Magnetism and Electromagnetism

Question 1

State the SI units for the following
Current
Voltage
Power
Amps
Volts
Watts

Answer 1

I
V
P
A
V
W

Question 2

What do magnets do?

Answer 2

Repel and attract other magnetic substances

Opposite poles attract and poles that are alike repel

Question 3

What substances can be magnetised

Answer 3

Nickel
Iron
Cobalt
and their alloys e.g. Steel

Question 4

What do you call…
A magnet that can be temporarily magnetised
A magnet that can be permanently magnetised

Answer 4

Magnetically Soft

Magnetically Hard

Question 5

What to magnetic field lines show

Answer 5

The direction of a magnetic field (from north to south)

The spacing between adjacent field lines indicate the strength of the magnetic field

Question 6

Describe an experiment to investigate the magnetic field pattern around a bar magnet using iron filings

Answer 6

• Place a bar magnet under a piece of waxy paper
• Lightly sprinkle iron filings onto the waxy paper
• Then gently heat the waxy paper over a Bunsen burner -until the iron filings are set into the paper
• This will show you the field line but not the direction

Question 7

How can we produce a uniform magnetic field pattern?

Answer 7

Using two permanent magnets (2 wide and close opposite poles (field lines go from north to south)

Question 8

What does an electric current in a wire produce?

Answer 8

It produces a magnetic field around it

Question 9

Describe the construction action of an electromagnet and how the strength of it can be increased

Answer 9

-When current flows in a conductor (wire) it produces a magnetic field around it
-We can work out the way the current goes using the right hand grip rule
We can increase the strength by:
-larger current
-More coils
-Adding an iron core

Question 10

Describe the Motor Effect

Answer 10

• Current flows in a conductor (wire or coil)
• This creates a magnetic field around the wire which interacts with the permanent magnetic field
• This creates a force (movement) on the wire
• For a motor this creates am upwards force on one side and a downwards force on the other side
• We can find out the direction of the force using Flemings left hand rule

Question 11

We can use Flemings left hand rule to…

Answer 11

Predict the direction of the resulting force when a wire carries current perpendicular to a permanent magnetic field

Question 12

What causes there to be no force (movement) during the motor effect

Answer 12

If the current is parallel to the magnetic field

Question 13

Describe how the strength of the force can be increased

Answer 13

Increasing the current

And in a motor adding more coils

Question 14

How can the motor effect be used in loudspeakers

Answer 14

• Current flows in the coil
• This creates a magnetic field around the wire, which interacts with the permanent magnetic field
• This creates a force (movement) on the wire
• An AC current is used, so the direction of the force is constantly changing
• This makes the could vibrate (50Hz AC makes the coil vibrate at 50Hz)
• The vibrations cause sound (a longitudinal wave)

Question 15

Describe how electromagnetic induction creates a voltage in a wire

Answer 15

• A wire (or coil) cuts through magnetic field lines (we can either move the coil, or create a moving magnetic field, but one must move relative to the other)
• This induces a voltage across the wire (if the wire is part of a circuit, this creates a flow of current)

Question 16

How do we use electromagnetic induction in a power station?

Answer 16

We use electromagnetic induction to generate electricity either by:
-By rotation of a magnet within a coil of wire
or
-Rotation of a coil of wire with a magnetic field

Question 17

The voltage which is induced through electromagnetic induction can be increased by…

Answer 17

• Moving ht wire relative to the field faster
• Using more coils
• Using a stronger magnetic field

Question 18

Describe the construction of a transformer

Answer 18

• Two coils are wound around an iron core (or a pair of ‘C’ cores that are held together
• Current in the primary coil creates a magnetic field (the presence of the ironware increases the strength of the field)
• The current must be AC (won’t work with DC)
• The AC current crates a changing magnetic field, which causes the secondary coil to cut field lines (as the field moves)
• This induces a voltage in the secondary coil

Question 19

Explain why electrical power is transmitted at high voltage and low current

Answer 19

• When large current flows electrons collide with the ions in the metal lattice (that make up the wire)
• The electrons transfer (kinetic) energy to the ions, and the wire heats up
• This means that some of the electrical energy is lost as heat energy
• The larger the current the more energy is lost as heat

Question 20

What is the effect of AC current on a magnetic field

Answer 20

• Creates a changing magnetic field which causes the secondary coil to cut field lines (as the field moves)
• This induces a voltage in the secondary coil

Question 21

What different transformers are there

Answer 21

Step-up transformer - increases voltage

Step-down transformer - decreases voltage

Question 22

What is the equation for the relationship between input (primary) and output (secondary) voltages and the turns trio for a transformer

Answer 22

Vp. =. np
_. _
Vs. ns

Question 23

What is the equation for the relationship between input and output current and voltage for 100% efficient transformer

Answer 23

Input power = output power
and since P=IxV
Ip x Vp = Is x Vs