P1 Conservation and dissipation of energy Flashcards
Name 8 energy stores.
Chemical, elastic potential, thermal, kinetic, gravitational potential, electrostatic, nuclear, magnetic.
What is the unit of energy?
Joules (J).
State the 4 ways in which energy can be transferred.
- By a force moving an object
- By an electric current
- By waves e.g. light and sound
- By heating.
What is a system?
An object or a group of objects.
What is a closed system?
A system where no energy can be transferred to or from the surroundings – the total energy in the system stays the same.
What is the principle of the conservation of energy?
Energy cannot be created or destroyed. (It can only be transferred into different forms.)
What is the term used when energy transfers between stores?
When energy transfers between stores we say ‘work is done’.
What is the unit of work done?
Joules (J).
What is the equation for mechanical work done?
Mechanical work done = Force x distance moved (along the line of action of the force)
W = F s; Work done (J), Force (N), distance (m).
What is the equation for electrical work done?
Energy transferred = charge flow x potential difference
E = Q V;
Energy transferred (J), Charge flow (C), Potential difference (V).
What is the equation for change in gravitational potential energy?
Change in gravitational potential energy = mass x Gravitational Field Strength x change in height
∆Ep = m x g x ∆h;
Energy (J), mass (kg), Gravitational Field Strength (N/kg), Height (m).
What is the equation for kinetic energy?
Kinetic energy = ½ x mass x (speed)²
Ek = ½ m v²;
Energy (J), mass (kg), speed (m/s).
What is the equation for elastic potential energy?
Elastic potential energy = ½ x Spring Constant x (Extension)²
Ee = ½ k e²;
Energy (J), Spring Constant (N/m), Extension (m).
What happens when energy is transferred for a purpose in a system?
Some of the energy is transferred usefully (it goes where you want it to go in the form we need). Some of the energy is wasted (transfers to a store we don’t want).
What eventually happens to wasted energy?
It is transferred to the thermal store of the surroundings, causing them to get warmer – we say it is dissipated.
What is friction?
Friction is a force at the surface of an object as it moves, or tries to move, through or over other particles. Friction always opposes the motion.
How is energy dissipated when objects move?
When objects move there is friction between surfaces. Work is done overcoming the frictional force. This transfers some energy into the thermal energy store of the objects. This thermal energy is dissipated into the thermal energy store of the surroundings by heat and sound.
How is energy dissipated when it is transferred between stores by an electric current?
Electric cables warm up when an electric current flows through them. This thermal energy is dissipated into the thermal energy store of the surroundings by heat.
What equation links total energy transferred, useful energy and wasted energy?
Total energy transferred =
useful energy + wasted energy.
What is efficiency?
The proportion of energy that is usefully transferred.
What equation links efficiency, total energy input and useful energy output?
efficiency = useful output energy transfer / total input energy transfer
Useful output energy (J), Total input energy (J), Efficiency does not have any units – it is a number between 0 and 1.
How do you represent efficiency as a percentage?
Multiply the efficiency formula by 100.
Why can efficiency not be greater than 1 (or 100%)?
The law of conservation of energy states that you can’t get more energy out of a device than you put into it.
What is efficiency of a device?
A measure of how good a device is at transferring energy input to useful energy output.
