Work, Power & Efficiency Flashcards

All 23 cards in this deck

Name the energy stores involved when a ball falls towards the ground (ignoring air resistance).

Gravitational potential store → kinetic store.

When a moving vehicle brakes to a stop, where is the energy from the kinetic store transferred to?

To the thermal store of the brakes and surroundings, mechanically (by friction).

When an electric kettle heats water, name the energy transfer taking place.

Energy is transferred electrically from the mains to the thermal store of the water.

True or false? Energy is a substance that can be used up.

False. Energy is transferred between stores; it is never created or destroyed.

What are the three ways the energy of a system can be changed?

By work done by forces, electrically (in electrical equipment), and by heating.

In a Sankey diagram, what does the width of each arrow represent?

The amount of energy transferred in that pathway.

What happens to the total energy in a closed system when energy transfers occur?

There is no net change — the total energy stays the same.

True or false? In a closed system, the useful output energy equals the total input energy.

False. Total output (useful + wasted) equals the input; useful output is less.

What is the relationship between work done and energy transferred?

Work done (J) = energy transferred (J) — they are equal and measured in joules.

State the equation for work done.

work done (J) = force (N) × distance moved in the direction of the force (m), $E = F \times d$

True or false? If a force acts at right angles to the direction an object moves, that force does no work on the object.

True. Only distance moved in the direction of the force counts.

State the equation for change in gravitational potential energy.

$\Delta GPE = m \times g \times \Delta h$ (mass in kg, gravitational field strength in N/kg, change in vertical height in m)

State the equation for kinetic energy.

$KE = \tfrac{1}{2} \times m \times v^2$ (mass in kg, speed in m/s)

True or false? Doubling the speed of a car doubles its kinetic energy.

False. KE depends on $v^2$, so doubling the speed gives four times the kinetic energy.

What does it mean to say energy is 'dissipated'?

It is transferred to the surroundings and stored in less useful ways, usually as thermal energy.

Why do mechanical processes that cause a rise in temperature become wasteful?

Energy is dissipated by heating the surroundings, so less is transferred usefully.

Define power.

The rate at which energy is transferred (the work done per second).

State the equation for power in terms of work done.

power (W) = work done (J) ÷ time taken (s), $P = E \div t$

One watt is equal to how many joules per second?

1 W = 1 J/s.

True or false? Two devices that transfer the same energy in different times have the same power.

False. The one taking less time has the greater power, since power = energy ÷ time.

State the equation for efficiency.

efficiency = useful energy transferred by the device ÷ total energy supplied to the device

How do you express an efficiency as a percentage?

e.g. efficiency = 0.7

Multiply by 100.
0.7 × 100 = 70%

In physics, what is meant by a 'system'?

The single object, or group of objects, that you have chosen to study.