2.4 Energy transfers Flashcards
Electrical power
The rate of energy transfer or the amount of energy transferred per second.
The power of a device depends on:
The voltage of the device.
The current of the device.
P = VI
P = power
V = potential difference
I = current
Power and resistance
The voltage across an electric device depends upon the current and resistance of that device
Using the equations P = VI with V = IR, power can be written in terms of resistance, R:
P = I^2 x R
Energy and power
Everyday appliances transfer energy electrically from the mains to energy stores within the appliance.
The amount of energy transferred to and from an appliance depends on:
The time the appliance is switched on for.
The power of the appliance.
Energy transfers in appliances
As charge (electrons) flows around a circuit, energy is transferred from the power source 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.
Calculating energy transfer
Work is done when charge flows through a circuit.
Work done is equal to the energy transferred.
E = Pt
E = energy transferred in joules (J)
P = power in watts (W)
t = time in seconds (s)
The energy transferred electrically also depends on the charge and potential difference:
E = Q × V
Q = charge in coulombs (C)
V = potential difference in volts (V)
Power ratings
The power of an appliance is the amount of energy it transfers by electrical working every second.
Every electrical appliance has a power rating which tells you how much energy it needs to work.
The power rating for domestic electrical appliances is normally given on a label. This will include:
The potential difference required to make the device work (eg. 230 V 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 national grid
The National Grid distributes electricity across the UK.
It consists of a system of cables and transformers linking power stations to consumers (houses, factories and buildings).
Electrical power is transferred from power stations to consumers using the National Grid.
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.
Benefits of the national grid
The National Grid system is an efficient way to transfer energy due to the use of step-up and step-down transformers.
The current generated by power stations is 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, the resistance in the wires causes heating, which results in wasted energy transfers.
By increasing the potential difference at which the current is transmitted, the same amount of power can be transferred using a much smaller current (due to the equation P = IV).
This results in less heating in the wire and hence less wasted energy
Therefore:
High potential difference means low current (less energy dissipated) for the same power.
Low potential difference means high current (more energy dissipated) for the same power.
The potential difference is increased using the step-up transformers and decreased using the step-down transformers.
Use of transformers
The type of current produced in power stations is alternating current (AC) which is transferred to homes via the National Grid.
Transformers are used to increase and decreases the potential difference of the current before and after transmission across the National Grid.
They are made up of two coils of wire, called the primary and secondary coils, around a magnetic iron core.
A step-up transformer has more turns on the secondary coil than the primary
A step-down transformer has more turns on the primary coil than the secondary.
Step-up transformers are used to increase the potential difference from the power station to the transmission cables.
Step-down transformers are used to decrease the potential difference, to a much lower value, from transmission cables for domestic use (houses, offices, shops).
