Work, Power and Efficiency Flashcards
Includes chapter 1
what is a closed system
a system where no energy can be transferred to or from the surroundings - the total energy stays the same
what is work done
energy transferred when a force moves an object
what is the unit for energy
joules (j)
what is one joule of work
the work done when a force of 1N causes an object to move 1m in the direction of the force
describe the energy transferred when a car slows down
energy is transferred mechanically from the kinetic store of the car to the thermal store of the brakes - some energy is dissipated to the thermal store of the surroundings
describe the transfer of energy when an electric kettle is used to heat water
the electric current in a kettle transfers energy to the heating elements thermal store - energy is then transferred by heating from the heating elements thermal store to the store of the water
describe the energy transferred when a battery powered toy car is used
energy is transferred electrically from the chemical store of the battery to the kinetic store of the car - some energy is dissipated to the thermal store of the surroundings
describe the energy transferred when a falling apple hits the ground
energy is transferred from the kinetic store of the apple and dissipated to the thermal store of the surroundings by sound waves
name the unit that represents one joule transferred per second
watt (W)
a motor is 30% efficient what does that mean
30% of the energy is usefully transferred and 70% is dissipated
How do we calculate work done?
Work done = force × distance
W = Fd
What units do we use for the work done equation
Work (Joules, J), Force (Newtons, N), Distance (metres, m)
What properties does a moving object have?
A moving object has mass and velocity (speed in a direction)
What is the kinetic energy store of an object?
The kinetic store of an object relates to its motion. The faster an object moves, the more energy it has in its kinetic store.
A moving object also has mass and if a larger mass travels at the same speed it would have more energy in its kinetic store.
How would we calculate the amount of energy in the kinetic store of an object?
Kinetic energy = 1/2 × mass × velocity2
Ek = 1/2mv2
What units do we use for kinetic equation
Kinetic Energy (Joules, J), Mass (kilograms, kg), Velocity (metres per second, m/s)
What is the elastic potential energy store of an object?
The elastic potential store of an object relates to how stretched it is. The more stretched (extended) it is, the more energy it has in its elastic store.
How would we calculate the amount of energy in the elastic potential store of an object?
Elastic potential energy = 1/2 × spring constant × extension2
Ee = 1/2ke2
what units do we use for elastic potential equation
Elastic Potential Energy (Joules, J), Spring Constant (Newtons per metre, N/m), Extension (metres, m)
What is the gravitational potential energy store of an object?
The gravitational potential store of an object relates to how high above the Earth’s surface it is. The higher it is, the more energy it has in its gravitational potential store.
How would we calculate the amount of energy in the gravitational potential store of an object?
Gravitational potential energy = mass × gravitational field strength × height
Ep = mgh
Units for GPE equation
Gravitational Potential Energy (Joules, J), Mass (kilograms, kg), Gravitational field strength (Newtons per kilogram, N/kg), Height (metres, m)
gravitational field strength on the Earth
9.8 N/kg
gravitational field strength on the moon
1.6 N/kg
What is power?
Power is the rate of transferring energy, also considered as the rate of doing work.
How would we calculate the power of an energy transfer?
power = work done/time or energy transferred/time
P = W/t or E/t
Units for power equation
Power (Watts, W), Work done (Joules, J), Time (seconds, s)
What does it mean if a kettle has a 3 kW rating?
This kettle transfers 3000 J of energy to the thermal store of the water in it per second.
This kettle does 3000 J of work to the water in it every second.
What is the law of conservation of energy?
Energy cannot be created or destroyed, only stored and transferred.
How do we apply the law of conservation of energy
The total energy output from a device is equal to the total energy input.
What do we mean for a device to be
efficient?
If a device is efficient it transfers its energy usefully.
If a device is inefficient what does it do?
If a device is inefficient it loses energy to the surroundings (usually by heating), we call this energy dissipated or wasted, as it is transferred to less useful stores
How do we calculate the efficiency of an
energy transfer?
efficiency = useful output transfer/total input
for a percentage ×100
This equation can be used with both energies and powers.
