P1: Energy and energy sources Flashcards
What are the 5 different energy stores?
Chemical energy
Kinetic energy
Gravitational potential energy (GPE)
Elastic potential energy
Thermal energy
How can energy be transferred?
Energy can be transferred from one store to another by:
1. Heating
2. Waves
3. Electric current
4. When a force moves an object
What happens when an object falls and gains speed?
Its gravitational potential (GPE) energy decreases
Its kinetic energy store increases
What happens when a falling object hits the ground without bouncing back?
It’s kinetic energy store is transferred:
-> By heating to thermal energy store of the object and its surroundings
-> By sound waves moving away from the point of impact
Describe the principle of the conservation of energy?
Energy cannot be created or destroyed
Energy can only be transferred usefully, stored or dissipated
What is a ‘closed system’?
An isolated system where no energy transfers take place into or out of the system
What can happen to energy within a closed system?
It can be transferred between energy stores within the closed system
What happens to the total energy in a closed system when there is transferred?
The total energy of the closed system is always the same before and after any transfers of energy
What happens in terms of energy transfers when someone stretches an elastic band?
The chemical store energy in their muscles decreases and is transferred to the elastic potential energy of the elastic band which increases
What happens in terms of energy transfers in a pendulum swing?
What happens at the middle? What happens at the end?
When the pendulum is at one end, it’s gravitational potential energy (GPE) is at it’s maximum
When the pendulum swings to the middle it’s gravitational potential energy (GPE) decreases and its kinetic energy is at its maximum
When the pendulum moves away from the middle to the other end, its kinetic energy decreases and the gravitational energy (GPE) increases and is at it’s maximum when it arrives at the end
What needs to happen to a stationary object for it to move?
A force needs to be applied to a stationary object to make it move
What happens when a force moves an object? what is this called?
When a force moves an object - energy is transferred to an object
We can say that WORK is done on the object
What is the work done on an object equal to?
The work done on an object is equal to the energy transferred to the object
What is the units for work and energy?
Work and energy are both measured in the same units:
J - Joules
How can you work out the work done on an object?
Work done = Force X Distance
W = F S
Where work is measured in J (Joules), Force is measured in N (Newtons) and distance moved is measured in m (metres)
How much work is done on an object if it doesn’t move when energy is transferred to it?
No work is done on the object
W = F x 0 (as the distance moved is 0)
Hence
W = 0
What force opposes the motion of two surfaces in contact with each other?
Friction
What happens to work that is done to overcome friction?
It is mainly transferred to thermal energy stores by heating
When does the gravitational potential energy (GPE) of an object change?
It changes due to the objects position in relation to earth’s gravitational field
When an object moves up - its gravitational potential (GPE) increases
When an object moves down - its gravitational potential (GPE) decreases
What is the work done when an objects gravitational potential (GPE) energy changes?
When an object moves up, the work done on the object by the lifting force is equal to the increase in gravitational potential energy (GPE)
When an object moves down, the work done on the object by the gravitational force is equal to the decrease in gravitational potential energy (GPE)
What is the equation for change in gravitational potential energy when an object is moved up or down?
Δ E = m x g x Δh
Where:
ΔE = Change in GPE measured in J (Joules)
m = mass of the object in kg (kilograms)
g = gravitational strength in N/kg (Newtons per kilogram)
Δh = change in height m (metres)
What is the gravitational field strength of earth and of the moon?
The gravitational field strength at earths sufrace = 9.8 N/kg
The moon’s gravitational field strength is 1/6th of the earth’s
What energy do all objects that move have?
All moving objects have kinetic energy
What is the equation for kinetic energy?
Ek = 1/2 x m x v2
Where:
Ek = Kinetic energy measured in J
m = mass of object measured in kg
v = speed of the object in m/s (metres per second)
What two things can increase the kinetic energy has?
Increasing mass
Increasing speed
What two things can decrease the kinetic energy an object has?
Decreasing mass
Decreasing speed
What is elastic potential energy?
This is the energy that is stored in an elastic object that has been stretched or squashed
When can you describe an object as being elastic?
An object is elastic if it regains its original position after being stretched / squashed
What happens when an elastic object is stretched or squashed?
When an elastic object is stretched / squashed the work done on it is stored as elastic potential energy
What happens when an elastic object returns to its original shape?
When an elastic object returns to its original position, the energy becomes available for other transfers
How can you work out the elastic potential of an object?
Ee = 1/2 x k x e2
Where:
Ee = Elastic potential energy in J
k = Spring constant of the spring in N/m (Newtons per metre)
What do machines do to energy?
They transfer energy for a purpose
What is useful energy?
Useful energy is energy that is transferred to where it is wanted, in the form that it is wanted
What is wasted energy?
Wasted energy is energy that is either transferred to where it is not wanted or in a form that is not wanted
Describe dissipated energy
This is energy that is not usefully transferred and is stored in less useful ways
What happens to both useful and wasted energy over time?
Both useful and wasted energy will be transferred into the surroundings and be dissipated - warming up the surroundings
What happens as energy dissipates
As energy dissipated as goes further away it becomes harder to use it for future energy transfers
How is energy often wasted with moving objects?
Energy is often wasted due to friction with moving object or the moving parts of a machine
What happens in terms of energy transfer when a bike / car brakes?
When bike / car brakes:
The kinetic energy store of the vehicle decreases and the thermal energy stores of the brakes and surroundings increase
What is the energy supplied to a machine called?
The energy supplied to a machine called - input energy
What is the energy that is usefully transferred by the machine called?
The energy that is usefully transferred by the machine called - the useful output energy
What is the input energy equal to?
According to the principle of the conservation of energy: energy cannot be created or destroyed
Hence:
Input energy = useful output energy + wasted energy
How can a machine be more efficient?
A machine is more efficient if it wastes less energy
How can you calculate the efficiency of a device? What are the units?
Efficiency = useful energy transferred ÷ total energy supplied x 100
There are no units as efficiency is a ratio
What is the maximum efficiency a machine can have?
The maximum efficiency of a machine is 1 (100%) - it cannot be more than 100% as this would mean that energy out is greater than energy in
What are some ways machines can waste energy?
Friction between moving parts
Air resistance
Electrical resistance
How can machines be made more efficient?
Machines can be made more efficient by reducing how much energy they waste
How can a machine with moving objects be made more efficient?
Using lubrication decreases friction between the moving objects and makes it more efficient
How can a vehicle be made more efficient?
Using a stream-lined design reduces air resistance and makes it more efficient
How can electrical devices be made more efficient?
Using copper wires decreases electrical resistance and makes it more efficient
What is the only device that can be 100% efficient and why?
A heater as it can transfer all the energy supplied to it as useful heat energy into its surroundings
How is most energy in the home supplied as?
Most energy in the home is supplied as electricity, gas or oil
What is the useful energy transfer in a:
light bulb
Electric energy into light waves emitted from the filament
What is the useful energy transfer in a:
electric mixer
electric energy into work done by blades of the mixer
What is the useful energy transfer in a:
speaker
Electric energy into sound waves from vibrations of the speaker
What is the useful energy transfer in a:
television
Electric energy into light and sound waves
What should electrical appliances aim to do in terms of efficiency?
They should aim to waste as little energy as possible and be as efficient as possible
What is the power of an appliance?
The power of an appliance is the rate at which it transfers energy
What is the unit of power?
Power is measured in Watts (W)
What is 1 watt equal to?
1 watt is equal to transferring 1 joule in 1 second
1 W = 1 J/s
What can be used to measure power instead of watts and why?
Watts are too small to be useful so power is often measured in kilowatts (kW)
Where 1000 W = 1 kW
What is the equation for power
P = E / t
Where:
P = Power measured in W
E = Energy transferred in J
t = Time measured in s
How can you work out power wasted?
Power wasted = total power supplied - useful power output
How can efficiency of a device be calculated in relation to power?
efficiency = (useful power output ÷ total power input) x 100
