Physics GCSE Notes!

Energy
Energy is never used up. When a system changes, the energy only transfers. A
system just refers to an object(s).
A closed system: when energy (or matter) cannot enter or exit the system. So,
the overall change in the total energy of a closed system is zero.
During the heating process, energy is transferred.
For example, when a kettle is heated, energy is transferred from electrical to
thermal (heat).
Energy can also be transferred by doing work. ‘Work done’ is the same as
saying energy transferred. There are a number of ways in which work can be
done:
 When current flows in an electric circuit
 When a force causes an object to move
For example, when a ball is dropped from the sky. The energy transfers from
gravitational potential energy to the kinetic (movement) energy store.
So, energy transfers refer to the different stores that energy goes through.
The different energy stores are:
 Thermal energy
 Kinetic energy
 Gravitational potential energy
 Elastic potential energy
 Chemical energy
 Magnetic energy
 Electrostatic energy
 Nuclear energy


We need to also know how to calculate the stored energies:
 Kinetic energy store:
To calculate kinetic energy, you need to use the formula below:

,Kinetic energy depends on an object’s speed (velocity) and mass. So, the higher
the mass and the faster the object is going, this means there will be more
energy in the kinetic energy store.
Let’s do a question:
A motorbike weighs 1000kg and is travelling at a speed of 10 m/s. Calculate the
kinetic energy of the motorbike.
1
x 1000 x 10² = 50,000 J
2

 Gravitational potential energy store
Lifting an object into the air causes energy to be stored in the gravitational
potential energy stores.
The higher the object is lifted into the air; the more energy is transferred to
this store.
The amount of energy in the gravitational potential energy stores is dependent
on the height, mass and strength of the gravitational field in the object. Below
is the equation to work out the gravitational potential energy.




When objects fall, energy is also transferred to kinetic energy (movement
energy) from gravitational potential energy.
Air resistance is a force that tends to work against falling objects, so some
energy is transferred to other stores.

,However, when there is 0 air resistance, all the energy lost from the
gravitational potential energy store is the same as the energy gained in the
kinetic energy store.
 Elastic potential energy store
Stretching an object also transfers energy to a store known as elastic potential
energy store.
Below is the equation to work out the energy in an elastic potential energy
store.




Power: rate of doing work
In other words, the rate at which energy transfers per second
Power is measured in watts. Note: one watt = 1 joule




Above is the formula to calculate power (W).
Remember: energy is never destroyed but can be wasted.

, Conservation of energy:
This principle tells us that energy can be stored or dissipated (wasted) but can
never be destroyed or created.
Between stores, energy is transferred. However, some of the energy is wasted.
In a closed system, the energy total change is always 0.


To calculate energy efficiency for energy transfers:
A device is said to be ‘useful’ if wastes a small amount of energy. To calculate
the efficiency of a device, we use the formulas below:




Let’s look at an example:
If a microwave is 0.7 efficient and has a total power input of 600W, work out
the useful power output.
0.7 x 600 = 420 W
Note: no device is 100% efficient.
Renewable and non-renewable energy sources
Renewable resources are those which never run out.