Energy Transfers

What is power defined as?

The rate of energy transfer or the amount of energy transferred per second

What does the power of a device depend on?

the voltage of the device and the current of the device

what is the equation for the power of an electrical component (appliance)?

• P = IV

• power (w) = potential difference/voltage (V) x current (A)

Power is just energy per …

second

Since P = VI and V = IR, what is the equation linking power current and resistance?

What is the equation linking power resistance and voltage?

• P = I²R

• P = V² / R

How do everyday appliances transfer energy from mains to energy stores within the appliance?

give an example of the energy transfers taking place for a heater

• electrically

• in a heater, energy will be transferred to the thermal store of the heating element, and then to the thermal store of the surroundings

What does the amount of energy transferred to and from an appliance depend on?

• the time the appliance is switched on for

• the power of the appliance

• A 1 kW iron uses the same amount of energy in 1 hour as a 2 kW iron would use in … …

• A 100 W heater uses the same amount of energy in 30 hours as a 3000 W heater does in … …

• 30 minutes

• 1 hour

As charge (electrons) flows around a circuit, to where is energy transferred from the power source?

How/why is this so?

• to the various components

• as the electrons pass through the power supply, energy is transferred to the electrons

• as the electrons pass through each component they transfer energy to the component

Where do most domestic appliances such as a remote control, transfer energy electrically from?

the chemical store of the DC cells and batteries

Where do most large household appliances transfer energy electrically from?

the AC mains

what store can this electricity be transferred to of an electric motor?

the kinetic energy store

What are motors used in?

• vacuum cleaners - to create the suction to suck in dust and dirt off carpets

• washing machines (or tumble dryers) - to rotate the drum to wash (or dry) clothes

• refrigerators - to compress the refrigerant chemical into a liquid to reduce the temperature

Or, what store could the energy be transferred of heating devices?

thermal store

give 3 things heating is used in

• toasters - to toast bread

• kettles - to boil hot water

• radiators - hot water is pumped from the boiler so the radiator can heat up a room

When is work done in a circuit?

What is work done equal to?

• when charge flows through a circuit

• equal to the energy transferred

What does the amount of energy transferred by electrical work depend on?

• the current

• the potential difference

• the amount of time the component is used for, t

When charge flows through a resistor, for example, what makes the resistor hot?

the energy transferred

What equation can we use to find the energy transferrred?

• E = P x t

• E = energy transferred in joules (J)

• P = power in watts (W)

• t = time in seconds (s)

Since P = IV, how else can the equation be written as?

E = I x V x t

The energy transferred electrically also depends on the charge and potential difference, what equation shows this?

• E = Q x V

• Q = charge in coulombs (C)

• V = potential difference in volts (V)

When charge flows around a circuit for a given time, the energy transferred from the cell or battery is … … the energy transferred to all the components in the circuit

equal to

What is the power of an appliance?

the amount of energy it transfers by electrical working every second

Every electrical appliance has a power rating which tells you what?

how much energy it needs to work

where is the power rating for domestic electrical appliances given?

what does it include?

• normally given on a label

• potential difference required to make the device work (e.g. 230V in the UK)

• the frequency of the supply (eg.50 Hz in the UK)

• the power rating in Watts (this varies for each device)

The higher the power rating the what?

The faster the energy is transferred

• e.g. 2000 W kettle means the kettle transfers 2000J of energy per second

What are the power ratings in Watts of the following appliances:

clock

lamp

drill

iron

kettle

hot water heater

electric oven

• 10

• 50

• 800

• 1250

• 2400

• 3000

• 12000

Care must be taken not to plug an appliance into a mains supply that has a voltage much higher than stated on the label, for example in another country that has a higher mains voltage, why?

This could cause the appliance to fuse or set on fire and become damaged

What does the National Grid do?

It distributes electricity across the Uk

What does the National Grid consist of?

A system of cables and transformers linking power stations to consumers (houses, factories and buildings)

Where is electrical power transferred from and to?

from power stations to consumers using the National Grid

What do the transformers include?

• step-up transformers which increase the voltage (and reduces the current) through the wires

• step-down transformers which decrease the voltage (and increases the current) through the wires

Why is the national grid an efficient way to transfer energy?

due to the use of step-up and step-down transformers

What is the current generated by power stations greater than and so what must it be transmitted through?

greater than that which is required for homes and other buildings, and so it must be transmitted through a network of wires that travel across the country

when electricity is transmitted over large distances, what does the resistance in the wire cause which results in wasted energy transfers?

heating

By increasing the potential difference at which current is transmitted, the same amount of power can be transferred using what?

what does this result in?

• a much smaller current (due to the equations P = IV)

• less heating in the wire and hence less wasted energy

Therefore:

… potential difference means low current (less energy dissipated) for the same power

… potential difference means high current (more energy dissipated) for the same power

• high

• low

How is this potential difference increased?

how is it decreased?

• using the step-up transformers

• using the step-down transformers

Explain how the step-up transformer increases the efficiency of the National Grid.

• The lower the current, the less heating due to resistance there will be in the wires 

• From the diagram, the step-up transformer increases the voltage and decreases the current in the cables

• By decreasing the current, the energy dissipated to the thermal store of the surroundings through the power cables is reduced

• Therefore, there is more energy being transferred to homes which increases the efficiency of the National Grid

What is the type of current produced in power stations?

via what is it transferred to homes?

• alternating current (AC)

• the national grid

What are transformers used for?

to increase and decrease the potential difference of the current before and after transmission across the National Grid

What are transformers made up of?

2 coils of wire, called the primary and secondary coils, around a magnetic iron core

A step-up transformer has more turns on the … coil than the …

A step-down transformer has more turns on the … coil than the …

• secondary, primary

• primary secondary

… transformers are used to increase the potential difference from the power station to the transmission cables

… transformers are used to decrease the potential difference, to a much lower value, from transmission cables for domestic use (houses, offices, shops)

• step-up

• Step-down