Magnets, motor effect, etc

The fields from turns pass through centre of coil and add together to form a very strong almost uniform magnetic field

- current goes above certain value
- electromagnet becomes strong enough to such a degree that it attracts an iron rocker, making it move past a spring catch
- exerts a non contact force
- opens switch contacts in circuit 2
- breaks circuit 2, stopping the flow of current
- prevents circuit from overloading due to a surge
- when the iron rocker moves past a spring catch, it cannot return to its original position until the user resets it

The field lines cancel to give a weaker field

More turns
Increased current size
Iron core (becomes an induced magnet when current flows. The magnetic field from solenoid and iron core is stronger overall)

Field which has the same strength and direction at all points

A wire with current flowing through it is placed in a magnetic field and experiences a force

More current (magnetic field around wire)
Stronger magnets (magnetic field between poles)
90 degrees ( increase interaction between fields)

F =BIL

- consists of a coil of wire free to move positioned in a uniform magnetic field

Current creates a magnetic field around the coil

Results in force exerted on coil

As the current flows in opposite directions on each side of the coil, the force produced pushes on side up and the other side down
This causes rotation to occur
The coil continues to rotate until a vertical position

- force acting upwards and force acing downwards
- split ring commutator swaps the contacts of the coil
- this reverses the current direction
- this reverses the force direction
- so, the coil continues to rotate

The commutator reverses the direction of the current every half turn
This keeps the coil rotating as long as current flows

Increase current
Increase field strength

Reverse current
Reverse poles

- increase current
- increase mag field
- add turns

A voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it

Generator effect

If there is relative movement between the conductor and the magnetic field

Current is induced in the conductor

Alternator

Speed of wire, coil or magnet ( rate of cut)
Number of turns ( each coil cuts field lines - total is result of all coils)
Size of coils ( increased area of coils - more wire to cut field lines)
Strength of magnetic field )

Orientation of magnet poles
Direction of movement

The direction of an induced current always opposes the change that produces It

- due to generator effect, pd induced in coil
- induced pd always opposes the change that produces it
- coil applies a force to oppose magnet pushed in p
- therefore, end closest becomes north
- so it repels north of magnet

And vice versa

Rotating coil in a magnetic field with slip rings and brushes

The slip rings are connected to the coil. The brushes make continous contact between the external circuit and the slip rings (rotor and stator)

Current is both in positive and negative regions

Direct current generator

Split ring commutator and brushes

changes direction of a current every half turn

As the coil spins, it cuts field lines
A potential difference, and current is induced

Rotating magnet placed inside a coil

Produces ac

Pressure variations in sound waves into current variations

1. When sound waves reach microphone, pressure variations cause diaphragm to vibrate
2. This causes the coil to move back and forth through the magnetic field
3. The coil cuts field lines, inducing an Alternating potential difference in the coil and ergo AC
4. AC is induced as the coil is constantly changing direction

Electrical signals into sound

A coil of wire wrapped around one pole of a permenant magnet

1. Ac passes through coil of loudspeaker
2. This creates a changing magnetic field around the coil
4. The magnetic field produced around the coil interacts with the permanent magnet field
5. The interacting fields exert a force on the coil
6. As the field is changing, the force on the coil is changing
7. The coil oscillates
9. The air oscillates and sound waves are made

An electrical device used to increase or decrease the voltage of an Alternating current. This is achieved using generator effect

Primary coil
Secondary coil
Iron core (easily magnetised)

AC in the first coil creates a changing magnetic field
This field cuts through the secondary coil
This induces a current in the secondary coil
if the number of loops is the same accross both coils then so will the induced voltage

Vp/Vs = Np/Ns

Results in less heat being produced in the wire
This reduces energy loss
This improves efficiency

25kV

400kv

230 volts

For domestic uses, voltage must be much lower and current must be higher

Pp = Ps

Collisions between electrons in the conductor and the lattice of metal ions in the metal conductor
The ions resist the current flow
The electron collisions make the ions vibrate more
Therefore the metal heats up
Increasing resistance increases this effect

Record field at one location by placing dots on either side of needle
Move compass to a new position such that bottom of needle is in same place as tip of needle was previously
Repeat to find how the field continues
Connect dots to reveal shape

Place filings on paper
Tap paper so that filings align with field
Observe pattern

Place iron in contact with magnet
Paper clip attracts to iron
Remove magnet and iron no longer attracts clips

DC flows in one direction
AC reverses direction back and forth

A changing field is needed to induce a current
Only a changing current can produce a changing field

Change frequency

● The primary coil has fewer turns
● An a.c. current produced a magnetic field in the primary coil
● The iron core passes the magnetic field to the secondary coil
● The magnetic field induces a higher voltage in the secondary coil ( magnetic field cuts more wires)
● Power is conserved, so this produces a lower current

A region where other magnets or magnetic materials experience a non contact force

Voltage which changes from positive to negative

current changes direction and therefore current in the external circuit changes direction

A permanent magnet induces its own field whereas an induced magnet becomes magnetic when placed in a magnetic field
Induced magnetism always causes a force of attraction whereas two permanent magnets can repel one another with the same poles

As it rotates the coil cuts the magnetic field
as rotation continues, one side of the coil moves down, so the induced potential difference reverses direction- current changes every 180*
Greatest when coil perpendicular and vice verse, can by increased by inc rate of rotation, strength of field, num of turns

- The coil rotates in a magnetic field, cutting field lines
- a voltage is induced acrosss the coil
- The split ring commutator switches the connections between the coil and the brushes every half turn
- This reverses the direction of current flow in the circuit
- This ensures that the current output is always in the same direction as the coil rotates
- The current varies from zero to its maximum value twice every cycle