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