Electrodynamics

Question 1

Magnetic field

Answer 1

a region of space where another magnet or ferromagnetic material will experience a force.

Question 2

Ferromagnetic

Answer 2

materials which can be magnetised.

Question 3

Right hand wire rule

Answer 3

Thumb - point right thumb in the same direction as conventional current, the rest of your fingers will curl in the direction of the magnetic field around the wire

Question 4

3-D direction convention

Answer 4

dot - out of page towards you
x - into page

Question 5

Right hand solenoid rule

Answer 5

curl fingers in direction of conventional current, thumb points to direction of north pole of solenoid.

Question 6

Electromagnetic induction

Answer 6

when current is induced when a solenoid and magnet move relative to each other.

Question 7

Electromagnetism theory

Qualitatively explain the factors that affect the magnitude of the
force on a current-carrying conductor using the equation
F =IℓBsinθ where ℓ is the length of the conductor experiencing
the magnetic field and θ is the angle between the current and the
magnetic field (no calculations required)

Question 8

DC motors

Answer 8

convert electrical energy to mechanical energy.

Question 9

Explain why a current-carrying coil placed in a magnetic field will
turn by referring to the forces exerted on the sides of the coil
perpendicular to the field

Answer 9

• current carrying coil is in magnetic field
• power source is direct current
• when coil parallel to magnetic field:
  -> current enters coil end connected to positive (+) terminal, through carbon brush “y”, through half split ring “2”, and flows from D to C
  -> side DC experiences force downwards (use left hand motor rule) because of motor effect
  -> similarly, side BA experiences force upwards
  -> ∴ coil rotates clockwise
• when coil perpendicular to the magnetic field (later referred to as “perpendicular position”):
  -> split rings lose contact with carbon brushes
  -> ∴ no current in coil
  -> coil’s momentum rotates it past perpendicular position
• each half of split ring now connected to different brush and different terminal, i.e. side AB (connected to half split ring “1”) connects to carbon brush “y” (“+ “ )
  -> ∴ direction of force on individual arms changes
  -> ∴ coil continues to rotate
• split ring commutator: enables arms of coil rotating on one side of “perpendicular position” to be connected to one terminal and when rotating on other side of “perpendicular position”, to be connected to other terminal
• speed of rotation: increases as current in coil increases

Question 10

Given a diagram of a direct current (d.c.) motor, explain the basic
principles of operation including why a d.c. motor has a split-ring
commutator

Answer 10

coil: the current carrying conductor in the magnetic field
magnet: provide constant magnetic field
slip rings: keep continuous contact between one arm of the coil and one of the brushes
carbon brushes: make electrical contact between the static and rotating parts of the circuit (terminals and slip rings)
AC source: provide alternating current to the coil

Question 11

Left hand motor rule

Answer 11

Thumb - motion
Index finger - field
Middle finger - current

Question 12

Magnetic flux (Φ)

Answer 12

the product of the magnetic field and the area perpendicular
to the field.

Question 13

Magnetic flux density (B)

Answer 13

a representation of the magnitude and direction of the magnetic field

Question 14

Φ = BA cos Θ described

Answer 14

for a loop of area (A) in the presence of a uniform
magnetic flux density (B), the magnetic flux (Ф) passing through
the loop is Ф = BAcos θ where θ is the angle between the
magnetic flux density (B) and the normal to the loop of the area
(A).

Question 15

Magnetic flux linkage

Answer 15

the product of the number of turns on the coil and the flux through the coil (NΦ).

Question 16

Faraday’s law of electromagnetic induction

Answer 16

the emf induced is directly proportional to the rate of change of magnetic flux (flux linkage).

Question 17

Lenz’s law

Answer 17

the induced current flows in a direction so as to set up a magnetic field to oppose the change in magnetic flux.

Question 18

Magnetic flux is changed by

Answer 18

• changing magnetic field strength
• changing area inside coils
• rotating coil

Question 19

Electromagnetic induction theory

Explain simple applications of electromagnetic induction (e.g. the
induced current and its direction when a magnet is passed through
a coil)

Question 20

AC generators

Answer 20

• convert mechanical energy to electrical energy.
• has slip rings.

Question 21

Operation of generators and transformers using Faraday’s Law equation

Use the equation emf = -N ΔФ/Δt
for Faraday’s law to explain
qualitatively the operation of generators and transformers. (No
calculations required)

Question 22

Right hand generator rule

Answer 22

Thumb - motion
Index finger - field (points towards North)
Middle finger - current

Question 23

Factors that affect induced emf

Question 24

Basic principle of AC generator
Explain the basic principle of an AC generator
(alternator) in which a coil is mechanically rotated in a magnetic
field

Answer 24

• rotating mechanism: to provide the mechanical energy to rotate the coil
• magnet: to provide constant B-field
• coil: to enclose area, A, which B-field lines can pass through
• slip rings: to allow current to pass from the coil through the carbon brushes to the external circuit
  -> each slip ring rigidly attached to one side of coil and rotates with it
  -> carbon brush presses against slip ring as slip ring rotates
• carbon brush: to form electrical contact between rotating parts of generator and external circuit

Question 25

Transformer

Answer 25

converts electrical energy in the primary coil to magnetic energy in the core and back to electrical energy in the secondary coil.

Question 26

Mutual induction

Answer 26

the induction of a voltage in one coil in response to a change in current in another coil (caused by flux linkage).

Question 27

Principle of operation of a simple iron-cored transformer

Question 28

Ideal transformer

Answer 28

for an ideal transformer, input power is equal to output power.

Question 29

Diode

Answer 29

a component that only allows current to flow in one direction.

forward bias - “arrow” of diode points in direction of current.
reverse bias - “arrow” of diode points against direction current would flow.

Question 30

Half-wave rectification
Explain how a single diode is used for the half-wave rectification of
an alternating current

Answer 30

• when current flowing in forward bias direction, diode allows current to flow
• when current changes direction (to reverse bias), diode “cuts off” current (i.e. no current flows)
• all “–” current values are prevented, ∴ half-wave rectified current has alternating “+” and zero values

Question 31

AC theory

Discuss the scientific and economic advantages of high voltages
and low currents for the transmission of electrical energy through
the national grid