Work, Power & Efficiency Flashcards
All 23 cards in this deck
What are the three ways the energy of a system can be changed?
By work done by forces, by electrical equipment, and by heating.
Describe the energy transfer when an object falls towards the ground.
Energy is transferred from the gravitational potential store of the object to its kinetic store.
Describe the energy transfer when a moving car brakes to a stop.
Energy is transferred from the kinetic store of the car to the thermal store of the brakes and surroundings.
Describe the energy transfer when a kettle heats water.
Energy is transferred electrically from the mains to the thermal store of the water.
What does the width of each arrow on a Sankey diagram represent?
The amount of energy transferred — the wider the arrow, the more energy.
On a Sankey diagram, how are useful and wasted energy transfers shown?
The useful energy continues straight on; the wasted energy branches off, usually downwards.
What happens to the total energy in a closed system when energy transfers take place inside it?
There is no net change — the total energy stays the same.
True or false? Doing 500 J of work on an object transfers 500 J of energy to it.
True. Energy transferred (J) is equal to work done (J).
What is the equation for work done?
work done = force × distance moved in the direction of the force, — unit: joule (J), also written N m.
How can you measure the work done by a force pulling a block along a bench?
Measure the force with a newtonmeter and the distance moved in the direction of the force with a ruler, then work done = force × distance.
True or false? To calculate the work done against friction when a box is pushed up a slope, you use the vertical height of the slope.
False. You use the distance moved along the slope, because friction acts along the slope.
What is the equation for the change in gravitational potential energy?
— mass (kg) × gravitational field strength (N/kg) × change in vertical height (m).
What is the equation for kinetic energy?
— mass (kg) and speed (m/s), giving energy in joules.
What does it mean to say energy has been dissipated?
It has been transferred to less useful stores — usually the thermal store of the surroundings.
What makes a mechanical process wasteful?
It causes a rise in temperature, dissipating energy by heating the surroundings instead of transferring it usefully.
What is power?
The rate at which energy is transferred, i.e. the energy transferred each second.
What is one watt equal to?
One joule per second (1 W = 1 J/s).
What is the equation for power in terms of work done?
power (W) = work done (J) ÷ time taken (s), .
What is the equation for efficiency?
efficiency = useful energy transferred by the device ÷ total energy supplied to the device.
True or false? A device can have an efficiency greater than 100%.
False. The useful energy transferred can never be more than the total energy supplied.
What three measurements must be taken to find a student's power output as they climb a flight of stairs?
The weight of the student (N), the vertical height climbed (m) and the time taken (s).
Name any four energy stores.
e.g. kinetic, gravitational potential, thermal and chemical (others: elastic potential, magnetic, electrostatic, nuclear).
What is meant by work done on an object?
The energy transferred when a force moves the object through a distance in the direction of the force.