2- Chapter 15 - Electromagnetism

Question 1

Emf generated=

Answer 1

• Change in flux/time * Coils
• NΔΦ/Δt = -NBACosθ/t

Question 2

Flux linkage=

Answer 2

NΦ = NBACosθ

Question 3

What is magnetic flux measured in?

Answer 3

Tm² or Wb

Question 4

What is field strength measured in?

Answer 4

Telsa’s (T)

Question 5

What is θ in magnetic flux equation?

Answer 5

The angle between field direction and line perpendicular to coils.

Question 6

What is a magnetc field?

Answer 6

A region in which a magnet or moving current experiances a force.

Question 7

What is a magnetic domain?

Answer 7

a polar section of a magnetic material which aligns with the pole when the matial gets magnatised.

Question 8

Wha happens when you magnatise a material

Answer 8

the magnetic domains inside the material line up and form an overall pole,

Question 9

rules for field lines

Answer 9

1. right angle from/into things
2. dont cross
3. no kinks
4. Magnetic is from N—> S , Electric is from +ve —> -ve , Gravitation is towards mass.

Question 10

Left hand rule

Answer 10

Thumb is the motion

First finger is Field

Centre finger is current

Question 11

FAradays law?

Answer 11

Any change in the magnetic environment of a coil will cause a voltage to be induced in the coil.

Question 12

lenz’s Law

Answer 12

The direction of any magnetic induction is such to oppose the cause of the effect.

Question 13

How does a transformer work?

Answer 13

1. Current in primary coil induces field around coil in the core.
2. THis field is changing directions as its ac current.
3. The soft iron core amplifies the field
4. The changin in flux in the core induces an emf in the secondary coil proportional to number of coils

Question 14

transformer equation?

Answer 14

V_p/V_s=N_p/N_s=I_s/I_p

Question 15

Why is the core of a transformer laminated

Answer 15

To reduce the formation of eddy currents

Question 16

Step down / step up transformer?

Answer 16

Step Down = N_s<n>p so Volatage decreases</n>

Step up = N_s>N_p so Volatage Increases

Question 17

What is magnetic permeability?

Answer 17

The ability of a material to become magnitised

Question 18

Relative permability?

Answer 18

μr=μ/μ₀

=Material permability/Vaccume permability

Question 19

Types of magnetic material

Answer 19

• Feromagnetic Material : μ_r>1 - Stongly attractive
• Paramagnetic Material : μ_{r Aprox} 1 - weakly attractive
• Diamagnetic Material : μ_r<1 - Repelled by a magnet

Question 20

Permance(Λ)=

Answer 20

μA/L

in WbA^-1 (webers per amphere turn)

Question 21

Force on a straght wire=

Answer 21

IlBsinθ

Question 22

Forces on a staght wire explained?

Answer 22

When current flows in a wire and an extranal field is appied across the flow, the wire expericances a force perpendicular to field and current

Question 23

How does a dc motor work?

Answer 23

• Magnetic field across a wire carrying dc current induces a movement
• The commutator ensure that the current flips every half a roatation so it keeps spinng the same way (or could us ac current)

Question 24

Torque in a dc motor=

Answer 24

NIABsinθ

Question 25

What is back emf?

Answer 25

The emf induced in the motor coil in the opposite direction to the cell emf( caused by the change in flux as the wire spins). THis slows down the motor as the current has decreased therefore so has torque.

Question 26

What problems happen due to back emf?

Answer 26

When back emf= applied emf at very high speed the motor does not work. This means the motor has to slow down so that a current can flow

Question 27

If I is proportinal to Sinwt then what happens when you double the frequency?

Answer 27

Twice the emf, twice as fast as Emf= wsinwt

Question 28

What is B

Answer 28

Field strength or flux density = Flux/Area

Question 29

What is a three phase generator?

Answer 29

Has three sets of coils 120 degrees around the rotor. These three coils are joined by a common wire so 4 wires are used not 6. The 4th wire can be very thin due to the phase difference causing nearly 0 current in it. This saves loads of wire and money

Question 30

What are eddy currents?

Answer 30

Currents that appose the change in flux that are created on the surface of conductors.

Question 31

What is ampere turns?

Answer 31

NI The diriving current turns

Question 32

What makes good transformer design?

Answer 32

1. High conductance 2. High permance

Question 33

flux=

Answer 33

Permeance x Current Turns ΛNI

Question 34

Equation for working out flux density from curent turns?

Answer 34

B=μNI/L