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Physics > Magnetic Fields COPY > Flashcards

Magnetic Fields COPY Flashcards

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1
Q

Which way do magnetic field lines go?

A

Away from a north pole of a magnet and towards a south pole.

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2
Q

What are magnetic fields produced by?

A

Permenant magnets and current carrying conductors

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3
Q

Draw the field lines around a magnet

A
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4
Q

What does each finger represent in Fleming’s left hand rule?

A

First Finger Field

seCond finger Current

THumb THrust

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5
Q

What happens if the current and field are in the same direction?

A

The current will get no force on it

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6
Q

What is the force on a conductor in a magnetic field at right angles to?

A
  • The direction of the current
  • The direction of the field
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7
Q

Describe the experiment to investigate F = BIl

A
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8
Q

What does the force on a conductor in a magnetic field depend on?

A
  • The magnetic field strength / flux density
  • The current flowing
  • The length of the wire
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9
Q

What is the condition for F = BIl to work?

A

The current and the B field are at right angles.

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10
Q

Define Tesla

A

A magnetic field has a strength of 1 Tesla when 1 metre of wire put at right angles to the field with a current of one amp through it has a force of one newton on it.

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11
Q

What does a simple d.c. motor consist of?

A

A rectangular coil wound on an iron core between the ples of a strong U-shaped permenant magnet

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12
Q

Describe how a simple d.c. motor works

A
  • The two ends of the coil are soldered onto the two halves of a split ring comutator
  • When a current flows, forces act on each side in opposite directions of the coil making it rotate. The forces put a couple on the coil
  • The turning effect is the biggest when the coil is horizontal, it decreases as the coil rotates
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13
Q

When is the turning effect biggest on a simple d.c. motor?

A

When the coil is horizontal, it decreases as the coil rotates

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14
Q

What would happen to the simple d.c. motor if the current remained in the same direction?

A

The turning effect would reverse in direction when the coil passed through the vertical position

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15
Q

How would you avoid the turning effect reversing in direction in a simple d.c. motor?

A
  • When the coil reaches a verticle position the brushes touch the gap between the two halves of the commutator, cutting off the current
  • The coild keeps rotating past the vertical positiondue to momentum
  • The current then reverses so that the turning effect continues to act in the same direction as previously.
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16
Q

What are the conditions that need to be met in order for a charged particle in a B field to have a force exerted on it?

A
  • The particle needs to be moving
  • It must not be moving parallel to the field
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17
Q

What is the formula for the force on a charged particle moving with velocity v at right angles to a magnetic field of strength B?

A

F = Bqv

18
Q

What is the direction of the current is the charged particle flowing is positive?

A

In the direction of the charged particles

19
Q

What is the direction of the current is the charged particle flowing is negitive?

A

It is opposite to the direction of flow of the charged particles

20
Q

Draw the path of a particle of charge +q as it enters a magnetic (direction is into the page). Draw the force that acts on the particle on the diagram.

A
21
Q

Why does a positive particle undergo circular motion after it enters a magnetic field?

A
  • The force on it acts as a centripetal force
  • Speed is constant
  • The force does no work on the particle so the K.E doesn’t change
22
Q

What is the equation for the energy of an accelerated charged particle in a magnetic field?

A

qV = 1/2 mv^2

23
Q

What are the assumptions made when using the formula qV = 1/2 mv^2?

A

The particle is accelerated from rest.

24
Q

What is the radius of the motion of the charged particle as it undergos circular motion in a magnetic field?

A

r = mv / Bq

25
Q

How does a cyclotron work?

A
  • A strong uniform magnetic field is used to make charged particles travel on a spiral path inside two hollow semi-circular “dees”
  • An alternating p.d is applied to the “dees” at a certain frequency
26
Q

Why does the frequency have to be right in a cyclotron?

A
  • The frequency of the ac p.d. is chosen so that whenever the charged particle gets to a gap the polarity of the ac changes so that the particle is attracted to the other “dee”, and accelerated to a higher velocity
27
Q

What is the same, no matter the radius or the velocity, in a cyclotron?

A

The time taken to travel a half cycle

28
Q

Derive a formula for the time taken for a charged particle of mass m, charge q moving at velocity v in a radius r in a magnetic field B to do half a cycle in a cyclotron.

A
29
Q

What does the time period of a charged particle to complete a hald cycle in a magnetic field in a cyclotron not depend on?

A

USING T = pi m/Bq

It doesnt depend on the radius of the motion or the velocity of the particle.

30
Q

Why does the time period get bigger when the particle travels near to the speed of light in a cyclotron?

A
  • Its mass increases significantly due to its kinetic energy having extra mass
  • The time period depends on the mass and so the particle would take longer and would be out no longer be in step with the ac supply
31
Q

Why can a cyclotron not be used to get charged particles to very high velocities near to the speed of light?

A
  • As the particles get near to the speed of light their mass increases significantly
  • This is due to kinetic energy having extra mass
  • The time period depends on the mass and so the particle would travel slowly
  • The particle would no longer stay in step with the ac supply.
32
Q

What happens in a velocity selector? Use this top derive the formula for mula fo a charged particle the will pass straight through the crossed fields

A
  • A charged particle enters crossed fields (magnetic and electric field)
  • The particle experiences a force of qV/d upwards due to the e field and a force of Bqv down due to the magnetic field
  • For a velocity to pass through Fe=Fm
  • Vq/d = Bqv

so v = V/Bd

33
Q

Derive an equation for the velocity of the charged particle that will pass straight through the combined field.

A
34
Q

What are crossed fields?

A

Where an e field and a B field are at right angles

35
Q

Which way does a the B field act in a velocity selector?

A

Into the page

36
Q

Derive a formula for the velocity of charged particles that can go through the slit in terms of v, B and E

A

BqV = Eq

Bv = E

v = E/B

37
Q

How is the mass of a charged particle measured in a mass spectrometer?

A
  • Ions with velocity E/B (or V/Bd) will pass through the slit
  • A magnetic field is applied in the mass spec and causes them to move in a circle.
  • since r = mv/Bq then m = Bqr/v
  • Since v=E/B and E,B,q and r can be measured, the mass can be measured.
38
Q

Derive a formula for the mass of a charged particle in mass spec using V, d, B, q & r

A
39
Q

What is likely the cause of ions arriving at r at slightly different points?

A

The ions are different isotopes and so ahve slightly different masses..

This is due to r being proportional to m.

40
Q

What is likely the cause of the radius measured being half the radius?

A

The ion will be doubly charged as r is proporional to 1/q

Physics (15 decks)

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