Physics - 1 Conservation and Dissipation of Energy

How many energy stores are there?

8

What are the 8 energy stores?

- magnetic
- thermal
- chemical
- kinetic
- electrostatic
- elastic potential
- gravitational potential
- nuclear

How many energy transfers are there?

4

What are the 4 energy transfers?

- mechanical work
- electrical work
- heating
- radiation

Sankey diagram

Represents useful and wasted energy with arrows

What is the law of conservation of energy?

energy cannot be created or destroyed, only transferred

Work

applied force that causes an object to move

'Work done' is another way of saying...

energy transferred

Equation for work done:

work done = force x distance

What is work measured in?

J (or Nm)

Where does work done to overcome friction mostly go?

into thermal energy stores via heating

What does friction do?

oppose motion

Why do brakes work? [2]

- friction opposes the motion of the wheel
- the kinetic energy of the wheels and vehicle is transferred by heating to the thermal stores of the brakes and the wheels

In equations, gravitational energy store is:

Eₚ

The force you need to lift an object is equal and opposite to the...

gravitational force of the object

Equation for gravitational potential energy:

Δgravitational potential energy = mass x gravitational field strength x Δheight

gravitational potential energy is measured in:

Joules, J

Equation for weight:

weight = mass x gravitational field strength

kinetic energy

the energy an object has due to its motion

Equation for kinetic energy:

kinetic energy = ½ x mass x speed²

Kinetic energy is also written as:

Eₖ

Symbol equation for kinetic energy:

Eₖ = ½mv²

Elastic potential energy

the energy of stretched or compressed objects

Spring constant

a parameter that is a measure of a spring's resistance to being compressed or stretched

Hooke's Law

extension is directly proportional to force until the spring reaches its elastic limit

Hooke's Law equation:

Force = spring constant x extension [F=ke]

Equation for elastic potential energy:

elastic potential energy = ½ x spring constant x extension²

Symbol equation for elastic potential energy:

Eₑ = ½ke²

What is spring constant measured in?

N/m

Useful energy

energy transferred to where it is wanted in the form it is wanted.

Wasted energy

energy that is not usefully transferred

dissipated energy

wasted energy that has 'spread out'

What are examples of wasted energy? [2]

- thermal energy (friction etc)
- sound waves

Combating friction in machinery [3]:

- lubrication
- removing irregularities (smoother)
- using smoother materials

Input energy

energy supplied to a device

Useful output energy

useful energy transferred by the device

Equation for efficiency:

efficiency = useful output energy ÷ total input energy

Efficiency limits

No device can be more than 100% efficient because you can never get more energy from a machine than you put into it

How could we investigate efficiency?

attach a joulemeter into the circuit of an electric winch and use a mass's increase in GPE to figure out its efficiency

Improving efficiency [4]:

- lubricate moving parts in machinery
- use wires with little electrical resistance
- streamline moving objects to reduce air resistance
- cut out noise (tighten loose parts to decrease vibration)

Light bulb [U:W]

- useful: light emitted from glowing filament
- wasted: energy transfer from filament heating surroundings

Electric heater [U:W]

- useful: energy heating surroundings
- wasted: light from glowing filament

Electric toaster [U:W]

- useful: energy heating bread
- wasted: energy heating toaster case and surroundings

Electric kettle [U:W]

- useful: energy heating water
- wasted: energy heating kettle and surroundings

Hairdryer [U:W]

- useful: kinetic energy of air from fan, energy heating air from filament
- wasted: sound of fan motor, energy heating hairdryer

Electric motor [U:W]

- useful: kinetic energy of object driven by motor, GPE of objects lifted
- wasted: energy heating motor, sound waves from motor

How does a clockwork radio function?

a spring is wound inside, increasing its elastic potential energy. This energy is transferred to its KE store as it turns an electric generator.

Power is...

the rate at which energy is transferred

The more powerful an appliance...

the faster it transfers energy

Power is measured in:

Watts, W

1 Watt =

1 J/s

Equation for power:

power = energy transferred OR work done ÷ time taken for transfer

Symbol equation for power:

P = E OR W ÷ t