Physics

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Energy

Energy Stores and Systems

Energy is never used up - it is transferred between different energy stores and objects.

Energy stores - Thermal, Kinetic, Gravitational Potential, Elastic Potential, Chemical, Magnetic, Electrostatic and Nuclear.

Energy - transferred mechanically (force doing work), electrically (work done by moving charges), heating or radiation.

When a system changes, energy is transferred.

System - single object/group of objects.

Can be transferred into or away from system, between different objects in system or between different types of energy stores.

Closed systems - matter not energy can enter or leave, net change in the total energy = 0.

Energy can be transferred by heating. For example: kettle.

Water - system. Energy transferred to water - from kettle's heating element, by heating, into water's thermal energy store: temperature rise.

Could think of kettle's heating element and water together as a two-object system - energy transferred electrically to thermal energy store of kettle's heating element, transfers energy by heating to water's thermal energy store.

Energy can be transferred by doing work.

Work done = energy transferred. Can be done when current flows or by force moving an object.

Kinetic and Potential Energy Stores

Movement - energy is in object's kinetic energy store.

Energy transferred to this store when object speeds up, and taken away when object slows down.

Energy in kinetic store depends on mass and speed. High mass + high speed = more energy.

Formula: Ek=1/2mv^2. Ek = Kinetic energy (J), m = Mass (Kg), v^2 = Speed^2 (m/s^2).

Raised objects store energy in gravitational potential energy stores.

Lifting an object in a gravitational field requires work - causes transfer of energy to gravitational potential energy of raised object. Higher the object = more energy transferred.

Amount of energy depends on object's mass, height and strength.

Formula: Ep=mgh. Ep = G.P.E (J), m = Mass (Kg), g = Gravitational Field Strength (N/Kg), h = Height (m).

Falling objects also transfer energy.

When something falls - energy from gravitational potential energy store transferred to kinetic energy store.

For a falling object when there's no air resistance: Energy lost from the G.P.E store = Energy gained in the kinetic energy store.

Air resistance acts against all falling objects - causes some energy to be transferred to other energy stores.

Stretching can transfer energy to elastic potential energy stores.

Stretching or squashing objects.

Formula: Ee=1/2ke^2. Ee = Elastic Potential Energy, k = Spring constant (N/m), e^2 = Extension (m).

Specific Heat Capacity

Specific heat capacity - how hard to heat something up.

Different materials have different heat capacities.

More energy needs to be transferred to thermal energy store of some materials to increase temperature than others. For example: you need 4200 J to warm 1 Kg of water by 1 degree but only 139 J to warm 1 Kg of mercury by 1 degree.

Materials that need a lot of energy to warm up also need a lot of energy to cool down. Can store a lot of energy.

Specific heat capacity - amount of energy needed to raise temperature of 1 Kg of a substance by 1 degree.

Equation: (change in) thermal energy (J)=mass (Kg) * Specific heat capacity (J/kg Degrees Celsius) * (change in) temperature (Degrees Celsius)

Practical

To investigate a solid material, you will need a block of the material with two holes in it for the heater and thermometer to go into.

Measure mass of block, then wrap it in an insulating material (for example: thick layer of newspaper) to reduce energy transferred from block to surroundings. Insert the thermometer and heater.

Measure initial temperature of block and set potential difference, V, of power supple to 10 V. Turn on power supply and start stop watch.

When you turn on the power, the current in circuit does work on heater, transferring energy electrically from power supply to heater's thermal energy store. Energy then transferred to material's thermal energy store by heating - temperature increases.

Block heats up - take readings of temperature and current every minute for 10 minutes. Should find that current through circuit doesn't change as block heats up.

When collected enough reading - turn off power supply, using measurement of current and potential difference of the power supply, calculate power using P=VI. Use this to calculate how much energy has been transferred to heater at the time of each temperature reading using E=Pt, where t is the time in seconds since experiment began.

If you assume all the energy supplied to heater has been transferred to the block, you can plot a graph of energy transferred to the thermal energy store of the block against temperature.

Find gradient of straight part of graph. This is (change in) temperature/(change in) energy. Specific heat capacity of block is: 1/(gradient*mass of block).

Can repeat experiment with different materials.

Conservation of Energy and Power

Energy is never destroyed.

Conservation of energy principle - energy can be transferred usefully, or stored or dissipated but can never be created or destroyed.

Dissipated energy = wasted energy - energy being stored in a way that is not useful.

Mobile phone - system. Using phone - energy usefully transferred from chemical store to battery but some energy dissipated into thermal energy store.

Cold spoon dropped into an insulated flask of hot soup, then sealed. Energy transferred from thermal energy store of soup to useless thermal energy store of spoon.

Power - rate of doing work.

Power is the rate of energy transfer, or the rate of doing work.

Measured in watts. One watt = 1 joule of energy transferred per second.

Equations: P=E/t or P=W/t. P = Power (W), E = Energy transferred (J), t = Time (s), W = Work done (J).

A powerful machine is one which transfers a lot of energy in a short space of time.

Conduction and Convection

Conduction - process where vibrating particles transfer energy to neighbouring particles. Mainly in solids.

Energy transferred to object by heating is transferred to thermal store of object.

Particles in part of the object being heated vibrate more and collide with each other. Collisions cause energy to be transferred between particles' kinetic energy stores - conduction.

Process continues throughout object until energy is transferred to other side of the object, then it is usually transferred to the thermal energy store of surroundings.

Thermal conductivity - measure of how quickly energy is transferred through a material in this way. Materials with high thermal conductivity transfers energy between particles quickly.

Convection - energetic particles move away from hotter to cooler regions. Only in liquids and gases.

Energy is transferred by heating to the thermal store of liquid or gas. Energy shared across kinetic energy store of the gas or liquid's particles.

Particles in liquids and gases are able to move. When heated up - particles move faster and the space between individual particles increases - causes density of region being heated to decrease.

Because liquids and gases can flow, the warmer and less dense region will rise above denser, cooler regions. If there is a constant heat source, a convection current can be created.

Radiators create convection currents.

Heating a room with a radiator relies on creating convection currents in the air of a room.

Energy transferred from radiator to nearby air particles by conduction.

Air by radiator becomes warmer and less dense.

Warm air rises, replaced by cooler air. Cooler air is heated by radiators.

At the same time, previously heated air transfers energy to surroundings. It cools, becomes denser and sinks.

Cycle repeats, causing a flow of air to circulate around the room - convection current.

Reducing Unwanted Energy Transfers

Lubrication reduces frictional forces.

Something moves - usually at least one frictional force acting against it. Causes some energy in system to be dissipated, for example: air resistance can transfer energy from a falling object's kinetic energy store to its thermal energy store.

For objects being rubbed together, lubricants used to reduce friction between object's surfaces when they move. Lubricants usually liquid so they can flow easily between objects and coat well.

Insulation - reduce rate of energy transfer by heating. Prevent energy losses through heating:

Thick walls made from low thermal conductivity. Thicker walls + lower thermal conductivity = slower rate of energy transfer (cool down slower).

Use thermal insulation:

Cavity walls - inner and outer wall with air gap in middle. Air gap - reduces amount of energy transferred by conduction through walls. Cavity wall insulation - where cavity wall air gap filled with foam, reduce energy transfer by convection.

Loft insulation - reduce convection currents being created in lofts.

Double-glazed windows - work in same way as cavity walls, air gap between two sheets of glass to prevent energy transfer by conduction through windows.

Draught excluders - around doors and windows reduce energy transfer by convection.

Practical

Boil water in kettle. Pour some of the water in a sealable container to a safe level. Measure mass of water in container.

Use thermometer to measure initial temperature of water.

Seal container and leave for five minutes. Measure time using stopwatch.

Remove lid, measure final temperature of water.

Pour away water and allow container to cool to room temperature.

Repeat experiment but wrap container in different material. Same amount of water and initial temperature each time.

Lower temperature difference = material is better thermal insulator.

Also investigate how thickness of material affects the effectiveness of material. Thicker materials = better thermal insulators.

Efficiency

Most energy transfers involve some waste energy.

Useful devices - useful as transfer energy from one store to another.

Useless energy store - usually a thermal energy store.

Less energy wasted = more efficient device.

Improve efficiency by insulating objects, lubricating them or making them more streamlined.

Equation: Efficiency = (Useful output energy transfer) / (Total input energy transfer).

Might not know energy input so equation: Efficiency = (Useful power output) / (Total power input).

Useful energy input isn't usually equal to total energy output.

No device is 100% efficient.

Electric heaters are 100% efficient - all energy in the electrostatic energy store transferred to useful thermal energy stores.

Energy Resources their Uses

Non-renewable energy resources will run out one day, all do damage to environment but provide most of our energy. Three main fossil fuels:

Coal.

Oil.

(Natural) Gas.

Renewable energy resources will never run out, do less damage to environment that non-renewable resources but do not provide much energy and could be unreliable since they depend on weather. Renewable resources:

Sun (Solar).

Wind.

Water waves.

Hydro-electricity.

Bio-fuel.

Tides.

Geothermal.

Energy resources can be used for transport.

Non-renewable energy resources:

Petrol and diesel powered vehicles use fuel created from oil.

Coal used in old-fashioned steam trains to boil water to produce steam.

Renewable energy resources:

Vehicles that run on bio-fuels.

Can also be used for heating.

Non-renewable energy resources:

Natural gas - heating homes in UK, heat water which is pumped into radiators.

Coal - burnt in fireplaces.

Electric heaters use electricity generated from non-renewable energy sources.

Renewable energy resources:

Geothermal heat pump uses geothermal energy resources to heat buildings.

Solar water heaters - uses sun to heat water which is pumped into radiators.

Burning bio-fuel or using electricity generated from renewable resources, can be used for heating.

Wind, Solar and Geothermal

Wind power:

Lots of wind turbines in exposed places. For example: on moors or around coasts.

Each turbine has generator inside, rotating blades turn generator and produce electricity.

No pollution.

Spoil view - need about 1500 wind turbines to replace one coal-fired power station, cover a lot of ground.

Very noisy.

Turbines stop when wind is too weak or strong. Impossible to increase supply when there is extra demand.

Average: produce electricity 70-85% of time.

Initial costs = high, but no fuel costs and minimal running cost.

No permanent damage to landscape.

Solar cells:

Generate electric currents from sunlight. Best source of energy to charge batteries in calculators and watches.

Often used in remote places where there isn't much choice and to power electric road signs and satellites.

No pollution.

Sunny countries - reliable, only in daytime. Could by cost-effective in cloudy countries.

Cannot increase power output when there is extra demand.

Initial costs = high, but after energy is free and running costs almost 0.

Usually generate electricity on small scale.

Geothermal power:

Possible in volcanic areas or where hot rocks lie near surface. Source: slow decay of radioactive elements.

Free energy, reliable and does little damage to environment.

Can be used to generate electricity, or to heat buildings directly.

Not many suitable locations for power plants and cost to build is high compared to amount of energy produces.

Hydro-electricity, Waves and Tides

Hydro-electricity power:

Requires flooding of valley by building big dam. Water allowed out through turbines, no pollution.

Big impact on environment - flooding. Possible loss of habitat, reservoirs could look unsightly when dried up.

Provides immediate response to an increased demand for electricity.

Reliable unless there is a drought.

Initial costs = high, but no fuel costs and minimal running costs.

Useful way to generate electricity on a small scale in remote areas.

Wave power:

Lots of small wave-powered turbines located around coast. Moving turbines connected to generator.

No pollution.

Disturbs seabed and habitats of marine animals, spoiling view and a hazard to boats.

Fairly unreliable - waves tend to die out when wind drops.

Initial costs = high, but no fuel costs and minimal running costs. Provides energy on small scale, useful on small islands.

Tidal barrages:

Tidal barrages - big dams built across river estuaries, with turbines. As tide comes in, it fills estuary. Water allowed out through turbines at controlled speed.

Tides produced by gravitational pull of sun and moon.

No pollution.

Prevents free access by boats, spoils view and alters habitat of wildlife.

Tides pretty reliable - happen twice day and always near to predicted height. Lower tides = less energy. Doesn't work when water level is same either side of barrage.

Initial costs - moderately high, but no fuel costs and minimal running costs. Can only be used in some of the suitable estuaries, tidal power has potential to create significant amount of energy.

Bio-fuels and Non-renewables

Bio-fuels are made from plants and waste.

Can be solid, liquid or gas and can be burnt to produce electricity or run cars in same way as fossil fuels.

Supposedly carbon neutral.

Fairly reliable, crops take short time to grow and different crops can be grown around the year. Cannot respond to immediate energy demands. Continuously produced and stored for when needed.

Cost to refine bio-fuels = very high.

In some regions, large areas of forest have been cleared to make room to grow bio-fuels, habitats lost. Decay and burning of this vegetation also increases carbon dioxide and methane emissions.

Non-renewables are reliable.

Enough fossil and nuclear fuels to meet current demand, power plants can respond quickly to changes in demand.

Fuels slowly running out. If no new resources found, some fossil fuel stocks may run out within a hundred years.

Set-up costs of power plants can be quite high compared to other energy resources, running costs are not that expensive. Cost effective.

Non-renewables create environmental problems.

Release carbon dioxide in atmosphere when burned. Carbon dioxide adds to greenhouse effect and contributes to global warming.

Burning coal and oil also releases sulphur dioxide, causes acid rain - harmful to trees and soils, bad effect in ecosystems.

Acid rain can be reduced by taking sulphur out before fuel is burned or cleaning up emissions.

Coal mining - makes mess of landscape, especially 'open-cast mining'. View can be spoilt by fossil fuel power plants.

Oil spillages - affect mammals and birds that live in and around the sea.

Nuclear power - clean but nuclear waste is dangerous and difficult to dispose.

Nuclear fuel - cheap but overall cost of nuclear power is high due to cost of power plant and final decommissioning.

Nuclear power always carries risk of major catastrophe.

Trends in Energy Resource Use

Currently we still depend on fossil fuels.

Over 20th century - electricity use of UK increased as population grew and people began to use electricity more.

Since beginning of 21st century - electricity use in UK decreasing as we become better at making appliances more efficient and become more careful with energy use in homes.

Most electricity produced from fossil fuels and nuclear power.

Oil used to fuel cars, gas used to heat homes and cook food.

People want to use more renewable energy resources.

Burning fossil fuels - very damaging to environment.

Non-renewables will run out, better to learn to get by without non-renewables before this happens.

Pressure.

The use of renewables is limited by reliability, money and politics.

Building new renewable power plants cost money, so some energy providers reluctant to do this. Cost of switching to renewable power will have to be paid - either by customers in bills or through government and taxes. Some people can't or don't want to pay and there are arguments about whether it is ethical to make them.

Even if new power plants are built, there are arguments about where to put them.

Some energy resources are not reliable whilst others cannot increase their power output on demand.

Research on improving reliability and cost of renewables takes time and money.

Making personal changes can be expensive. Hybrid cars are generally more expensive than petrol cars and solar panels for your home are expensive.