StemUp: OCR A A level Physics 3.3: Work, energy and power

What is meant by the work done by a force? (1)

This is the transfer of energy to an object by a force that causes the motion of the object.

What is the equation for the work done? (3)

- The work done is given by the magnitude of the force * the distance moved by the object

- The distance moved by the object must be in the same direction as the direction of the force.

- If the force (F) is at an angle to the horizontal (theta), the work done (W) is given by: W = F x cos θ where x is the distance moved.

What is the unit and S.I. base unit for energy? (1)

The unit for energy is the Joule (J), the base unit is kg·m²·s⁻²

​Energy is a scalar quantity.

What is the principle of conservation of energy? (2)

The principle of conservation of energy states that in a closed system energy cannot be created or destroyed but only transferred from one form to another.

What are the different forms of energy? (9)

- Kinetic

- Gravitational

potential

- Elastic potential

- Electric potential

- Sound

- Internal

- Electromagnetic

- Nuclear

- Chemical

What is meant by kinetic energy? (1)

This is the energy which arises from the motion of a mass.

What is meant by gravitational potential energy (GPE)? (1)

This is the energy stored by an object at a point in the gravitational field.

What is meant by elastic potential energy? (1)

This is the energy stored in an elastic object when it changes shape into a shape that can be reversed.

What is meant by electric potential energy? (1)

This is the energy of charges due to their position in an electric field.

What is meant by sound energy? (1)

This is the energy of a sound wave due to the movement of the atoms as it passes through.

What is meant by internal energy? (1)

This is the sum of kinetic and potential energies of the molecules in a substance.

What is meant by electromagnetic energy? (1)

This is the energy carried by electromagnetic waves.

What is meant by nuclear energy? (1)

This is the energy stored in nuclei which is released in processes such as fission.

What is meant by chemical energy? (1)

This is the energy stored in chemical bonds in atoms which is released when they rearrange.

What is the relation between energy transfers and the work done? (1)

The work done in a system is equal to the amount of energy transferred between objects in the system. (e.g An object falling transfers energy from gravitational potential to kinetic energy whilst doing work against the air).

What is the formula for the kinetic energy of an object? (1)

If an object has a mass (m) and velocity (v) the kinetic energy is:

How is the formula for kinetic energy derived? (3)

- Take the equation of motion v² = u² + 2as and as u = 0 this reduces to v² = 2as and s = v² / 2a​

- Use the work done formula W = Fx to find the work done (kinetic energy) to be Fv²/ 2a

- As F = ma F/a can be replaced by m so the kinetic energy is: 1/ 2 mv²​

What is the formula for the gravitational potential energy (GPE) of an object? (1)

If the object has mass (m) and is h meters above the ground in a gravitational field with acceleration of free fall g, the GPE is given by:

PE = mgh

How is the formula for gravitational potential energy (GPE) derived? (2)

- The work done is W = Fx and in a gravitational field the force is weight (mg) and the distance travelled is the height (h).

- So W = Fx can be rewritten to be PE = mgh​

How is energy transferred from gravitational potential energy to kinetic energy? (1)

When an object falls, its gravitational potential energy is transferred to kinetic energy as the object does work against the air.

How is energy transferred from gravitational potential energy to kinetic energy in a closed loop? (3)

- In a closed loop the total change in KE is equal to the total change in PE.

- Therefore mgh = 1/ 2 mv²​

- 2gh =​ v²​ and the final speed can be found to be v = √ 2gh

What is the definition of power? (1)

Power is the rate of which energy is transferred. It can also be defined as the rate work is done at.

What is the equation for power? (1)

Power is given by the equation P = E / t where E is the energy transferred (J) and t is the time taken for this to happen (s)

What is the unit and S.I. unit for power? (1)

The unit for power is the Watt (W) and the S.I. units are kg·m²·s⁻³

What is the relation between power and velocity? (3)

- A car will drive at a constant driving force (F) at a velocity (v).

- As P = E/t and E = Fx the power can be written to be P = Fx / t

- As x/t is the velocity the power is written as P = F v​

What is the definition of efficiency? (2)

Efficiency is a measure of how much total input energy is conserved as useful energy. A greater efficiency means less energy is dissipated and wasted to the environment.

What is the equation of efficiency? (1)

Efficiency = (Useful energy output / total energy input) x 100%.