Physics IGCSE Oxford AQA generating and distributing electricity and household use

A potential difference (pd) is induced across the ends of a conductor when [2 things]

• the conductor moves relative to a magnetic field
• the conductor is in a changing magnetic field.

How can you increase the size of the induced potential difference?

move the coil more rapidly
increase the number of turns in the coil
if we use a stronger magnetic field

A potential difference is induced across the ends of a coil of wire when

• a permanent magnet is moved into or out of the coil
• the coil is moved relative to the magnet.

If the conductor is part of a complete circuit, what happens in the wire

a current flows in the wire

The magnetic field produced by the induced current supports/opposes the field of the permanent magnet

opposes

what needs to happen for the polarity of the induced potential difference and direction of flow of any induced current to be reversed

If the direction of motion, or the polarity of the magnet, is reversed

The size of the induced potential difference increases when

• the speed of the movement increases
• the strength of the magnetic field increases
• the number of turns on the coil increases
• the area of the coil increases.

Students should be able to explain how an alternator generates ac and a dynamo generates dc, including graphs of potential difference generated in the coil against time. However, detailed knowledge of slip rings and split rings are not required.

How do power stations generate electricity?

Power stations use turbines to turn wire coils between magnets to generate electricity.

Electricity is distributed from power stations to consumers along _____________________________

transmission cables

Electricity is distributed from power stations to consumers along transmission cables with ____________ at both ends.

transformers

Students should be able to identify and label the essential parts of an electric power transmission and distribution system.

For a given power rating, a high distribution voltage increases/reduces the current flowing

reduces

For a given power rating, a high distribution voltage reduces the current flowing, therefore _________ energy losses due to heating and making the system more ____________.

reducing
efficient

A basic transformer consists of 2 coils called

primary and secondary coil

A basic transformer consists of a primary coil and a secondary coil wound on a ____________________

soft iron core

An alternating current in the primary coil of a transformer produces a ____________

changing magnetic field

An alternating current in the primary coil of a transformer produces a changing magnetic field in___________ and hence in __________

the iron core
the secondary coil

An alternating current in the primary coil of a transformer produces a changing magnetic field in the iron core and hence in the secondary coil. This induces a ______________

changing potential difference

An alternating current in the primary coil of a transformer produces a changing magnetic field in the iron core and hence in the secondary coil. This induces a changing potential difference across the ends of the __________ coil

secondary

An alternating current in the primary coil of a transformer produces a changing magnetic field in the iron core and hence in the secondary coil. This induces a changing potential difference across the ends of the secondary coil and an__________________flows.

alternating current

Describe how a transformer works

An alternating current in the primary coil of a transformer produces a changing magnetic field in the iron core and hence in the secondary coil. This induces a changing potential difference across the ends of the secondary coil and an alternating current flows.

What are step-up transformers used for?

are used to increase the voltage before the distribution lines

remember a higher voltage (for a given power rating) means that there is a smaller current, thus reducing energy loss from heating

What are step-down transformers used for?

reduce it at the end to produce a safer voltage for local consumers

In a step-up transformer the potential difference across the secondary coil is smaller/greater than the potential difference across the primary coil.

greater

In a step-down transformer the potential difference across the secondary coil is less than/greater the potential difference across the primary coil.

less than

Switch mode transformers operate at which frequencies?

operate at a high frequency, often between 50 kHz and 200 kHz

Which is heavier, a traditional or switch-mode transformer

traditional

which is smaller, a traditional or switch-mode transformer
which frequency supply do they work from

switch-mode
50 Hz

Where are switch-mode transformers used ususally

phone chargers

What is special about the power consumption of switch-mode transformers

use very little power when they are switched on but no load is applied

Cells and batteries supply current that always passes in the same direction/alternating direction

the same

What do you call a current that always passes in the same direction

direct current

What is an alternating current

An alternating current (ac) is one that is repeatedly changing direction

Students should be able to determine the period, and hence the frequency, of a supply from diagrams. They should be able to compare and calculate potential differences of dc supplies and the peak potential differences of ac supplies from diagrams

Mains electricity is an ac/dc supply, which has a set frequency and voltage

ac

Give an example of safety features that can be incorporated in electrical systems and appliances

earthing

What does it mean when an appliance is earthed?

if the metal body of an appliance becomes live through a fault, the current is harmlessly conducted away.

What safety measure can be used If an electrical fault causes too great a current to flow

fuse/circuit breaker

What do fuses/circuit breakers do?

disconnects the circuit when too large a current passes through

What would happen to a fuse when too large a current passes through?

The current will cause the fuse wire to overheat and melt

What would happen to a circuit breaker when too large a current passes through?

trip

what are two advantages of circuit breakers against fuses

A circuit breaker operates much faster than a fuse and can be reset

Detail how a metal body can harmlessly conduct a fault away

If a fault develops a large current flows from the live wire to earth. This melts the fuse and disconnects the live wire

What is an exception for appliances so that they do not have to be earthed?

Students should be aware that some appliances are double insulated and therefore have no earth wire connection

A current in a wire causes a ___________

magnetic field to be induced

What is the motor effect

When a current carrying conductor is placed in a magnetic field so that it cuts lines of magnetic force, the magnet and conductor exert a force on each other

What is the condition for which The conductor will not experience a force?

The conductor will not experience a force if it is parallel to the magnetic field

. The size of the force can be increased by:

• increasing the strength of the magnetic field
• increasing the size of the current
• increasing the length of the conductor in the magnetic field.

The direction of the force is reversed for 2 reasons:

if either the direction of the current or the direction of the magnetic field is reversed

Students should be able to identify the direction of the force using Flemings left-hand rule

A coil of wire carrying a current in a magnetic field tends to ___________. This is the basis of an electric motor

rotate

Electrical appliances are designed to transfer ____. Students should be able to give examples of such devices and identify the energy transfers

energy

The amount of energy an appliance transfers depends on 2 things:

how long the appliance is switched on for
and its power rating