses of energy resources include transport, electricity generation and heating.

    Energy Resource Renewable/ Non- renewable Description Environmental impact Uses of energy resource Reliability
    Fossil fuels Non-renewable Coal, oil and gas can be burned to heat water, to make stream, to turn a turbine. Greenhouse gases Electricity generation, transport Reliable
    Nuclear Non-renewable Nuclear fission heats water, to make steam, to turn a turbine. Radioactive waste Electricity generation Reliable
    Biofuel Renewable Biofuel is burnt to heat water, to make steam, to turn a turbine. Carbon – neutral Electricity generation, heating, transport Reliable
    Wind Renewable Wind turns a turbine Noise Electricity generation Unreliable
    Hydroelectric Renewable Water through a dam turns a turbine Flooding of habitats Electricity generation Reliable
    Geothermal Renewable Heat from underground heats water, to make steam, to turn a turbine None Electricity generation, heating Reliable
    Quick Check 9

    Energy topic knowledge organiser

    Energy topic glossary