igcse physics: electromagnetic effects

ELECTROMAGNETIC EFFECTS

IGCSE PHYSICS

Electromagnetic Induction A magnetic field can be used to produce

current.When the wire is moved across the magnetic field a small EMF(voltage) is created. This is called electromagnetic induction.“EMF is induced”

Induced EMF increased by:-Moving wire faster-Using stronger magnet-Increasing length of wire.

Induced CurrentsFleming’s right hand rule:

Difference between the left hand and the right hand rule:-When current causes motion the left hand rule applies-When motion causes current the right hand rule applies

The current and EMF direction can be reversed by:• moving the wire in the opposite direction• turning the magnet round so that the field direction reversed.

How is Current Induced in a Coil of Wire?

When a magnet is moved towards (or inside) a coil of wire, a current is induced inside the wire. This can be shown by connecting the coil to a very sensitive ammeter called a galvanometer.

The size of the induced current can be made bigger by:1. Using a stronger magnet.2. Moving the magnet at a faster speed.3. Using more turns of wire on the coil.These all result in the pointer on the galvanometermoving further to the right.

The direction of the current can be reversed by:1. Moving the magnet in the opposite direction.2. Using a magnet facing the opposite way round(with North becoming South).These both result in the pointer on the galvanometermoving to the left.

If the magnet stops moving, even though it may still beinside the coil of wire, no current is induced in the wire.

Generators The coil rotates Magnetic fields are

cut EMF is generated Causes current to flow Coil rotates– upwards,

downwards, upwards causing the current to flow backwards, forwards, backwards.

Increasing EMF:- Increasing the number of

turns on coil- Increasing area of coil- Use stronger magnet- Rotate coil faster

Coils and Transformers Moving magnet induces EMF Magnetic field SAME effect. Mutual induction: when coils are

magnetically linked so that changing current in one coil causes an induced EMF in the other.

Simple Transformer- Alternating current flows

through primary coil- This sets up an altering

magnetic field in the core.

- Coils of the secondary coil ‘cut’ the altering magnetic field thus inducing an alternating voltage in the output coil.

Turns in output coil=

Input voltage

output voltage

Turns on input coil

Step-up and Step-down transformers

Step-up: this is when the number of output coils is greater than the number of input coils which means that there will be a greater output voltage as opposed to input voltage.

Step-down: this is when the number of output coils is less than the number of input coils which means that there will be less output voltage as opposed to input voltage.

Power Through a Transformer

= x Output current

Output voltage

Input voltage

x Input current

Magnetic Effects of Current When an electric current is passed

through a wire an magnetic field is produced. The features of this magnetic field are: They are circles Field is strongest close to the wire Increasing current increases strength of

field.

Right-hand grip rule

Electromagnets These are types of magnets that can be

switched on and off.

Iron core

Coils

The strength of the magnetic field can be increased by:- Increasing the current.- Increasing the number of turns in the coil

Magnetic Relay

When electricity is passed through the coil end wires, it induced a magnetic field in the iron ROD. This attracts the iron STRIP causing both metal contacts to touch.

Metal contact

s.

Circuit BreakerCircuit breaker- it is an automatic switch cutting off the current within a circuit if it rises above a specified value.- In the case on the left, the pull of the electromagnet has become so strong that it has attracted the soft iron armature. This causes the contacts to open and stop the current.

- If u press the reset button, the contacts close once again.

Magnetic force on the current

Copper is a non-magnet feels no force of the magnet

But.. If it has a current

passing through it, there will obviously be a force on the wire.

The wire moves ACROSS the field. It is not attracted to it.

Force is increased if:-Current is increased-Stronger magnet is used-Length of wire in field is increased.

Flemings Left Hand Rule

Electric motors

A motor is made up from a coil of wire which is positioned between the two poles of the magnet.

When the current flows through the coil, it creates a magnetic field. This magnetic field that is produced interacts with the magnetic field produced by the 2 permanent magnets.

The combination of these two magnetic fields exert a force, pushing the wire at right angles to the permanent magnetic field.

An electric motor transfers electrical energy to kinetic energy.

Improve turning effect

Increasing Turning Effect Increase the current Use a stronger magnet Increase the number of turns on the coil Increase the area of the coil.

Reversing the rotation can be done by:• reversing the battery• reversing poles