Electromagnetic Machines

Question 1

What is a magnetic field?

Answer 1

A region in space where a force is exerted on magnetic materials.

Question 2

How is a stronger field presented in a field diagram?

Answer 2

By increasing the line density of the field lines.

Question 3

What does the right-hand rule detail?

Answer 3

The magnetic field around a current-carrying conductor.

Question 4

How does a DC motor work?

Answer 4

The rotor coil carries a current which creates a magnetic field around the coil. This field interacts with the external field running across the rotor, causing the magnetic field lines to contract on opposite sides of the rotor coil on either side of the motor. Field lines tend to contract and straighten, therefore a force is produced on the coil. This force causes the rotor to rotate.

Question 5

What is Fleming’s left-hand rule?

Answer 5

The force on a rotor is perpendicular to the direction of current flow within that rotor and the magnetic field across that rotor.

Question 6

What is the force on a motor coil equivalent to?

Answer 6

F = BIl
Where: B = Magnetic field strength in Teslas [T]
I = Current in the coil
l = Length of the coil in the field

Question 7

What is field strength measured in?

Answer 7

Teslas

Question 8

What is 1 Tesla equivalent to?

Answer 8

1 Wb/m²

Question 9

What is a magnetic field strength?

Answer 9

The amount of magnetic flux per unit area (B = φ/A)

Question 10

How is a motor able to rotate smoothly?

Answer 10

By switching the direction of current using a split-ring commutator each time the rotor coil is vertical.

Question 11

What is an induction motor?

Answer 11

A motor which works by pulsing stator coils, which are arranged a central rotor coil, with alternating current. These current pulses create a changing magnetic field which induces a current within the rotor coil, further giving the rotor its own magnetic field. The rotor’s and stator’s fields interact, and the rotor rotates.

Question 12

What is flux density measured in/ what is flux density equivalent to?

Answer 12

Magnetic flux density is equivalent to magnetic field strength, which is measured in Teslas.

Question 13

What is total magnetic flux measure in?

Answer 13

Webers

Question 14

What is total magnetic flux (φ) equivalent to?

Answer 14

Magnetic field strength multiplied by field area.

φ = BA

Question 15

What is flux linkage?

Answer 15

Total magnetic flux multiplied by turn number (φN)

Question 16

What is a Weber?

Answer 16

A unit of measurement for total magnetic flux.

Question 17

Why is an emf induced in a conducting rod which passes through a magnetic field?

Answer 17

The magnetic field interacts with the rod’s electrons, forcing them to on end of the rod. This means that there is a charge gradient along the rod, further meaning that there is a potential difference across the rod.

Question 18

What is Faraday’s law?

Answer 18

That the induced emf in a conductor is proportional to the rate of change of flux linkage.
ε = -N dφ/dt

Question 19

What is Lenz’s law?

Answer 19

The emf induced in a rod is always in such a manner as to produce a magnetic field which opposes the field change that created it.

Question 20

How does Lenz’s law ensure that the conservation of energy is adhered to in electromagnetic systems?

Answer 20

The energy required to move the conductor through the magnetic field against the resistance of that field, is what produces the current within the conductor.

Question 21

What is a transformer?

Answer 21

A device which alters the voltage of a given circuit’s electricity.

Question 22

How does a transformer work?

Answer 22

By alternating the voltage across the primary coil, thereby inducing a changing magnetic field within the transformer core. This changing field induces an emf in the secondary coil, further creating a current in that coil.

Question 23

What is the relationship between the ratio of the number of turns in the primary coil and the number of turns in the secondary coil, and the voltage created across the secondary coil relative to the first coil’s voltage?

Answer 23

Ns/Np = Vs/Vp

Question 24

What is permeability?

Answer 24

The magnetic equivalent of conductivity - it is how easily a material aligns itself so as to support a magnetic field.

Question 25

What is permeance?

Answer 25

The magnetic equivalent of conductance - it is how easily a given object passes a magnetic field.

Question 26

What is the permeance (Λ) of an object equivalent to?

Answer 26

The permeability of the object multiplied by its c.s.A, over its length.
Λ = μA/L

Question 27

What are desirable features for a transformer to have?

Answer 27

A high core permeance - high permeability, large cross-sectional area, short length.
A high coil conductivity - high conductivity, large cross-sectional area, short length (tightly wound).

Question 28

What is the main form of power loss in a transformer?

Answer 28

Heat loss - eddy currents in the core create heat.

Question 29

How can heat loss be prevented/reduced in a transformer core?

Answer 29

By laminating the core - this reduced the amount of flux per unit area in the core, further reducing the magnitude of the currents induced within the core. The smaller sections also have a higher resistance, reducing the currents further.

Question 30

What is the transmittance and domestic usage voltage of mains electricity in the UK?

Answer 30

Transmittance: ~ 400kV
Domestic: 230V

Question 31

What is an alternator?

Answer 31

A generator of alternating electric current.

Question 32

How is an alternator able to provide sustained current to an external circuit?

Answer 32

Via. slip rings connected to each end of the coil.

Question 33

How often does the output voltage direction of an alternator change?

Answer 33

Each half-turn (T/2)