PHYSICS UNIT 4: Energy resources and energy transfers

(a) Units (b) Energy transfers (c) Work and power (d) Energy resources and electricity generation

What is the SI unit of mass?

kilogram (kg). Mass measures the amount of matter in an object.

What does the symbol kg represent?

kilogram, the SI unit for mass.

What is the SI unit of energy?

joule (J).

What does the symbol J represent?

joule, the SI unit for energy and work done.

What is one joule?

One joule is the energy transferred when a force of 1 newton moves an object 1 metre in the direction of the force.

What is the SI unit of distance?

metre (m).

What does the symbol m represent?

metre, the SI unit for distance or displacement.

What is the SI unit of speed?

metre per second (m/s).

What does m/s mean?

Metres travelled every second; the SI unit of speed.

What is the SI unit of acceleration?

metre per second squared (m/s²).

What does m/s² represent?

The change in velocity in metres per second every second.

What is the SI unit of force?

newton (N).

What does the symbol N represent?

newton, the SI unit of force.

What is one newton?

The force required to give a 1 kg mass an acceleration of 1 m/s².

What is the SI unit of time?

second (s).

What does the symbol s represent?

second, the SI unit of time.

What is the SI unit of power?

watt (W).

What does the symbol W represent?

watt, the SI unit of power.

What is one watt?

One watt is one joule of energy transferred per second.

How are joules and watts related?

Power in watts shows how quickly energy is transferred: 1 W = 1 J/s.

Why must physics calculations use SI units?

Because equations are designed to work correctly using standard units.

Convert 1 kJ into joules.

1 kJ = 1000 J.

Convert 1 MJ into joules.

1 MJ = 1,000,000 J.

Convert minutes into seconds.

Multiply by 60.

Convert hours into seconds.

Multiply by 3600.

Convert centimetres into metres.

Divide by 100.

Convert kilometres into metres.

Multiply by 1000.

What unit should mass be converted into before using equations like GPE = mgh?

kilograms (kg).

What unit should height be converted into before using GPE = mgh?

metres (m).

What unit should time be converted into before using power equations?

seconds (s).

What unit should force be converted into before using work done equations?

newtons (N).

What unit should distance be converted into before using work done equations?

metres (m).

What unit should energy be converted into before calculating efficiency?

joules (J).

What unit should power be calculated in?

watts (W).

A car travels at 20 m/s. What does this unit mean?

The car travels 20 metres every second.

An acceleration of 5 m/s² means what?

The velocity increases by 5 metres per second every second.

Why is acceleration measured in m/s² rather than m/s?

Because acceleration measures how speed changes over time.

What is the relationship between newtons, kilograms and acceleration?

Force = mass × acceleration, so N = kg × m/s².

Why is the joule used for both work done and energy transferred?

Because doing work transfers energy.

Why is watt not a unit of energy?

A watt measures the rate of energy transfer, not the amount of energy transferred.

What is an energy store?

A way of describing where energy is stored in a system.

What are the 8 energy stores required for IGCSE Physics?

Chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic and nuclear.

What is a chemical energy store?

Energy stored in chemical bonds.

Give examples of a chemical energy store.

Fuels, food, batteries and explosives.

What happens to the chemical energy store of fuel when it burns?

It decreases as energy is transferred to other stores, mainly thermal energy.

What is a kinetic energy store?

The energy stored in a moving object due to its motion.

What factors affect the kinetic energy store of an object?

The mass and speed of the object.

What is a gravitational potential energy store?

The energy stored in an object because of its position in a gravitational field.

Give an example of increasing a gravitational potential energy store.

Lifting an object higher above the ground.

What is an elastic energy store?

Energy stored in a stretched or compressed elastic object.

Give examples of elastic energy stores.

A stretched spring, a compressed spring or a stretched elastic band.

What is a thermal energy store?

The energy stored in an object due to the random movement of its particles.

What happens to a thermal energy store when an object is heated?

The particles gain kinetic energy and the thermal energy store increases.

What is a magnetic energy store?

Energy stored due to the position of magnetic materials in a magnetic field.

Give an example of a magnetic energy transfer.

Two magnets being pushed apart or attracted together.

What is an electrostatic energy store?

Energy stored due to the positions of electrically charged objects.

Give an example of an electrostatic energy store.

Two charged objects repelling each other.

What is a nuclear energy store?

Energy stored in the nucleus of atoms.

Where is nuclear energy released from?

Changes in atomic nuclei during fission, fusion or radioactive decay.

What does an energy transfer pathway describe?

How energy moves from one store to another.

What are the four energy transfer pathways?

Mechanically, electrically, by heating and by radiation.

What is mechanical energy transfer?

Energy transferred when a force moves an object and does work.

Give an example of mechanical energy transfer.

A person lifting a box transfers energy mechanically from their chemical store to the box's gravitational potential energy store.

What is electrical energy transfer?

Energy transferred by moving charges through a circuit.

Give an example of electrical energy transfer.

A battery transfers energy electrically to a motor.

What is energy transfer by heating?

Energy transferred due to a temperature difference between objects.

What are the three methods of thermal energy transfer?

Conduction, convection and radiation.

What is energy transfer by radiation?

Energy transferred by waves, such as light, sound or infrared radiation.

Give an example of energy transfer by light radiation.

The Sun transferring energy to Earth.

Give an example of energy transfer by sound radiation.

A loudspeaker transferring energy to the surroundings.

Describe the energy transfers in a torch.

Chemical energy store in the battery decreases; energy is transferred electrically to the lamp; light and thermal energy are transferred to the surroundings.

Describe the energy transfers in a car.

Chemical energy store of fuel decreases; energy is transferred mechanically; kinetic energy store of the car increases with some energy wasted as thermal and sound energy.

Describe the energy transfers in a falling object.

Gravitational potential energy store decreases and kinetic energy store increases; some energy may be transferred to thermal energy due to air resistance.

State the principle of conservation of energy.

Energy cannot be created or destroyed; it can only be transferred between stores.

What does conservation of energy mean in calculations?

The total energy before a transfer equals the total energy after the transfer.

Why do real devices become warmer when operating?

Some energy is transferred to unwanted thermal energy stores due to friction or electrical resistance.

What is useful energy?

The energy transferred to the intended output store.

What is wasted energy?

Energy transferred to unwanted stores, usually the surroundings.

What is efficiency?

The ratio of useful energy output to total energy input, expressed as a percentage.

Write the equation for efficiency.

efficiency = (useful energy output ÷ total energy input) × 100%

Why can efficiency never be greater than 100%?

Because useful output energy cannot be greater than the total energy supplied.

A machine has 500 J input energy and 400 J useful output energy. Calculate efficiency.

(400 ÷ 500) × 100 = 80%

Why are efficient devices better?

They transfer a greater proportion of input energy into useful energy and waste less energy.

What does a Sankey diagram show?

The energy transfers into useful and wasted energy.

What does the width of an arrow in a Sankey diagram represent?

The amount of energy transferred.

What does a wider arrow mean on a Sankey diagram?

A greater amount of energy is transferred.

How is efficiency shown on a Sankey diagram?

By comparing the width of useful output energy with total input energy.

What happens to wasted energy in a Sankey diagram?

It leaves the main pathway, often going to the surroundings.

A device has a large wasted energy arrow on a Sankey diagram. What does this show?

The device has low efficiency.

How can a device be made more efficient?

Reduce unwanted energy transfers, for example using insulation or lubrication.

Describe energy transfers in an electric motor.

Electrical energy is transferred into a kinetic energy store, with some energy wasted as thermal energy due to resistance.

Describe energy transfers in a speaker.

Electrical energy is transferred into sound energy through vibrations.

Describe energy transfers in a kettle.

Electrical energy is transferred to the thermal energy store of the water, with some energy wasted to the surroundings.

Describe conduction.

The transfer of thermal energy through a substance without the substance moving overall.

How does conduction occur in metals?

Free electrons transfer kinetic energy through the metal, and vibrating ions transfer energy by collisions.

Why are metals good thermal conductors?

They contain free electrons that transfer energy quickly.

Why are plastics good thermal insulators?

They have no free electrons and limit particle collisions.

Describe convection.

The transfer of thermal energy through liquids and gases by the movement of the fluid.

Why does convection only happen in fluids?

Particles in liquids and gases can move from one place to another.

Explain a convection current.

Heating makes a fluid expand, become less dense and rise; cooler denser fluid sinks, creating a circular current.