Work, Power, Efficiency Flashcards
Define work
When a force acts on an object, causing it to move, then work is done.
Write out the word equation for work
Work = force x distance moved in direction of force
or
Work = force x displacement
Write out the symbol equation for work
W = F x s
Prove
The increase in gravitational potential energy equals what in terms of work done.
In an instant of walking up some stairs or the roller coaster car being raised
e.g. mass = 50kg
height = 20m
The increase in gravitational potential energy equals the work done
mass
Ep = mgh
Ep = 50 x 10 x 20
Ep = 10000J
W= F x s Force = weight (the force needed to walk up stairs is the weight) = m x g
W= (50 x 10) x 20
W= 10000
Units for:
Work
Force
Displacement
Work in Joules = J
Force in Newtons = N
Displacement in metres = m
Give examples that illustrate the definition of power
Two electric motors can lift the same weight through the same height but one does it faster
than the other. The work done is the same but the one which took the least amount of time has the larger power output
Fill in the gap
The more powerful an appliance is, the _____ the rate at which it transfers ________
The more powerful an appliance is, the faster the rate at which it transfers energy
Define power
Power is the work done or energy used in one second
Energy and work are the same thing
Energy and work are the same thing
Write out the word equation which links power work done and time
Power = work done / time
Write out the word equation which links power energy and time
Power = energy / time
Write down the word equation for power
Power = work done / time
Power = energy / time
Write down the symbol equation for power
P = W/t or P=E/t
Units for:
Power
Work done
Energy
Time
Power = Joules per second - J/s or Watts - W
Work done = Joules, J
Energy = Joules, J
Time - Seconds, s
what does 50 watts mean
50 joules of electrical energy transferred per second
What does 1 joule per second equal to
1 joule per second is equal to a power of 1 watt
What is a 1 joule
1 Joule is the amount of energy needed to move 1N, 1 metre
Define efficiency
The efficiency is the percentage of the input energy which is converted into a useful form
Write down the word equation for efficiency
Efficiency = useful energy output / total energy input x100
efficiency = useful output energy transfer / total input energy transfer
It is multiplied by 100 to convert the answer to a percentage.
A decimal efficiency is fine
Write down the word equation for efficiency
efficiency = useful power output/total power input x100
What does this statement mean?
Fridge A is more efficient than Fridge B
It means that Fridge A converts more of it’s input energy into a useful form
Describe how friction causes devices wastes energy
Friction causes devices to waste energy as Friction between the moving parts causes heating
Describe how the resistance wires causes devices wastes energy
The resistance of a wire causes the wire to get hot when a current passes through it
Describe how air resistance causes devices wastes energy
Air resistance causes a force on a moving object that opposes its motion
Energy transferred from the object to the surroundings by this force is wasted.
Describe how sound causes devices wastes energy
Sound created by machinery causes energy transfer to the surroundings
How to increase the efficiency of a motor and reduce the problem of friction
To increase efficiency, lubricate the moving parts of the motor to reduce friction
How to increase the efficiency of a circuit and reduce the problem of the resistance of wires
To increase efficiency, in circuits, use wires with as little electrical resistance as possible
How to increase the efficiency of a moving object and reduce the problem of air resistance
To increase efficiency, streamline the shapes of moving objects to reduce air resistance
How to increase the efficiency of machinery and reduce the problem of sound
To increase efficiency, cut out noise (for example, tighten loose parts to reduce vibration)
Explain why the percentage efficiency of a machine can never be more than 100%
efficiency = useful energy delivered/ input energy supplied
A machine can never be more than 100% efficient because the useful energy delivered can never be greater than the total energy supplied.
Explain why the efficiency of a machine can never be equal to 100% (4 marks)
A machine can never be 100% efficient since the useful energy delivered is always less than the total energy supplied
due to electric currents in circuit wires/components
and friction between moving parts,
energy is dissipated by transferring thermal energy to the surroundings
Fridge A is more efficient is more efficient than Fridge B
What does this statement mean?
This statement means:
Fridge A converts more of the input energy into a useful form
