What is a system?
An object or group of objects
What happens to the system when the energy changes?
The system will then change
Explain what happens and the changes when you throw a ball up to the air
- The initial force exerted by a person to throw a ball upwards does work.
- It causes an energy transfer from the chemical energy store of the person’s arm to the kinetic energy store of the ball and arm.
Explain what happens and the changes when a moving object hits an obstacle
- In a collision between a car and a stationary object, the normal contact force between the car and the object does work.
- It causes energy to be transferred from the car’s kinetic energy store to other energy stores, e.g. the elastic potential and thermal energy stores of the object and the car body.
- Some energy might also be transferred away by sound waves.
Explain what happens and the changes when an object is accelerated by a constant force
- A ball dropped from a height is accelerated by gravity.
- The gravitational force does work.
- It causes energy to be transferred from the ball’s gravitational potential energy store to its kinetic energy store.
Explain what happens and the changes when a vehicle slows down
- The friction between a car’s brakes and its wheels does work as it slows down.
- It causes an energy transfer from the wheels’ kinetic energy stores to the thermal energy store of the surroundings.
Explain what happens and the changes when bringing a water to boil in an electric kettle
- The water as the system.
- Energy is transferred to the water (from the kettle’s heating element) by heating, into the water’s thermal energy store (causing the temperature of the water to rise).
Explain what happens and the changes when bringing a water to boil in an electric kettle
- You could think of the kettle’s heating element and the water together as a two-object system.
- Energy is transferred electrically to the thermal energy store of the kettle’s heating element, which transfers energy by heating to the water’s thermal energy store.
What is the equation for kinetic energy?
Kinetic energy(J) = 0.5 x mass(kg) x speed^2(m/s)
E(k) = 1/2mv^2
What is the equation for elastic potential energy? (Assuming the limit of proportionality has not been exceeded)
Elastic potential energy(J) = 0.5 x spring constant(N/m) x extension^2(m)
E(e) = 1/2ke^2
(Equation is in the physics equation sheet)
What is the equation for gravitational potential energy?
Gravitational potential energy(J) = mass(kg) x gravitational field strength(N/kg) x height(m)
E(p) = mgh
How do you find the change in thermal energy?
Change in thermal energy(J) = mass(kg) x specific heat capacity(J/kg C) x temperature change(C)
△E = mc△θ
Practical / Just in case for later
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What is power?
The rate at which energy is transferred or the rate at which work is done
State the two equations to calculate power
Power(W) = Energy transferred(J) / Time(s)
P = E/t
Power(W) = Work done (J) / Time(s)
P = W/t
What is an energy transfer of 1 Joule equal to?
1 Watt
Give an example that illustrates the definition of power
- Take two cars that are identical in every way apart from the power of their engines
- Both cars race the same distance along a straight race track to a finish line.
- The car with the more powerful engine will reach the finish line faster than the other car- i.e. it will transfer the same amount of energy but over less time.
What is the conservation of energy principle?
Energy can be transferred usefully, stored or dissipated,but can never be created or destroyed.
What is a ‘closed system’?
- Closed systems are systems where neither matter nor energy can enter or leave.
- The net change in the total energy of a closed system is always zero.
Give an example of an energy transfer for closed systems
- A cold spoon is dropped into an insulated flask of hot soup, which is then sealed.
- You can assume that the flask is a perfect thermal insulator so the spoon and the soup form a closed system.
- Energy is transferred from the thermal energy store of the soup to the useless thermal energy store of the spoon (causing the soup to cool down slightly).
- Energy transfers have occurred within the system, but no energy has left the system - so the net change in energy is zero,
Why is dissipated energy called ‘wasted energy?’
As the energy is being stored in a way that is not useful (usually energy has been transferred into thermal energy stores).
Give an example of how the system changes when there is dissipated energy
- A mobile phone is a system.
- When you use the phone, energy is usefully transferred from the chemical energy store of the battery in the phone.
- But some of this energy is dissipated in this transfer to the thermal energy store of the phone (you may have noticed your phone feels warm if you’ve been using it for a while).
What can lubricants do to frictional forces?
- For objects that are being rubbed together, lubricants can be used to reduce the friction between the objects’ surfaces when they move.
- Lubricants are usually liquids (like oil), so they can flow easily between objects and coat them.
The higher the thermal conductivity of a material the higher….?
The rate of energy transfer by conduction across the material
