7: Electric and Magnetic Fields

Question 1

How to draw uniform electric fields?

Answer 1

3 Equally spaced lines
Arrows from higher V to lower V
Lines must touch both plates

Question 2

How to draw radial electric fields?

Answer 2

All lines through centre
Lines evenly spaced
Arrows point away from positive charge; towards negative charge

Question 3

What is Faraday’s law?

Answer 3

Faraday’s law states that when there is a rate of change of flux linkage, there is an equal induced emf .

Question 3

Definition of uniform electric fields

Answer 3

• Region in space where a charged particle experiences an electric force
• Field strength is constant

Question 4

Definition of radial electric fields

Answer 4

• Area where charged particle will experience an electric force
• Field strength depends on distance to surface of object.
• Decreases as distance increases according to inverse square law

Question 5

What is Lenz’s law?

Answer 5

Lenz’s law states that the induced emf from rate of change of flux linkage

Question 6

How can u tell if question is asking about magnet moving through a coil?

Answer 6

Key words
- current produced
- emf induced
- magnet moves
- magnet falls
- electromagnetic induction

Question 7

Model answer for magnet moving through coil

Answer 7

• As magnet moves through the coil, there is change in magnetic flux linkage
• Change of flux linkage induces an emf in coil according to Faraday’s law
• Emf induced causes induced current to flow in coil
• Direction of induced current is in opposite direction of flux linkage due to Lenz’s law

Question 8

Model answer for magnet on a spring moving through a coil

Answer 8

- As magnet moves through the coil, there is change in magnetic flux linkage
- Change of flux linkage induces an emf in coil according to Faraday’s law
- Emf induced causes induced current to flow in coil
- Direction of induced current is in opposite direction of flux linkage due to Lenz’s law

• When magnet changes direction, induced emf/current changes direction producing alternating current
• Magnitude of emf/current changes depends on rate of change of flux linkage
• which depends on position and speed of magnet

Question 9

Model answer - magnetic field of wire interacts with another magnetic field

Answer 9

• A current-carrying wire generates a magnetic field around it
• Magnetic field of wire interacts with magnetic field of magnet/other current carrying wire exerting a force on it

Question 10

Model answer - Magnet dropped through copper tube

Answer 10

• As magnet moves/falls through coil, there is change in magnetic flux linkage
• Change of flux linkage induces emf in coil according to Faraday’s Law
• Induces a current to flow in coil as closed circuit which opposes direction of change in flux linkage due to Lenz’s law
• Copper tube generates a magnetic field as it is carrying a current
• The magnetic field of the copper tube interacts
  with the magnetic field of the magnet exerting a
  force opposing its motion, upwards.
• Magnet’s resultant force downwards is less and acceleration is less than g due to 2nd law

Question 11

Model answers - transformers/iron core

Answer 11

• Alternating current in primary coil produces changing magnetic field
• Iron core becomes magnetised; there is changing magnetic field in the core
• Rate of change of flux in secondary coil is equal to rate of change in primary coil
• Changing flux linkage induces emf in second. coil
• As (formula of emf induced);
• In order to step down emf, second coil must have fewer turns than first coil

Question 12

Magnet dropped through coil- graph (accelerating)

Answer 12

• As magnet falls, change of magnetic flux
• This induces emf in coil; Faraday’s Law
• Magnitude of emf is proportional to rate of change of flux linkage
• Initial increase in emf as magnet gets closer to coil
• As magnet goes through coil gradient become negative
• Magnet velocity increases as it falls hence maximum neg value greater than max positive value; time for second pulse shorter
  -Area of two pulses is same as change in flux linkage is equal and opposite.

Question 13

Model answer - coil moved through uniform field

Answer 13

• As coil moves closer to magnet, there is change of magnetic flux linkage
• This induces an emf due to Faraday’s Law
• Magnitude of emf is proportional to rate of change of flux linkage
• When the coil enters the uniform magnetic field there is no change in flux linkage so no induced emf
• As coil’s velocity is constant, two peaks are of equal magnitude and their duration same
• Area of pulses is same as flux linkage; constant

Question 14

How does thermionic emissions work?

Answer 14

The metal disk is heated due to a high current
This causes electrons to gain energy and heat up
If enough energy is gained, electrons will leave the surface of metal
In a process called thermionic emission

Question 15

How will you know if a question is asking about thermionic emissions?

Answer 15

Key words
- Metal disk
- Emit electrons
- High p.d