Energy changes in a system, and the ways energy is stored before and after such changes

What is a system in physics?

A system is an object or a group of objects that you choose to focus on. Defining the system sets the starting point and stopping point for the energy transfers you consider.

State the principle of conservation of energy.

Energy cannot be created or destroyed, it can only be transferred from one store to another. In a closed system the total amount of energy remains constant.

Name the eight main energy stores.

Kinetic, gravitational potential, elastic potential, magnetic, electrostatic, chemical, nuclear and thermal.

What is meant by the kinetic energy store of an object?

It is the energy an object has because of its mass and its speed. Any moving object has energy in its kinetic store.

When does an object gain energy in its gravitational potential store?

When it is lifted through a gravitational field, i.e. raised to a greater height. Energy is transferred away from this store when the object falls.

What does the elastic potential store describe?

The energy stored in an object that has been stretched, squashed or bent. It depends on the spring constant and the extension.

Name the four energy transfer pathways.

Mechanical working, electrical working, heating (by particles) and heating by radiation.

Describe energy transfer by mechanical working.

A force acts on an object and moves it over a distance, for example pulling, pushing, stretching or squashing. The force does work on the object.

Describe energy transfer by electrical working.

A charge moves through a potential difference, for example a current flowing from a power supply through the components of a circuit.

Describe energy transfer by heating.

Energy is transferred from a hotter object to a colder one, for example by conduction. This raises the energy in the kinetic store of the particles, increasing the object's thermal store.

How is energy transferred by radiation?

Energy is carried by electromagnetic waves, such as visible light or infrared. No particles are needed for this transfer.

What is the equation for work done by a force?

E = F × d, where E is work done or energy transferred in joules (J), F is force in newtons (N) and d is distance in metres (m).

What does 'work done' mean?

Work is done when a force moves an object over a distance in the direction of the force. Work done equals the energy transferred.

What happens to the work done if a force is applied but the object does not move?

No work is done, because there is no movement in the direction of the force. No energy is transferred by that force.

Give the equation for gravitational potential energy.

ΔGPE = m × g × Δh, where m is mass in kg, g is gravitational field strength in N/kg and Δh is the change in vertical height in metres.

Give the equation for kinetic energy.

KE = ½ × m × v², where m is mass in kg and v is speed in m/s. Remember to square the speed and include the factor of one half.

Give the equation for elastic potential energy.

Ee = ½ × k × e², where k is the spring constant in N/m and e is the extension in metres. This assumes the limit of proportionality has not been exceeded.

What is the equation linking power, energy transferred and time?

P = E ÷ t, where P is power in watts (W), E is energy transferred or work done in joules (J) and t is time in seconds (s).

Define power.

Power is the rate at which energy is transferred, or the rate at which work is done. One joule transferred per second equals one watt.

What is the unit of power and what does it mean?

The watt (W). 1 W = 1 J/s, so a device with a power of 1 W transfers 1 joule of energy every second.

Two motors lift the same weight through the same height, but one does it faster. Which has more power?

The motor that lifts the weight faster has more power. Both transfer the same amount of energy, but the faster motor transfers it in less time.

What is efficiency?

Efficiency is the ratio of the useful energy output from a system to the total energy supplied to it. It can be written as a decimal between 0 and 1, or as a percentage.

Give the two equations for efficiency.

efficiency = useful energy transferred ÷ total energy supplied, and efficiency = useful power output ÷ total power input. Efficiency has no units.

What does it mean if a system has low efficiency?

Most of the energy transferred is wasted rather than useful. A high-efficiency system transfers most of its energy usefully.

What is meant by dissipated energy?

Energy that is spread out to the thermal store of the surroundings, often by heating or radiation. It cannot easily be gathered for a useful purpose, so it is described as wasted energy.

Why is energy never described as 'lost'?

Energy is conserved, so it is never destroyed. It is transferred to the surroundings, often by heating, where it is dissipated and no longer useful.

What causes wasted energy transfers in machines?

Friction. It transfers energy from the kinetic store of the moving parts to the thermal store of the objects and surroundings by heating, so more work is needed to keep the machine moving.

How can friction in machine parts be reduced?

By lubricating the parts that rub together. This reduces the wasted energy transferred to the thermal store of the surroundings.

What is a Sankey diagram used for?

To represent energy transfers, with the width of each arrow proportional to the amount of energy transferred. The straight arrow shows useful output and the bent arrows show wasted energy.

State the energy balance shown by a Sankey diagram.

Total energy in = useful energy out + wasted energy. This follows from the conservation of energy.

Describe the energy transfers when a bat hits a ball.

Energy is transferred from the kinetic store of the bat to the kinetic store of the ball. Some is transferred to the thermal store of the bat, ball and surroundings by heating, and is dissipated.

Describe the energy transfers when an electric kettle boils water.

Energy is transferred electrically from the mains supply to the thermal store of the heating element, then by heating to the thermal store of the water. Some energy is wasted to the thermal store of the kettle and the surroundings.

Describe the energy transfers when a moving object hits an obstacle.

Energy is transferred from the kinetic store of the moving object to the kinetic store of the obstacle, and some to the thermal store of both objects and the surroundings by heating and sound.

Describe the energy changes for an object projected upwards.

Energy is transferred mechanically from the kinetic store of the object to its gravitational potential store as it rises. On the way down, energy is transferred back from the gravitational potential store to the kinetic store.

Describe the energy transfers when a vehicle slows down.

Energy is transferred from the kinetic store of the vehicle to the thermal store of the brakes and surroundings by heating due to friction. The brakes do work against the motion.

What is the equation for the change in thermal energy of a substance?

ΔE = m × c × Δθ, where m is mass in kg, c is specific heat capacity in J/kg °C and Δθ is the temperature change in °C.

Define specific heat capacity.

The amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius. Its unit is J/kg °C.

What does the symbol Δ mean in equations such as ΔGPE = m × g × Δh?

Δ is the Greek letter delta and stands for 'change in'. So Δh is the change in vertical height.

When can energy values be equated in a calculation?

When the energy transfer is assumed to be perfect, for example when a question says to ignore air resistance or friction. Then the energy lost from one store equals the energy gained by another.

How do you find the speed of an object from its kinetic energy?

Rearrange KE = ½ × m × v² to give v = √(2 × KE ÷ m). Substitute the kinetic energy in joules and the mass in kilograms.

Why does a ball bounce back to a lower height than it was dropped from?

Some energy is transferred to the thermal store of the ball, floor and surroundings by heating and sound on impact. This energy is dissipated, so less is available to transfer back to the gravitational potential store.

What is the difference between energy and power?

Energy is the total amount transferred, measured in joules. Power is the rate at which energy is transferred, measured in watts, so power = energy ÷ time.

How does the gravitational potential energy of an object change with height?

It increases linearly with height, because ΔGPE = m × g × Δh. A graph of GPE against height is a straight line through the origin.