IGCSE Physics : Topic 4 Electricity and Magnetism

Topic 4.1- Phenomena of Magnetism

Properties of magnets:

-Has a magnetic field around it - 2 opposite poles

-Induce magnetic materials - Exert no force on non-magnetic materials

Ferrous materials: iron, nickel, cobalt

Non-Ferrous materials: Copper, glass

Magnetising method- Insert metal into a DC solenoid, induced magnetism from magnet

Demagnetising method- Stroke with hammer, insert into AC solenoid, heat it

Iron= soft ferrous metal ( non-permanent magnet) Steel= hard ferrous metal

Topic 4.2- Electric Charge

Induced Charge: a charge that “appears” on an uncharged object because of a charged object nearby, for example if a positively charged rod is brought near a small piece of aluminium foil. Electrons in the foil are pulled towards the rod, which leaves the bottom of the foil with a net positive charge. The attraction is stronger than the repulsion because the attracting charges are closer than the repelling ones.

Current, Voltage and resistance

Voltage= current x resistance Voltage= energy/charge Charge=current x time

Resistance factors High temp (semi conductor) = Less resistance

Double the length = Double resistance Double cross-section = Half resistance

Better conductor = Less resistance High temp (conductor) = more resistance

Electric Field Lines

Topic 4.3- Electrical circuits

Use of circuit components

Thermistor-Less resistance at high temp LDR- resistance decrease at high intensity

Capacitor- Store charges Relay- Switch operated by electromagnet

Diode- Only allow current flow in one direction, used as rectifier (convert AC into DC)

Transistor- Used for amplifying signals and for switching

4.3 Dangers of electricity

Damaged insulation: contact with the wire (live wire especially) due to gap in the insulation causes electric shock which can cause serious injury or shock.

Overheating of cables: when long extension leads are coiled up, they may overheat. The current warms the wire, but the heat has less area to escape from a tight bundle. This might cause a fire.

Damp conditions: water can conduct a current, so if electrical equipment is wet someone might get electrocuted

Fuses: a thin piece of wire which overheats and melts (the fuse ‘blows’) if the current is too high. It is placed on the live wire before the switch. This prevents overheating and catching fire. A fuse will have a specific current value (e.g. 13A) so when choosing a suitable fuse you must use the one which can have the lowest current value but over the current value of the appliance.

Digital Electronics

Topic 4.5-Electromagnetic Effects

Electromagnetic Induction

If a wire is passed across a magnetic field, a small EMF is induced, this is electromagnetic induction. If the wire forms part of a complete circuit, the EMF makes a current flow. This can be detected using a galvanometer. The EMF induced in a conductor is proportional to the rate at which the magnetic field lines are cut by the conductor.
The induced EMF can be increased by:

-moving the wire faster

-using a stronger magnet

-increasing the length of wire in the magnetic field

Current direction could be reversed by:

Turning the magnet around, Moving wire in opposite direction

An induced current always flows in a direction such that it opposes the change which produced it. When a magnet is moved towards a coil the pole of the coil and magnet next to each other are the same. When the magnet is moved away the poles are opposite (opposite poles attract). The pole-type (north or south) is controlled by the direction in which the current is induced. Right hand grip rule is used.

AC generator

• The coil is made of insulated copper wire and is rotated by turning the shaft. The slip rings are fixed to the coil and rotate with it. The brushes are 2 contacts which rub against the slip rings and keep the coil connected to the outside part of the circuit, usually made of carbon. When the coil is rotated, it cuts magnetic field lines, so an EMF is generated, which makes a current flow. Each side of the coil travels upwards then downwards then upwards etc. so the current flows backwards then forwards then backwards etc. so it is an alternating current. The current is maximum when the coil is horizontal since field lines are being cut at the fastest rate and 0 when the coil is vertical, since it is cutting NO field lines.
The EMF can be increased by:

-increasing the number of turns on the coil

-increasing the area of the coil

-using a stronger magnet

-rotating the coil faster

Transformers

How does it work?

When alternating current flows through the primary coil, it sets up an alternating magnetic field in the core and, therefore, in the secondary coil

Step up transformers- Increase voltage in Secondary coil

Step down transformers- Decrease voltage in Primary coil

Formulas:

Output voltage/ Input voltage = Turns on output coil/ Turns on input coil

Input voltage x Input current= Output current x Output voltage

Transformers are transmitted in a high voltage to save cost so thinner, lighter and cheaper cables can be used. Current is reduced thus less resistance.

Magnetic field effect of a current

1. Increasing the current increases the strength of the field

2. Increasing the number of turns of a coil increases the strength.

3. Reversing the current direction reverses the magnetic field direction (right-hand rule).

Force on current-carrying conductor

If a current carrying conductor is in a magnetic field, it warps the field lines. The field lines from the magnet want to straighten out naturally. This causes a catapult like action on the wire creating a force. The direction of the force, current or magnetic field is given by Fleming’s left-hand rule

DC Motor

When a current-carrying coil is in a magnetic field, it experiences a turning effect. A DC motor runs on a direct current. The coil is made of insulated copper wire. It is free to rotate between the poles of the magnet. The commutator, or split-ring, is fixed to the coil and rotates with it. When the coil overshoots the vertical, the commutator changes the direction of the current through it, so the forces change direction and keep the coil turning. The brushes are two contacts which rub against the commutator and keep the coil connected to the battery. They are usually made of carbon. The maximum turning effect if when the coil is horizontal. There is no force when the coil is vertical (but luckily it always overshoots this position).

4.6 Cathode- Ray oscilloscope