Work, Energy and Power Flashcards
State the equation for Gravitational Potential Energy and define G.P.E.
G.P.E = mgh
G.P.E. is the energy a mass has due to its position in a gravitational field
Describe the change in G.P.E and K.E. for a falling object
As the object falls, its G.P.E decreases because it is converted to K.E. (in the form of velocity)
½mv² = mgΔh
Define work done
Work Done (energy transferred) = Force X distance moved in the direction of the force
W=Fs
Explain why the energy transferred in a falling object is equal to the work done.
Force (F) = Weight (mg)
Displacement (s) = change in height (h)
Therefore Fs = mgh
So W = mgh
Define a machine
Something that makes the work easier
Explain how a ramp/inclined plane acts as a machine
It allows a smaller force to be applied by ‘spreading it’ over a larger distance
What does the area under the curve show in a Force vs. Displacement graph?
Work Done (Energy Transferred)
Define Power and state the equation needed to calculate it
The rate of energy transfer
Power (J/s) = Work Done (J) / Time Taken (s)
P = W / t
Show that P = Fv
Power = Work / Time Power = (Force X Displacement) / Time Displacement / Time = Velocity Therefore Power = Force X Velocity P = Fv
State the equation for Kinetic Energy
K.E. = ½mv²
Give the 6 forms of energy
1) Potential energy
2) Kinetic energy
3) Thermal energy
4) Chemical/nuclear energy
5) Electrical energy
6) Elastic energy
Define joules
A measurement of energy where 1 joule is equal to the energy needed to raise a 1N weight through a vertical height of 1m
State the principle of conservation of energy
Energy cannot be created or destroyed
Give the equation for the work done to stretch a spring to extension ΔL
Work done = ½FΔL
Define kinetic energy
The energy of an object due to its motion
Define potential energy
The energy of an object due to its position
Define thermal energy
The energy of an object due to its temperature
Define chemical/nuclear energy
The energy associated with chemical or nuclear reactions
Define electrical energy
The energy of electrically charged objects
Define elastic energy
The energy stored in an object when it is stretched or compressed
Give the equation for the kinetic energy of a pendulum bob at height h, released from height h₀
kinetic energy = loss of potential energy from maximum height
½mv² = mg (h₀ - h)
Give 2 ways in which energy can be transferred from one object to another
1) Work done - by a force due to one object making the other object move
2) Heat transfer - from a hot object to a cold object. Heat transfer can be due to conduction, convection or radiation
Give the 3 ways heat can be transferred
Conduction
Convection
Radiation
State the name for the name for the output power of an engine
Motive power
Describe the resistive forces in relation to the motive force when an object moves at a constant velocity at constant height
The resistive forces (e.g. friction, drag, air resistance) are equal and opposite to the motive force
Give the equation for the power of an engine in terms of its velocity
Work done per second = force x distance moved per second
P = Fv
Describe the motion of an object when its output force exceeds the resistive forces on it
It speeds up
Give the equation for the motive power of an object in terms of energy wasted
Motive power = energy wasted per second (due to resistive forces) + Gain of kinetic energy per second
Give the equation for the output of a machine
P(out) = Work done by the machine / time taken
Output power, P(out) = Fv
where F = output force of machine and v = velocity of object
Define useful energy
The energy transferred for a purpose
Give the equation for efficiency of a machine
Efficiency = Useful energy transfered / energy supplied
= work done / energy supplied
= output power / input power
Give the equation for percentage efficiency of a machine
% Efficiency = efficiency x 100%
Give the average electrical power generated by a single wind turbine in a suitable location
2MW
Give the equation for the kinetic energy per second of the wind passing through a wind turbine
Kinetic Energy per second = ½ρv²vA = ½ρv³A
where ρ is the density of air, v is the speed of the wind and A is the area of the cylinder of air passing through the turbine
Describe the principle of how hydroelectricity and tidal power stations work
They make use of the potential energy released by water when it runs to a lower level
Describe the difference between a tidal power station and a hydroelectric power station
Tidal power stations drop more water but over a smaller distance than a hydroelectric power station
Tidal power stations release more energy per day
Explain why solar panels in space would absorb more solar energy than solar panels at the Earth’s surface
Because some solar energy is absorbed in the atmosphere
Describe how a solar panels produces electricity
A solar panel produces electricity directly
A potential difference is produced across each cell when light is incident on the cell
This converts light energy into electrical energy
Give 2 ways in which energy demands could be reduced
Better insulation in homes and buildings
More efficient machines
Give 3 ways in which carbon emissions could be cut significantly
- Using more renewable sources of energy
- Carbon capture of carbon emissions from fossil-fuel power stations
- Road transport to be switched from fossil fuels
