3.1 Energy Principles
Energy is the ability to do work. It is a scalar quantity measured in joules (J). Energy is not "used up"; it is just transferred from one store to another.
A system is just a word for the object or group of objects we are looking at. A closed system is one where no energy can enter or leave.
The Law of Conservation of Energy states that energy cannot be created or destroyed, only transferred, stored, or dissipated (spread out).
The 8 Energy Stores
You need to know these 8 ways energy can be stored:
- Kinetic: The energy of a moving object.
- Gravitational Potential (GPE): The energy stored in an object due to its height in a gravitational field.
- Elastic Potential: The energy stored in a stretched or squashed object (like a spring).
- Thermal: The energy of an object due to its temperature (the kinetic energy of its particles).
- Chemical: The energy stored in chemical bonds (e.g., in food, fuel, and batteries).
- Nuclear: The energy stored in the nucleus of an atom.
- Magnetic: The energy stored when repelling magnets are pushed together.
- Electrostatic: The energy stored when repelling charges are pushed together.
Energy is moved from one store to another through one of four pathways:
- Mechanically: A force moves an object (e.g., pushing a box, which is "work done").
- Electrically: A charge moves through a potential difference (e.g., a current in a wire).
- By Heating: Energy transfer from a hotter object to a cooler object.
- By Radiation: Energy transferred by waves (e.g., light from the Sun or sound waves).
Kinetic Energy (KE) is the energy of motion. The more mass and the more speed an object has, the more kinetic energy it has.
Kinetic Energy = 0.5 × Mass × (Speed)²
KE = ½ × m × v²
Where:
- KE is Kinetic Energy in joules (J)
- m is mass in kilograms (kg)
- v is speed in metres per second (m/s)
Gravitational Potential Energy (GPE) is the energy stored by lifting an object against gravity.
GPE = Mass × Gravitational Field Strength × Height
GPE = m × g × h
Where:
- GPE is Gravitational Potential Energy in joules (J)
- m is mass in kilograms (kg)
- g is Gravitational Field Strength (9.8 N/kg on Earth)
- h is the change in height in metres (m)
When a ball is thrown upwards (1), its kinetic energy is converted into gravitational potential energy as it rises. At its maximum height (2), GPE is at its highest, and KE is zero. As it falls back down (3), GPE is converted back into KE.
Work Done is the scientific term for energy transferred mechanically (by a force). When you push a box across the floor, you are doing work against the force of friction.
Work Done = Force × Distance
W = F × d
Where:
- W is Work Done in joules (J)
- F is the force in newtons (N)
- d is the distance moved in the direction of the force, in metres (m)
Power is the rate at which energy is transferred, or the rate at which work is done. It is measured in watts (W), where 1 watt = 1 joule per second.
Power = Energy Transferred / Time
P = E / t
You can also say:
Power = Work Done / Time
P = W / t
Where:
- P is Power in watts (W)
- E or W is Energy/Work in joules (J)
- t is time in seconds (s)