Chapter 24 - Magnetic fields

Question 1

What are magnetic fields produced by?

Answer 1

The movement of charge or permanent magnets.

Question 2

What is the direction of magnetic fields in terms of poles?

Answer 2

North → South

Question 3

Unlike poles ______ and like poles ______.

Answer 3

Unlike poles attract and like poles repel.

Question 4

For a current-carrying wire, the magnetic field lines are…

Answer 4

…concentric circles in a plane perpendicular to the length of wire.

Direction of magnetic field and CONVENTIONAL current are shown by the right hand grip rule (as if you were holding the wire).

Question 5

What is a solenoid?

Answer 5

A long coil of wire with a large number of loops.

Magnetic field lines flow straight through the centre of a solenoid.

Question 6

The closer the magnetic field lines (lines of flux), the _______ the magnetic field.

Answer 6

The closer the magnetic field lines (lines of flux), the stronger the magnetic field.

Question 7

What is magnetic flux density, B?

What is the unit?

Answer 7

The density of magnetic field lines (lines of flux) in a given region. It is a measure of the strength of a magnetic field.
Unit: Tesla, T

Question 8

What is shown by Fleming’s left hand rule?

Answer 8

Force (thumb), magnetic field and conventional current all perpendicular to each other.

The force represents the direction of motion (as all forces are accompanied by an acceleration which leads to motion).

Conventional current represents the direction of positive charges (opposite to electron flow).

Question 9

What is represented by F = BIL?

Answer 9

The equation for the force on a current-carrying wire in a magnetic field perpendicular to the wire (current).
(Use Fleming’s left hand rule for directions)

If the wire is parallel to a magnetic field the force exerted by the magnetic field F = 0 (minimum).

Question 10

Explain what happens to a current-carrying wire perpendicular to a magnetic field (e.g. that produced by a permanent magnet).

Answer 10

The magnetic field of the moving charges and magnet interact and the field lines from each source become distorted.
The interaction between the force fields causes the field of the magnet to exert a force on the wire.
(Magnitude F = BIL, direction shown by Fleming’s left or right hand rule depending on charge in wire (usually you can assume its conventional current, so left hand rule))

Question 11

Describe the motion of a charged particle in a magnetic field to which it is travelling perpendicular to.

Answer 11

The force on a charged particle due to a magnetic field is always perpendicular to the particle’s direction of motion.

As a result, the force on a charged particle will always be directed inwards, so it will move in a circular path (in the plane perpendicular to the magnetic field).

Question 12

What is represented by F = BQv?

another form of F = BIL

Answer 12

The force exerted on a charged particle moving in a magnetic field with a velocity perpendicular to the magnetic field.

The magnitude of the force is maximum when the charge is travelling perpendicular to the field and minimum (F = 0) if parallel.

Question 13

How can the radius of the path of a charged particle travelling perpendicular to the magnetic field be calculated?

Answer 13

The radius of the charged particle’s circular path can be calculated by equating BQv to the equation for centripetal force.

Question 14

What is magnetic flux, Ф?

What is the unit?

Answer 14

Ф = 𝐵A
where B is perpendicular to A (cross-sectional area).

Unit: Weber, Wb

Question 15

What equation must be used for magnetic flux and flux linkage if B is not perpendicular to A.

Answer 15

Ф = 𝐵Acosθ
NФ = 𝐵ANcosθ

Ф is maximum when B and A are perpendicular and minimum when B and A are parallel

Question 16

What is a transformer?

Answer 16

A pair of coils linked by an iron core. The primary coil is attached to an input AC supply and the secondary coil is connected to the output voltage.

Question 17

Describe the action of a transformer.

Answer 17

The primary coil provides a changing magnetic field (and changing NФ… maybe?), which passes through the iron core and interacts with the secondary coil, inducing an emf/voltage in the secondary coil, higher or lower than in the primary coil depending on the type of transformer.

Question 18

What is the role of a transformer?

Answer 18

To change voltage and there are two different types - step-up (Ns > Np) and step-down (Np > Ns) transformers.

Question 19

What are some causes of inefficiencies in transformers?

Answer 19

• Energy loss due to resistance in the coils as it causes heating
• Energy loss due to a core that is not easily magnetised and demagnetised
• Energy loss due to induced currents in the core (eddy currents)

Question 20

How do eddy currents cause energy loss in a transformer?

Answer 20

Due to Lenz’s law, the eddy currents (induced currents) oppose the field that produced them, reducing the fields flux density and generating heat.

Question 21

Why are transformers used in electrical power transmission by the national grid?

Answer 21

P = IV
therefore, to reduce power loss, the current should be reduced to a minimum value to prevent energy loss which is done by increasing potential difference.

Step-up transformers are used for long distance transmission of electricity to reduce current by increasing potential difference.

Question 22

What is magnetic flux linkage?

Answer 22

The amount of flux cut by a coil.

NФ = 𝐵ANcosθ

Question 23

What is the equation for power of an ideal transformer?

Answer 23

IpVp = IsVs