P15 Electromagnetism

Question 1

What is an induced magnet?

Answer 1

A material that becomes magnetic when placed in a magnetic field

(Induced magnets always produce a force of attraction)

Question 2

What are the 4 magnetic materials?

Answer 2

Iron
Steel (alloy of iron)
Cobalt
Nickel

(Can be made into permanent or induced magnets)

Question 3

What are permanent magnets?

Answer 3

Magnets that produce their own magnetic field

Question 4

Where is magnetic force felt strongest

Answer 4

• Closer to the magnetic material

- Towards the poles of the material

Question 5

Describe the ways that you could increase the strength of the magnetic field around a solenoid

Answer 5

• Increasing size of the current
• Increasing number of turns (while keeping length the same)
• Add an iron core (electromagnet)

Question 6

Benefits of electromagnets

Answer 6

• Can change the size of the magnetic field by changing the current
• Can be turned on and off

Question 7

What device is used in electric motors to allow coils to continue rotating

Answer 7

Split ring commutators

Question 8

What is the basis of an electric motor according to the specification?

Answer 8

A coil of wire carrying a current in a magnetic field tends to rotate.

Question 9

Describe ways to increase the induced potential w the generator effect

Answer 9

• Move wire/ magnet faster
• Use a stronger magnetic field
• Shape the wire into a coil (more coils = greater pd and current)

Question 10

Explain how work is done when u move a magnet through a coil in a circuit

Answer 10

• Moving the magnet cuts lines of magnetic flux, which induces a potential
  difference and current in the coil.
• This current produces its own magnetic field, which opposes the magnets
  movement (visualise)
• Makes it harder to move the magnet, which shows that work is being done
• Energy required to move the magnet is being transferred to movement of
  the current

Question 11

What is an alternator?

Answer 11

A coil of wire rotating in a magnetic field

Question 12

Describe how to increase the size of AC in an alternator

Answer 12

• Increase strength of magnetic field
• Increase number of turns/area of coil
• Increase rotational speed of coil (increases size and frequency)

Question 13

Why are there two peaks for each full rotation of the coil in a dynamo?

Answer 13

Each side of the coil passes through the magnetic field twice per rotation

Question 14

Why do transformers only work with AC?

Answer 14

• A changing magnetic field is needed to induce a potential difference
• DC produces a constant magnetic field, which wouldn’t work and no pd
  would be induced

Question 15

What not to forget abt transformers

Answer 15

Potential difference isn’t always exact as transformers may not be 100% efficient

Power must be conserved (V1I1=V2I2)
(only applies if transformer is 100% efficient)

Question 16

Describe how transformers are used in power transmission

Answer 16

• Power is wasted in national grid via heating
• Power lost depends on square of current or whatever
• Increasing voltage decreases the current, reducing energy transferred via
  heating

Question 17

Why must the coils of wire around a transformer be insulated?

Answer 17

without insulation, the path of least resistance would be straight across the coil (wire to wire to wire) rather than around the coil, through the wire. It would not create a magnetic field in the same way a coil would, making the transformer not work.