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physics > magnetic fields and induction > Flashcards

magnetic fields and induction Flashcards

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1
Q

how are magnetic fields produced

A

by permanent magnets and around conductors carrying a current

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2
Q

definition of magnetic flux density

A

the force per unit length per unit current carrying conductor at right angles to the magnetic field lines ( B = F/IL )

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3
Q

what does it mean if the field lines are closer together

A

the field is stronger

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4
Q

what is 1 tesla

A

the magnetic flux density that produces a force of 1 newton on a wire of length 1 metre with 1 ampere of current flowing through it

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5
Q

what happens when a current carrying conductor is placed in a magnetic field and what can be used to find the direction of this

A

the conductor experiences a force that can be found using flemmings left hand rule

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6
Q

where is the magnetic field going if there are x’s on the page

A

in to the page

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7
Q

where is the magnetic field going if there are dots on the page

A

out of the page

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8
Q

when can we use F=BIL

A

when the angle between the magnetic field and the current is 90 degrees

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9
Q

what happens when the current is parallel to the field lines

A

there is no force induced

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10
Q

what is the catapult effect

A

occurs when a current is passed through two wires connected by a loose wire or bar in a magnetic field

due to flemings left hand rule, the loose wire is then catapulted horizontally away from the magnetic field

this occurs due to the magnetic forces acting on the wires and in the magnetic fields itself

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11
Q

how is a magnetic field formed around a current carrying wire and what do we use to find the direction of the field lines

A

they form concentric circles around the wire and the direction can be found using the right hand grip rule

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12
Q

what happens when two wires carry current in opposite directions

A

the net effect of their magnetic fields is added together in the space between the two wires

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13
Q

what happens when two wires carry current in the same direction

A

the net effect is to cancel out the magnetic field the space between the wires

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14
Q

how to find the direction of current in a solenoid

A

anticlockwise = north
clockwise = south

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15
Q

what is magnetic flux measured in and what is the equation

A

magnetic flux = BA
measured in Wb (Weber)

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16
Q

what is the equation for flux linkage if it is at an angle to the normal of the coil

A

flux linkage = NBAcosx

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17
Q

equation for force on charged particles moving in a magnetic field and when is it valid

A

F = BQV

valid when the charge is moving perpendicular to the field lines

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18
Q

what happens if the particle is moving parallel to the field lines or if the particle is not moving

A

there is no magnetic force

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19
Q

equation for force on charged particles moving in a magnetic field that is not at 90 degrees

A

F = BQvsinx

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20
Q

what direction is current when there is a positive charge

A

the same direction as the positive charge

21
Q

what path will a charged particle take in a magnetic field and what is the equation

A

circular path
r = mv/BQ

22
Q

what happens if a particle moves through a cloud chamber

A

it will lose energy because it is giving energy to the molecules in the chamber so it will slow down and spiral inwards

23
Q

what is a cyclotron

A

a particle accelerator

24
Q

how does a cyclotron work

A
  • there is an electric field and therefore a potential difference horizontally between 2 dees
  • the protons accelerate when they cross the gap between the dees
  • the magnetic field is perpendicular to the motion of the protons
  • the protons move in circular motion when inside the dees
  • when the speed of the protons increases the radius of the path increases (r = mv/BQ)
  • we want the protons to speed up every time they cross the gap
  • we do this by switching the direction of the electric field every time the protons undergo one half circle
  • we can switch the direction of the electric field using an alternating power supply
  • the frequency of the supply should equal the frequency of the circular motion of the protons
  • v = rBQ/m
  • time for 1 half circle =
    pi r / v
  • f = BQ/ 2 pi m
  • the frequency does not depend on the radius of the path so we can use an ac supply of constant frequency
25
Q

what causes an induced emf

A

a change in magnetic flux linkage through a coil or when a conductor cuts across magnetic flux lines

26
Q

what is used to find the direction of the induced emf in a straight conductor moving through a magnetic field

A

flemings right hand rule

27
Q

what is lenz’s law

A

the induced emf and hence current, is in a direction that tends to oppose the change causing it

28
Q

what is lenz’s law a demonstration of

A

the conservation of energy
the work done by the movement of the magnet against the opposing force is transferred to the coil providing the energy for the flow of a current in the circuit (energy is dissipated as heat when the current flows.

29
Q

why is the north pole of the magnetic field on the same side as the north pole of the magnet

A

the magnetic field produced by the induced current is such that it opposes the movement of the magnet.

30
Q

what is Faraday’s law

A

the emf induced in a circuit is directly proportional to the rate of change of flux linkage

31
Q

what three things does the size of the emf depend on

A
  • the strength of the magnetic field
  • the rate at which the coil, magnet or conductor are moved, or the rate at which B changes
  • the number of wires loops or turns on the coil
32
Q

what does the magnitude of the emf depend on

A

the rate of change of flux linkage

33
Q

what is the equation for the magnitude of the induced emf when the conductor is perpendicular to the flux lines

A

e = Blv

34
Q

if the conductor is connected in a complete circuit what does the current do

A

flow

35
Q

if the conductor is not connected in a complete circuit what does the current do

A

the charge build up at either end of the rod until the potential difference caused by this charge build up is equal and opposite to the induced emf

36
Q

how do you get the graph for emf againt time from a flux time graph

A

the negative gradient

37
Q

as a magnet enters and leaves a coil, what will there be acting on it and what will this cause

A

there will be an upwards force on the magnet which causes its acceleration to be less than 9.81

38
Q

what will the magnets acceleration be when its falling through a coil but there is no induced current

A

9.81

39
Q

what is the equation for emf induced if the coil is rotated

A

e = N x change in flux linkage / change in time

40
Q

what would the flux linkage be if the angle is 0

A

maximum

41
Q

what would the flux linkage be if the angle is 90

A

0

42
Q

what is the equation for change in flux linkage

A

NBA

43
Q

what is an eddy current

A

when the magnetic flux linkage cutting a large block of conducting material changes, an induced emf is produced. This gives currents that flow within the block

44
Q

examples of eddy currents

A

in large vehicles as one of the braking systems - the biggest effect is produced when the vehicle is moving quickly

45
Q

what happens to the eddy currents if the block is moving

A

a force opposes it movement (a breaking force) - therefore, the kinetic energy of the wheel is converted into energy dissipated as heat by the eddy currents

46
Q

three ways to increase the induced emf

A
  • move the wire faster
  • use a stronger magnet
  • making the wire into a coil
47
Q

low speed =
(electric motor)

A

high current and emf is small

48
Q

high speed =
(electric motor)

A

low current and emf is high

49
Q

energy must be transferred as heat when the current flows through the wire, where does this energy come from

A

collisions between the electrons and the atoms that make up the wire - the electrons transfer some of their kinetic energy

physics (11 decks)

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