6 Magnetism and electromagnetism

Question 1

what do magnets do

Answer 1

attract or repel other magnetic substances

Question 2

what are the properties of a magnetically hard material

Answer 2

not easily magnitised or demagnitised

once it is magnetised it will stay magnetised for a long time even once the external field is removed

Question 3

what are the properties of a magneticallt soft material

Answer 3

easily magnetised and unmagnetised

won’t stay magnetised once the external field is removed

Question 4

examples of a magnetically hard material

Answer 4

steel

Question 5

examples of a magnetically soft material

Answer 5

iron

Question 6

what is a magnetic field line

Answer 6

The region around a magnet where a force acts on another magnet or on a magnetic material

Question 7

how is magnetism induced

Answer 7

it is induced in certain materials when they are placed in a magentic field

Question 8

practical: investigate the shape and direction of the magnetic field around a bar magnet

Answer 8

place the magnet on a piece of paper
Draw a dot at one end of the magnet (near its corner)

Place a plotting compass next to the dot, so that one end of the needle of the compass points away from the dot
Use a pencil to draw a new dot at the other side of the compass needle

Move the compass so that it points away from the new dot, and repeat the process above

Keep repeating the previous process until there is a chain of dots going from one end of the magnet to the other
Then remove the compass, and link the dots using a smooth curve – this will be the magnetic field line

Question 9

practical: investigate the shape of a magnetic field around a bar magnet

Answer 9

place the magnet on a piece of paper

gently scatter iron filings around the magnet

gently tap the piece of paper and the iron will move into the shape of the field

Question 10

how to use 2 permenant magnets to form a uniform magnetic field

Answer 10

place the 2 magnets a few centimeters apart

have the north of one magnet point to the south of the other magnet

Question 11

how to draw a uniform magnetic field

Answer 11

straight evenly spaced lines pointing from north to south

Question 12

what does an electric current in a conductor produce

Answer 12

a magnetic field

Question 13

construction of an electro magnet

Answer 13

a soft iron core wrapped with wire connected to a cell/battery

Question 14

what is the magnetic field pattern around a straight wire

Answer 14

field lines go in circles around the wire
use the right hand rule to work out the direction that the field lines are going in

Question 15

what is the magnetic field pattern around a flat circular coil

Answer 15

it is circles around the wire

Question 16

what is the magnetic field pattern around a soleniod

Answer 16

the same as a bar magnet

Question 17

what happens to a charged particle when it moves parallel to the field lines

Answer 17

nothing it doesn’t experience a force

Question 18

what happens to a charged particle when it moves perpendicular to the field lines

Answer 18

it will experience maxium force

Question 19

what is flemings left hand rule

Answer 19

left hand

thumb represents the direction in which the force will act on the wire
first finger - the direction the magnetic field runs in (north to south)
middle finger - the direction the current is going in (+ -> -)

Question 20

why is a force exerted on a current carrying wire in a magnetic field

Answer 20

the 2 magnet field (existing one + wire) interact
a force will act on the wire

Question 21

how is d.c. used to create simple electric motors

Answer 21

Current flows in the wire/coil.
This creates a magnetic field around the wire/coil.
This magnetic field interacts with the field from the permanent magnet.
This produces a force on the wire/coil which moves the wire/coil.
equal forces in opposite directions produce a turning movement so the coil spins
The split-ring commutator changes the direction of the current every half turn as it spins. This reverses the direction of the forces, allowing the coil to continue spinning.

Question 22

how is a.c. used to create simple loudspeakers

Answer 22

a cone is placed on the south pole of a circular magnet
the cone is wrapped in wire which is connected to a AC power supply
as current is passed through the wire it generates an magnetic field
the two fields interact and produce a force which causes the cone to move
when the current switches direction the force on the cone reverses
this causes the cone to move in and out and generate sound waves
by changing the frequency of the AC supply we can change the frequency that the cone vibrates
the vibrations create sound waves
by increasing the current we increase the amplitude and loudness

Question 23

how does the force on a current carrying conductor (eg wire) in a magnetic field change with the magnitude and direction of the field and current

Answer 23

If you increase the magnitude of the current through a wire or the size of the magnet being used, you increase the force on the wire.

If you change the direction of the current or reverse the poles of the magnet, you change the direction of the force on the wire

if you wind more coils of wire that will also increase the force