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A2 Physics > Electromagnetism > Flashcards

Electromagnetism Flashcards

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1
Q

What is magnetic flux density?

A
  • A measure of magnetic field strength

- Magnetic flux per m^2

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2
Q

What is the unit of magnetic flux density?

A

Tesla or Weber per metre

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3
Q

What is Fleming’s Left Hand Rule?

A
  • First Finger Field
  • Second finger current
  • Thumb gives motion
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4
Q

How is magnetic flux density calculated?

A

B = F / Il when the wire is perpendicular to the field lines.

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5
Q

How is the force on a wire calculated?

A 
F = BIlsinθ
F = Bqv
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6
Q

When a metal frame is balanced, how can magnetic flux density be calculated?

A

B = mg/Il

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7
Q

When is there no force acting on a particle moving through a magnetic field?

A
  • When the particle is moving parallel to the field lines.

- When the particle is stationery

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8
Q

How does an electron gun work?

A
  • The heater produces electrons through thermionic emission.
  • The electrons are repelled from the heater and attracted towards the anode.
  • The electrons are accelerated as they pass through the gun.
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9
Q

What is the electron gun equation?

A

v = (2eV/m)^(1/2)

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10
Q

How is the radius of the circular path of a particle calculated?

A

r = mv/Bq

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11
Q

How is specific charge calculated?

A 
e/m = v / Br
e/m = 2V/(Br)^2
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12
Q

How does an electron move in a magnetic field?

A

In a circular path

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13
Q

How does an electron move in an electric field?

A

In a parabolic curve towards the positive charge since y ∝ x^2

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14
Q

What is a cyclotron?

A

A particle accelerator

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15
Q

How does a cyclotron work?

A
  • Two dees are placed in a vacuum chamber between the poles of a powerful magnet so the dees are subjected to a perpendicular uniform magnetic field.
  • Protons move in a circular path within the dees due to the magnetic field.
  • A high frequency alternating voltage is applied between the dees creating an electric field causing the proton to accelerate.
  • Once the proton has reached the maximum radius, the proton is deflected out by a charged plate.
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16
Q

How is the time taken to move through a dee calculated?

A

t = πm/Bq

17
Q

How is the required frequency of a cyclotron calculated?

A

f = Bq/2πm

18
Q

What is a galvanometer?

A

An ammeter that is designed to measure small currents.

19
Q

What is the unit of magnetic flux?

A

Weber (Wb) or Tm^2

20
Q

How is magnetic flux calculated?

A

Φ = BA

21
Q

Why do the electrons in a wire become displaced when moved through a magnetic field?

A

The electrons experience a magnetic force.

22
Q

How is the force on an electron in a wire calculated?

A

F = Bev

23
Q

How is the electric force on an electron in a wire calculated?

A

F = eE = e(ε/l)

24
Q

How is the emf induced in a conductor moving through a magnetic field calculated?

A

ε = Blv

25
Q

When is an emf induced but no current?

A

When there is no external load across the wire.

26
Q

What is Lenz’s Law?

A

When an emf is generated by a change in magnetic flux in accordance with Faraday’s Law, the polarity of the induced emf is such that it produces a current to generate a magnetic force that opposes the change which produced the emf.

27
Q

Give four examples of Lenz’s Law.

A
  • Dynamo
  • Spinning copper disc
  • Aluminium plate pendulum
  • Magnet falling down a copper tube
28
Q

What happens to a magnetic brake if slits are cut in the disc?

A

The brake has no effect because no current can flow in the disc.

29
Q

What is Faraday’s Law of electromagnetic induction?

A

ε = -N(dΦ/dt)

30
Q

A wire carrying a current is held mid-way between two poles of a magnet on a top pan balance. If the balance shows X what does the balance show if the current is doubled and reversed?

A

-2X

31
Q

Give three causes of energy losses in a transformer.

A
  • Induced currents in the soft iron core
  • Reversal of magnetism in the soft iron core
  • Resistances in the coils
32
Q

For a rotating coil, at what value of magnetic flux is the emf greatest?

A

0

33
Q

For a rotating coil, when is the rate of change of magnetic flux greatest?

A

Parallel to the field

34
Q

For a rotating coil, when does the coil cut flux lines?

A

Perpendicular to the field

A2 Physics (9 decks)

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