section 6: Magnetism and electromagnetism

Question 1

units:

Answer 1

-ampere (A)
-volt (V)
-watt (W)

Question 1

magnetism:

Answer 1

-magnets repel and attract other magnets and attract magnetic materials, like poles of magnets repel and opposite poles attract
-non-magnetic materials are material that are not attracted to magnets and cannot be magnetised (e.g. glass, plastic)
-magnetic materials are materials that are attracted to magnets and can be magnetised (e.g. iron, steel, cobal, nickel)
-magnetism can be induced in magnetic materials by placing them in a magnetic field
-magnetic materials that can be permanently magnetised are described as magnetically hard (e.g. steel)
-magnetic materials that are only temporarily magnetised are described as magnetically soft (e.g. soft iron)

Question 2

magnetic field lines:

Answer 2

-magnetic field lines represent the magnetic force on a north pole at a given point
-the direction of a magnetic field line shows the direction of the force
-how close together the magnetic field lines are shows the magntidue of the force
-field lines from magnets point from north to south
-there is a uniform magnetic fielf between the opposite poles of two magnets placed close together, as the field lines move from the north pole of one straight towards the south pole of the other, the field lines are parallel and evenly spaced

Question 3

practical: investigate the magnetic field pattern for a permanent bar magnet and between two bar magnets

Answer 3

-method 1: iron fillings
1) sprinkle some iron fillings onto a sheet of paper
2) place the permanent bar magnet onto the paper and the fillings shouls move into the shape of the magnetic field
3) repeat this with two permanent bar magnets placed a short distance apart
-method 2: plotting compass
1) place the permanent bar magnet onto a plain piece of paper
2) place the plotting compass somewhere around the field and then draw the direction of the needle at that point
3) continue placing the needle in various positions on the paper and drawing arrows
-the arrows should show that the field lines go from the north pole to the south
5) you can also try this with two permanent bar magnets placed a short distance apart
-experiment with placing like poles and unlike poles facing eachother

Question 4

electromagnetism + Fleming’s left hand rule:

Answer 4

-an electric current passing through a conductor produces a magnetic field around it
-a force acts on a current-carrying conductor in a magnetic field
-Fleming’s left-hand rule shows the relative directions of the force, field and current…
-thumb = direction of force
-index finger = direction of field
-middle finger = direction of current

Question 5

The motor effect:

Answer 5

-if the current is reversed or the magnetic field is reversed, the force will be reversed
-if the magnitude of the current or the magnetic field is increased, the magnitude of the force will increase

Question 6

DC motors:

Answer 6

-DC motors consist of a coil of wire in between two permanent magnets
-direct current flows through the wire and it expericiences a turning effect due to the forces exerted on it in the magnetic field
-as the current flows in opposite directions on each side of the coil, the forces on each side are in opposite directions-making it turn
-the turning effect can be increased by:
-increasing the current
-using a stronger magnetic field
-increasing the number of turns on the coil

Question 7

a commutator:

Answer 7

-a split ring commutator is used to ensure that the direction that the current flows in the coil reverses every half turn

Question 8

loudspeakers:

Answer 8

-loud speakers consist of a coil attached to a cone in a magnetic field
-when alternating current flows through the coil, the cone is continuously pushed away and pulled back, making a sound
-the frequency (and therefore pitch) of the sound can be altered by changing the frequency of the alternating current used

Question 9

electromagnetic induction: the generator effect

Answer 9

-when there is relative movement between a conducting wire & a magnetic field, a voltage will be induced
-for example, if conducting wire moves across a magnetic field, a voltage is induced in it, if it is part of a complete circuit, this causes the a current to flow
-this is called the generator effect

Question 10

electromagnetic induction: voltage

Answer 10

-the induced voltage can be increase by:
-moving the wire more quickly
-using a stronger magnetic field
-increasing the length of the wire inside the magnetic field (e.g. by making it more coiled)
-a voltage is also induced in a coil with a changing magnetic field through it. For example, when a magnet is moved into a coil, a voltage is induced in it
-the more quickly the magnetic field changes, the greater the voltage

Question 11

electromagnetic induction: electricity

Answer 11

-electricity can be generated by rotating a magnet within a coil or by rotating a coil in a magnetic field
-as they rotate, the magnetic field through the coil changes, which induces a voltage and therefire a current in the coil, the voltage can be increased by:
-increasing the length of a wire inside the magnetic field (e.g. by using a larger area, more turns or a longer wire)
-using a stronger magnet
-increasing the speed of rotation