Electromagnetic Effects
Electromagnetic Effects
Electromagnetic Effects
As the wire moves downwards, it cuts through field lines, inducing an EMF in the wire
When the magnet enters the coil, the field lines cut through the turns, inducing an EMF
More generally, whenever the magnetic field passing through a loop of wire changes, an EMF is induced.
Exam Tip
When discussing factors affecting EM Induction:
Make sure you state:
“Add more turns to the coil”
And not just: “Add more coils”
(This second one means something slightly different).
Extended Only
The RIght-Hand Rule
When moving a wire through a magnetic field, the direction of the induced EMF can be worked out by using
the Right-Hand Dynamo rule:
The Right Hand Dynamo rule can be used to deduce the direction of the induced EMF
o Start by pointing the first finger (on your right hand) in the direction of the field.
(First Finger Field)
o Next, rotate your hand so that the thumb point in the direction that the wire is moving in.
(ThuMb Motion)
o Your Second finger will now be pointing in the direction of the current (or, strictly speaking, the
EMF).
(SeCond Current)
The direction of the induced EMF always opposes the change that produces it.
This means that any magnetic field created by the EMF will act so that it tries to stop the wire or magnet
from moving.
Two graphs showing the variation of current with time for alternating current and direct current
Exam Tip
If asked to explain the difference between alternating and direct current, sketch the graphs shown above.
A well sketched (and labelled) graph can earn you full marks.
A.C. Generator
Extended Only
A.C. Generator: Basics
A generator looks very similar to a motor, but instead of connecting it to a power supply, the coil is spun by
some mechanical process which then produces electricity.
When a coil is spun in a magnetic field, a voltage is induced between the ends of the coil
As the coil rotates, it cuts through the field lines.
This induces an EMF between the end of the coil
(which can then create a current).
The size of this EMF can be increased by:
Slip rings, attached to the ends of the coil, transfer the current to metal brushes whilst allowing the coil to
rotate freely.
The Output
The A.C. generator creates an alternating current, varying in size and direction as the coil rotates.
o The induced EMF is greatest when the coil is horizontal, as in this position it cuts through the field at
the fastest rate.
o The EMF is smallest when the coil is vertical , as in this position it will not be cutting through field
lines.
Diagram showing how the current varies with the position of the coil
Exam Tip
Transformers
What is a Transformer?
A transformer is an electrical device that can be used to increase or decrease the voltage of an alternating
current.
(Transformers only work with a.c.)
A transformer consists of two coils of wire wrapped around a soft iron core
Extended Only
How It Works
When an alternating current is supplied to the primary coil, a changing magnetic field is produced by the
primary coil.
This field passes through the soft iron core and through the secondary coil.
The changing field in the secondary coil induces an EMF.
This EMF is also alternating and has the same frequency as the original current.
Where IP and IS are the currents in the primary and secondary coils
Hence, if the voltage is increased by some factor, the current must decrease by the same factor.
Transmitting Electricity
High-Voltage Transmission
When electricity is transmitted along overhead cables, it is done at high voltages.
o A step-up transformer is used to raise the potential difference (voltage) before transmissions.
o A step-down transformer is then used to step the potential difference back down to normal levels
when it reaches its destination.
Electricity is transmitted at high voltage, which reduces both the current and the loss of power
Extended Only
How High Voltage Reduces Power Loss
When electricity is transmitted over large distances, the current heats the wires, resulting in energy loss.
By raising the voltage at which the electricity is transmitted at, the same amount of power (energy per
second) can be transmitted using a much smaller current (P=I×V).
This results in less heat being produced in the wire and hence there is less energy loss.
The magnetic field lines form concentric circles around the wire.
The direction of the field is given by the right-hand grip rule:
When the thumb is pointing in the direction of the current, the fingers will curl in the direction of the field
Extended Only
Magnetic Field Strength & Direction
The direction of a magnetic field is defined as being:
o The direction of the force on the north pole of a magnet placed at that point.
The strength and direction of the field depend on the size and direction of the current:
o If the current is increased, the field will get stronger.
o If the direction of the current is changed, the direction of the field will change.
The strength of the magnetic field is related to the distance between the field lines:
As the field lines spread out, the field gets weaker.
The field lines around a wire get further apart the further they are from the wire.
The magnetic field around a solenoid (a long coil) is identical to the magnetic field of a bar magnet
One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole.
Extended Only
Strength & Direction within a Solenoid
Inside the solenoid the field lines straighten up and are very close together – they form a strong uniform
field.
Solenoid Applications
A solenoid can be used as an electromagnet by adding a soft iron core.
(This increases the strength of the magnetic field significantly).
Electromagnets are used in a wide variety of applications including:
o Doorbells
o Electronic door locks
An electromagnet is also used in a relay:
When a current passes through the coil, it attracts the switch, closing it, which allows a current in the right-hand
part of the circuit
Extended Only
The Left-Hand Rule
The force is always directed at 90 degrees to both the field and the current.
The direction of the force on a current-carrying wire can be worked out by using the left-hand rule:
The left-hand rule can help you figure out the direction of the force on a current-carrying wire
When a charged particle (such as an electron) enters a magnetic field, it is deflected by the field
The force is always at 90 degrees to both the direction of travel and the magnetic field lines, and can be
worked out by using the left-hand rule.
o However:
If the particle has a negative charge (such as an electron), then the second finger (the current) must
point in the opposite direction to the direction of travel.
The deflection of charged particles can be demonstrated either by using a cathode ray tube and a pair of
magnets, or by passing a collimated beam of beta particles (high energy electrons) between the poles of a
horseshoe magnet.
(Note: A cathode ray tube fires electrons at high speed towards a target. Old TV sets contained cathode ray
tubes, but you must be careful using these, as holding a magnet to the screen can permanently affect the image.)
D.C. Motor
Simple D.C Motor: Basics
The simple d.c. motor consists of a coil in a uniform magnetic field:
When there is a current in the coil, the magnets exert a turning effect on the coil, causing it to rotate.
The force supplied by a motor can be increased by:
o Increasing the current in the coil.
o Increasing the strength of the magnetic field.
o Adding more turns to the coil.
Extended Only
How It Works
When a current passes through the coil:
o The current creates a magnetic field around the coil,
o which interacts with the field of the magnets,
o exerting a force on the coil,
o in accordance with the left-hand rule (see below).
o This pushes one side of the coil up and the other side down, causing it to spin.
The commutator:
o Reverses the direction of the current in the coil every half turn.
o This reverses the direction of the forces, which keeps the coil spinning.