p7-magnetism and electromagnetism

how many poles does a magnet have and what happens when they’re placed near each other?

how many poles does a magnet have and what happens when they’re placed near each other?

-2 poles (North and South)

-like poles(NN or SS) repel

-unlike/opposite poles (NS) attract

what do magnets produce?

A magnetic field

which is a region where magnetic materials experience a non contact force(of attraction or repulsion)

- are strongest at the poles so,forces are strongest at the poles

-magnetic field lines are drawn from N to S

what is a permanent magnet?

a material which has its own magnetic field

what is an induced magnet?

a material that becomes a magnet only when placed in a magnetic field

-induced magnetism only produces attraction forces

how do compasses show the direction of a magnetic field?

-inside compasses are bar magnets(needle)

-north pole of the magnet is attracted to the south pole of any magnet near

-compass needle points in direction of the magnetic field it’s in

-when not near a magnet, compass needles always point N because the Earth generates its own magnetic field-shows that the inside core of the Earth must be magnetic

Method for using a compass to plot magnetic field lines around a bar magnet

1) Place a magnet on a piece of paper and draw around it

2) Place the compass by a pole of the magnet and make a dot at the tip of the compass needle

3) move the compass tail to the dot and make a new dot at the tip

4) repeat until the compass reaches the other pole of the magnet

5) draw a line through the dot

6) repeat for different starting positions at the poles

7) add arrows showing the direction of the magnetic field lines(pointing from N to S poles)

8) mark N and S poles

Right hand thumb rule(1)

right hand thumb rule(2)

what happens when current flows through a wire?(a)

a magnetic field is created because MOVING CHARGE CREATES A MAGNETIC FIELD

the field is made of concentric circles perpendicular to the wire( wire in the centre )

what is a solenoid/ how do we make a solenoid?

coil of wire w/ current flowing

shaping the wire into a coil to increase the strength of the magnetic field

-the field lines around each loop of wire line up

means lots of field lines are very close and pointing in the same direction

the closer the field lines are stronger the field

what is the magnetic field of a solenoid like?

-similar to a bar magnet

-strong and uniform inside the solenoid because it has the same strength and direction at every point in that region

what is an Electromagnet?

-solenoid with an iron core

-a magnet whose magnetic field can be turned on and off with an electric current

(inserting an iron core in a solenoid will increase the magnetic field by confining field lines inside of itself)

-the iron becomes an induced magnet wherever the current is flowing

what happens when you stop the current?

the magnetic field stops/ disappears

how do you increase the strength of an electromagnet?

1) increase the number of turns of the wire

2) increasing the current

F=BIL

F=Force in Newtons[N]

B=Magnetic Flux Density/Magnetic Field in Tesla[T]

I=Current in Amps[A]

L=Length of the wire within the magnetic field in metres[m]

mA / 1000= A

how do you reverse the direction of the magnetic field?

1) swapping the polarity of the battery

2) reversing the current/ circuit

3) swapping the poles of the magnet

how do we increase the strength of a magnetic field?

1) increase current (using more powerful battery)

2) increase magnetic field(B) by choosing a stronger magnet

3) increase length by coiling the wire

the motor effect

-magnetic field around a wire(or any conductor)

-wire is placed between 2 poles

-magnetic field around the wire interacts with magnetic field around the poles

-magnet and wire/conductor exert a force on each other and the wire moves

complete the sentence:

if the conductor/wire is parallel to the magnetic field….

… the conductor/wire will NOT experience a force

complete the sentence:

if the conductor / wire is at a right angle to the magnetic field…

…..the conductor/ wire WILL experience a force

how can we show the direction of the force?

-applying a current to a set of rails inside a horseshoe magnet

-bar is placed on the rails, completing the circuit

-this generates a force which rolls the bar along the rails

fleming’s left hand rule

Electric motors

-2 side arms of a coil carrying an electric current

-current flowing in through the + terminal(right) and wire experiences a downwards force

-current coming out of the - terminal(left) and wire experiences an upwards force

(2 different forces because current flowing in opposite directions )

what happens as a result of the opposite forces on either side of the wire?

the coil will start to spin clockwise

what happens when the coil is rotated by 180°?

-the current travels in the opposite direction

-and forces acting on each side of the coil have swapped

(so left side experiences a downwards force and the right experiences an upwards force)

-THIS CAUSES THE COIL TO TURN ANTI CLOCKWISE

-the coil will keep rotating and will not turn 360°

how do we fix the coil so it rotates fully?

-change the direction of the current every half turn so the direction of the forces are always clockwise and the coil will rotate in the same way

-we use a SPLIT RING COMMUTATOR for this

what does a split ring commutator do?

-swaps the + and - connections every half turn

-causes the direction of the current to swap every half turn

-means forces acting on the coil will always be acting in the same direction so…

-coil will rotate in the same direction

how do we increase the speed of the rotations?

1) increase the current

2) add more turns to the coil

3) increase magnetic flux density(by using stronger magnets)

what happens when the compass is placed near the wire? (a)

It will trace the direction of the magnetic field

What does changing the direction of the current do the magnetic field? (a)

It changes the direction (of the mf)

What is the strength of the magnetic field affected by? (a)

The strength of the current and the distance from the wire

(e.g. the larger the current or the closer to the wire you are, the stronger the magnetic field

What does flemming's left-hand rule show?

If either the current on the magnetic field is reversed, then the direction of the magnetic field is reversed.

What does a dc motor do?

Forces act on the 2 sides of a coil of a wire caryinya current • because the current is is 2 differenSt, directions, the forces are opposite (going up or down), causing the wire to rotate . It rotates until 180° then spins the other way. We place a split ring commutator so the wire rotates one way only (by swapping the + and - connections every half turn).

what is the role of graphite brushes?

they are electrical conductors and complete the circuit as the wire rotates