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#A2 PHYSICS > 2- Chapter 15 - Electromagnetism > Flashcards

2- Chapter 15 - Electromagnetism Flashcards

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1
Q

Emf generated=

A
  • Change in flux/time * Coils
  • NΔΦ/Δt = -NBACosθ/t
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2
Q

Flux linkage=

A

NΦ = NBACosθ

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3
Q

What is magnetic flux measured in?

A

Tm2 or Wb

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4
Q

What is field strength measured in?

A

Telsa’s (T)

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5
Q

What is θ in magnetic flux equation?

A

The angle between field direction and line perpendicular to coils.

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6
Q

What is a magnetc field?

A

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

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7
Q

What is a magnetic domain?

A

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

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8
Q

Wha happens when you magnatise a material

A

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

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9
Q

rules for field lines

A
  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.
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10
Q

Left hand rule

A

Thumb is the motion

First finger is Field

Centre finger is current

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11
Q

FAradays law?

A

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

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12
Q

lenz’s Law

A

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

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13
Q

How does a transformer work?

A
  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
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14
Q

transformer equation?

A

Vp/Vs=Np/Ns=Is/Ip

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15
Q

Why is the core of a transformer laminated

A

To reduce the formation of eddy currents

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16
Q

Step down / step up transformer?

A

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

Step up = Ns>Np so Volatage Increases

17
Q

What is magnetic permeability?

A

The ability of a material to become magnitised

18
Q

Relative permability?

A

μr=μ/μ0

=Material permability/Vaccume permability

19
Q

Types of magnetic material

A
  • Feromagnetic Material : μr>1 - Stongly attractive
  • Paramagnetic Material : μr Aprox 1 - weakly attractive
  • Diamagnetic Material : μr<1 - Repelled by a magnet
20
Q

Permance(Λ)=

A

μA/L

in WbA-1 (webers per amphere turn)

21
Q

Force on a straght wire=

A

IlBsinθ

22
Q

Forces on a staght wire explained?

A

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

23
Q

How does a dc motor work?

A
  • 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)
24
Q

Torque in a dc motor=

A

NIABsinθ

25
Q

What is back emf?

A

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.

26
Q

What problems happen due to back emf?

A

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

27
Q

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

A

Twice the emf, twice as fast

as Emf= wsinwt

28
Q

What is B

A

Field strength or flux density = Flux/Area

29
Q

What is a three phase generator?

A

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

30
Q

What are eddy currents?

A

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

31
Q

What is ampere turns?

A

NI

The diriving current turns

32
Q

What makes good transformer design?

A
  1. High conductance
  2. High permance
33
Q

flux=

A

Permeance x Current Turns

ΛNI

34
Q

Equation for working out flux density from curent turns?

A

B=μNI/L

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