P1: Conservation and dissipation of energy

System

An object or group of objects

What happens when a system changes?

Changes in the way energy is stored

Law of conservation of energy

Energy can’t be created or destroyed

• Only transferred usefully, stored or dissipated

Kinetic energy

Energy stored in moving objects

How much KE does a stationary object have?

0

Kinetic energy equation

How can energy be transferred?

1. Heating

2. Waves

3. Electric current

4. Force when it moves an object

Energy transfers for a car moving at a constant speed

Chemical energy store (in petrol) → KE store (when car moves)

What does the energy in a KE store depend on?

• Its mass

• Its speed

What happens when a spring is stretched?

Work is done

• As applying a force to change the length of the spring

What is put in to stretch a spring?

Energy

• So stretched spring stores EPE

Elastic potential energy

Energy stored in an elastic object when work is done on the object

• Energy stored in a stretched spring

Extension (e)

Amount by which a spring’s length increases when a force is applied (compared to og length)

Spring constant

Force per unit extension of a spring

What is the extension of a spring directly proportional to (within limit of proportionality)

Force (N)

Limit of proportionality (recheck definition)

Max point an elastic object will stretch + return to its og shape?

Graph of force against extension when a spring has passed its limit of proportionality

Elastic potential energy equation

Gravitational potential energy

Energy stored in an object due to its position above the Earth’s surface

What causes GPE?

Force of gravity acting on an object

When does the GPE store of an object increase?

When it moves up

Why does the GPE store of an object increase when you move it up?

1. Chemical energy stored in muscles- used to lift object

2. CE of muscles → GPE in ball

Why does the GPE store of an object increase when it is lifted?

Work is done on it to overcome the gravitational force

When does the GPE store of an object decrease?

When it moves down

Why does a balls GPE store decrease when it falls from a shelf to the ground?

• GPE → TE + KE

• So ball has no GPE

Gravitational potential energy equation

Gravitational field strength

Measure of the force of gravity

Specific heat capacity

Amt of energy needed to raise the temp of 1 kg of the substance by 1°C

Specific heat capacity equation

Equation to calc the amt of energy stored in / released from a system as its temp changes

If an object has more mass, will it have more or less KE?

More

• More mass, more KE

Closed system

No energy can enter or leave

• So no net change to the total energy

What happens when a person stretched an elastic band?

CE store in muscles decreases → ET to EPE store of elastic band (increases)

Work done

When energy is transferred from 1 store to another

2 types of work

1. Mechanical- using a force to move an object

2. Electrical- current transferring energy

Energy transfers in this diagram:

Dog applies force to sledge → force causes sledge to move

CE in dog → KE in moving sledge → TE in sledge + ground (due to friction)

• So work is done

How do you know work is done?

ET betw diff stores

How do you know work is done when a force moves an object?

ET to object

Work done equation

Power

• Rate at which energy is transferred

• Or rate at which work is done

Power equation

1 W =

Energy transfer (or work done) of 1 J per second

The higher the power of an appliance…

The faster the rate of energy transfer is

1 kW =

1000 W

Example of comparing power

2 electric motors that both lift the same weight to same height but 1 does it faster than the other

Friction

Force that opposes the motion of 2 surfaces in contact w each other

Work done to overcome friction is mainly transferred to?

Thermal energy stores by heating

What happens when an elastic object is stretched / squashed?

WD on it is stored as EPE

Elastic object

Regains its og shape after being stretched / squashed

Efficiency

What fraction of input energy is transferred to useful forms of energy

Efficiency equation

Input energy

Energy supplied to a machine

Useful output energy

Energy transferred usefully by a machine

Input energy equation

IE = useful output energy + energy wasted

Efficiency can’t be greater than?

100%

• Can’t create energy

The more efficient a machine…

The less energy wasted

How do machines waste energy?

1. Friction betw moving parts

2. Air resistance

3. Electrical resistance

4. Noise

Principle on how to increase the efficiency of an energy transfer?

Reduce energy they waste

How to increase efficiency of a machine by reducing:

1. Friction betw moving parts

2. Air resistance

3. Electrical resistance

1. Lubrication

2. Make shape of vehicles more streamlined

3. Use copper wires

What do machines do?

Transfer energy for a purpose

Wasted energy

Energy not transferred usefully

What eventually happens to wasted energy?

Transferred to surroundings → surroundings become warmer

Dissipates

Spreads out

What happens to the usefullness of energy as it dissipates?

Becomes less useful

Electrical appliances should be designed to?

Waste as little energy as possible → increases efficiency

Uses of everyday electrical appliances?

1. Heating

2. Lighting

3. Making objects move (using an electrical motor)

4. Producing sound

5. Producing visual images

Describe all changes involved in the way energy is stored when a system changes:

1. An object projected upwards

2. Moving object hitting an obstacle

3. Object accelerated by a constant force

4. A vehicle slowing down

5. Bringing water to a boil in an electric kettle

What causes energy changes as a result of changing a system?

1. Heating

2. Work done by forces

3. Work done when a current flows

What happens when energy dissapates?

• Stored in less useful ways

• Wasted energy

Chemical energy