Work, Power & Efficiency Flashcards

All 39 cards in this deck

What are the main energy stores?

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

Which energy store empties and which fills as an object falls freely?

e.g. a ball dropped from a window

Energy is transferred from the gravitational potential store of the ball to its kinetic store.

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

The amount of energy transferred — the wider the arrow, the more energy.

In a Sankey diagram for a device, what do the arrows that branch away from the main arrow show?

The wasted (dissipated) energy, usually energy transferred to the thermal store of the surroundings.

True or false? When a car brakes, energy is transferred from the kinetic store of the car to the thermal store of the brakes and surroundings.

True. The work done by friction in the brakes fills thermal stores.

What does the principle of conservation of energy say about a closed system?

Energy can be transferred, stored or dissipated, but there is no net change to the total energy of the system.

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

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

Which of the three ways of changing a system's energy is involved when a crane lifts a load?

Work done by a force (the lifting force moves the load).

Which energy store empties and which fills as a ball thrown vertically upwards rises?

Energy is transferred from the kinetic store of the ball to its gravitational potential store.

True or false? In a closed system, energy is destroyed when it is dissipated to the surroundings.

False. The total energy stays the same; it is just stored in less useful ways.

What is the equation for work done?

work done (J) = force (N) × distance moved in the direction of the force (m)

What is the relationship between energy transferred and work done?

They are equal: energy transferred (J) = work done (J).

What is 1 joule written in newton metres?

$1\text{ J} = 1\text{ N m}$ — one joule of work is done when a force of 1 N moves an object 1 m in the direction of the force.

What are the steps to find the force from the work done and the distance moved?

e.g. a cyclist does 1600 J of useful work travelling 28 m

1. Write the equation: work done = force × distance, so $1600 = F \times 28$
2. Rearrange: $F = 1600 \div 28$
3. Force = 57 N

What are the steps to measure the work done by a force in the laboratory?

e.g. pulling a wooden block along a bench

1. Measure the pulling force with a newtonmeter: e.g. 5 N
2. Measure the distance moved in the direction of the force with a ruler: e.g. 2 m
3. Multiply: work done = $5 \times 2 = 10$ J

True or false? A porter carrying a suitcase horizontally at a steady height does work against gravity.

False. The upward force is at right angles to the motion, so no distance is moved in the direction of the force and the work done is zero.

What is the equation for the change in gravitational potential energy when an object is raised?

change in GPE (J) = mass (kg) × gravitational field strength (N/kg) × change in vertical height (m)

What is the equation for the kinetic energy of a moving object?

kinetic energy (J) = $\frac{1}{2}$ × mass (kg) × (speed)$^2$ (m/s)

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

False. Kinetic energy is proportional to speed squared, so it becomes four times as great.

What are the steps to find the height an object was raised from its gain in gravitational potential energy?

e.g. a 2 kg mass gains 60 J, $g = 10$ N/kg

1. Write the equation: $\Delta GPE = m g \Delta h$, so $60 = 2 \times 10 \times \Delta h$
2. Rearrange: $\Delta h = 60 \div 20$
3. Height = 3 m

What are the steps to find the speed of an object that has fallen from a height?

e.g. a ball dropped from 5 m, $g = 10$ N/kg

1. GPE lost = KE gained: $mg\Delta h = \frac{1}{2}mv^2$
2. Mass cancels: $v^2 = 2g\Delta h = 2 \times 10 \times 5 = 100$
3. $v = 10$ m/s

What are the steps to find the kinetic energy lost by a vehicle from the braking force and distance?

e.g. a braking force of 500 N acts over 20 m

1. Work done = force × distance = $500 \times 20 = 10\,000$ J
2. Work done by the brakes = kinetic energy lost
3. Kinetic energy lost = 10 000 J

What does it mean to say that energy is dissipated?

It is transferred to the surroundings and stored in less useful ways, so it is no longer available to do useful work.

Where is energy dissipated to when friction acts between the moving parts of a machine?

To the thermal store of the parts and then the surroundings, raising their temperature.

Why is a mechanical process wasteful when it causes a rise in temperature?

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

What happens to the energy supplied to a filament lamp that is not usefully transferred?

It is dissipated by heating — stored in the thermal store of the lamp and the surroundings.

True or false? Energy that has been dissipated to the surroundings can still be usefully transferred.

False. It is spread out among the surroundings and stored in less useful ways.

How is power defined?

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

One watt is equal to how many joules per second?

$1\text{ W} = 1\text{ J/s}$

What is the equation for power in terms of work done?

power (W) = work done (J) ÷ time taken (s)

What are the steps to find the time taken for a device to transfer a given amount of energy?

e.g. a 50 W lamp transferring 400 J

1. Write the equation: power = energy transferred ÷ time
2. Rearrange: time = $400 \div 50$
3. Time = 8 s

What are the steps to calculate a person's power output when climbing stairs?

e.g. a 60 kg person climbs 4 m in 10 s, $g = 10$ N/kg

1. Work done = $mg\Delta h = 60 \times 10 \times 4 = 2400$ J
2. Power = work done ÷ time = $2400 \div 10$
3. Power = 240 W

True or false? If two machines transfer the same amount of energy, the one that takes less time has the greater power.

True. Power is the energy transferred each second.

What is the equation for efficiency in terms of energy?

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

What is the equation for efficiency in terms of power?

efficiency = useful power output ÷ total power input

What are the steps to find the useful energy output of a device from its efficiency?

e.g. efficiency 0.70 with 6500 kJ supplied

1. efficiency = useful output ÷ total input
2. Rearrange: useful output = efficiency × input = $0.70 \times 6500$
3. Useful output = 4550 kJ

What are the steps to find the energy wasted by a device from its efficiency?

e.g. 200 J supplied, efficiency 0.25

1. Useful energy = $0.25 \times 200 = 50$ J
2. Wasted = total supplied − useful = $200 - 50$
3. Wasted energy = 150 J

What are the steps to write an efficiency given as a decimal as a percentage?

e.g. efficiency 0.35

1. Multiply by 100: $0.35 \times 100 = 35$
2. Add the percent sign: efficiency = 35%

True or false? A device can have an efficiency greater than 1 (100%).

False. The useful output can never be more than the total energy supplied.