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Physics A - Level > Fields (4): Magnetic Fields > Flashcards

Fields (4): Magnetic Fields Flashcards

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1
Q

What is an application of magnetic fields?

A

Particle accelerators

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2
Q

State the definition of a magnetic field

A

A force field surrounding a magnet or current carrying wire which acts on any other magnet or current-carrying wire placed in the field

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3
Q

Where is the magnetic field on a bar magnet strongest?

A

At its poles

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4
Q

What are the two ends of a bar magnet referred to as?

A

North seeking and south seeking ‘poles’ - these correspond to the direction each end of the bar magnet points when the magnet is free to aline itself with the horizontal component of earths magnetic field

ie. north pole of magnet will point towards south pole and vice versa

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5
Q

What do magnetic field lines show?

A

The line of force of a magnetic field - line along which a north pole would move in a magnetic field

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6
Q

In what direction are magnetic field lines drawn?

A

North to south

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7
Q

What happens when a current carrying wire is placed in a magnetic field?

A

If a current-carrying wire is placed at a non-zero angle to the lines of force (magnetic field lines) of an external magnetic field, it will experience a force due to the field. This is because the magnetic field around the current-carrying wire and the external magnetic field interact producing a force that acts perpendicular to the wire and lines of force.

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8
Q

What is the name of the effect when a current-carrying wire experiences a force when placed in an external magnetic field?

A

The motor effect

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9
Q

Factors affecting the strength of the force on a current-carrying wire

A
  1. Current through the wire
  2. Strength of the magnetic field
  3. Length of the wire
  4. Angle between the lines of force of the field and the current direction
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10
Q

When is the force experienced by a current-carrying wire in an external magnetic field = 0N?

A

When the wire is parallel to the magnetic field

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11
Q

When is the force experienced by a current-carrying wire in an external magnetic field greatest?

A

When the wire is at right angles to the magnetic field

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12
Q

How can you determine the direction of the force experienced due to the motor effect?

A

Using Flemings left hand rule

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13
Q

What does each finger represent in flemings left hand rule?

A

Thumb = Force
First finger = Field
Second finger = Conventional current

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14
Q

What do you have to look out for when using Flemings left hand rule?

A

Whether you are dealing with current, movement of positive particles or movement of negative particles

NB - If you are dealing with negative particles, conventional current is in the opposite direction to their movement

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15
Q

Describe the set up that can be used to investigate the force on a current-carrying wire in a magnetic field

A
  • Two magnets with opposite magnetic poles are placed in a metal cradle which is placed onto a top pan balance
  • A wire is suspended perpendicular to the lines of force between the two magnets - held up by two clamps
  • Connect the wire in series to a variable power supply (used to vary current), and ammeter (used to measure current)
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16
Q

What is the dependent and independent variable for the practical: Investigating magnetic fields in wires?

A

Dependent: Mass, m
Independent: Current, I

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17
Q

Describe briefly, the method for the practical: Investigating magnetic fields in wires

A
  1. Set up apparatus as shown
  2. Adjust the voltage of the supply so that the ammeter reads 0.5A
  3. Read and record the mass stated on the top pan balance
  4. Increase the voltage of the power supply so that the ammeter reading increases by 0.5A and record the mass again.
  5. Repeat step 4 until you have reached 5A
  6. Repeat the experiment through twice more and find the mean mass for each current
  7. Measure, using a ruler, the length of the magnets which will roughly be equal to the length of wire experiencing a force
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18
Q

Describe briefly, the graphs and calculations needed for the practical: Investigating magnetic fields in wires

A
  • Calculate force, F, for each current by doing mass x 9.81
  • F = BIL, so plot a graph of F against I
  • Your gradient = BL, therefore calculate gradient/L to get your value of B. magnetic flux density
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19
Q

Describe briefly, the safety risks in the practical: Investigating magnetic fields in wires

A
  • High currents will be flowing through the wire so do not touch it as it could cause burns -> to reduce this risk turn the power supply off between consecutive readings
  • Make sure no wires or connections are damaged or contain the appropriate fuses to avoid a short circuit and a fire
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20
Q

Describe briefly, the ways you can improve accuracy in the practical: Investigating magnetic fields in wires

A

RANDOM ERROR
- Ensure that no high currents run through the wire, otherwise the wires resistance will increase and affect the experiment

SYSTEMATIC ERROR
- Ensure that the top pan balance is zeroed to avoid a zero error

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21
Q

What value are you trying to determine in the practical: Investigating magnetic fields in wires?

A

B, the magnetic flux density/strength of the magnetic field

22
Q

Define the magnetic flux density, B, of a magnetic field

A

The force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines

23
Q

What is the equation that links force experienced by a wire of length, L, carrying a current, I, in a magnetic flux density,B?

A

F = BIL

24
Q

What is the magnetic flux density also known as?

A

Strength of the magnetic field

25
Q

When is only time when the equation F = BIl is true?

A

When the wire carrying the current is at 90 degrees to the field lines of the magnetic field

26
Q

What is the unit of magnetic flux density?

A

Tesla, T = 1 Nm^-1A^-1

27
Q

What is the equation for the force felt by a current-carrying wire when it is at an angle theta to the magnetic field?

A

F = BIlsin(theta)

28
Q

Describe briefly the set up of a simple electric motor and how it works

A

Electric motor - a force generated from electricity.magnetic field

SET UP
- A split ring commutator is connected to a wire all on the same plane that stretches out between two magnets (the two magnets have opposite polarities so that there is a uniform magnetic field between them)
- The split ring commutator is also connected to an external circuit by metal/graphite brushes which allow the commutator to be connected to the electric circuit but also allow it to turn

FUNCTION
- A current flows through the commutator and the wires from the +ve to the -ve terminal
- The two sides of current carrying wire between the permanent magnetic field experience opposite forces, one up one down
- When the wire-commutator part of the motor is vertical the commutator is not connected to the external circuit but the momentum of the force means that it continues turning
- When the commutator comes into contact with the brushes again the current direction through the commutator and wire switches which allows the mechanism to continue turning in the same direction.

29
Q

Why does a current carrying wire experience a force?

A

As it contains individual electrons that each experience a force due to the magnetic field

30
Q

What type of path will a moving charge in a magnetic field follow e.g an electron beam?

A

A circular path

31
Q

Why will a moving charge in a magnetic field follow a circular path?

A

The direction of the force on each individual charged particle is perpendicular to the motion of the charged particle

32
Q

Can we use flemings left hand rule to determine the direction of the force on an electron moving in a magnetic field?

A

Yes - provided we remember the convention that the current direction is opposite to the direction in which the electrons move

33
Q

The force on a charged particle in a magnetic field =…?

A

Centripetal force

34
Q

The movement of charged particles in fields can be modelled with…?

A

Circular motion equations

35
Q

What is the equation for the force on a charged particle, Q, moving through a uniform magnetic field at speed, v, perpendicular to the direction to the field?

A

F = BQv

36
Q

How do you get from F = BIl to F = BQv?

A

I = Q/t and l = vt

37
Q

Do stationary charges in a magnetic field experience a force?

A

No

38
Q

Is work done on a charged particle moving in a magnetic field? Why?

A

No work is done/no energy is transferred - this is because the force on the particle always acts at right angles to the velocity of the particle. This means the speed of the particle is unchanged only its direction is affected by the force.

39
Q

How does the kinetic energy of a charged particle change as it moves in a magnetic field?

A

Kinetic energy remains constant

40
Q

What two forces can we equate for a charged particle moving in a magnetic field?

A

Centripetal force and magnetic force

41
Q

Equate F = BQv and the centripetal acceleration of the charged particle in a magnetic field and rearrange to make r the subject

A

centripetal acceleration = v^2/r

Applying Newtons 2nd law in the form F = ma gives:

BQv = mv^2/r

therefore,

r = mv/BQ

42
Q

What 3 factors affect the radius/curvature of a charged particles path in a magnetic field?

A
  • B, magnetic flux density
  • v, velocity that the particle enters the field with
  • Specific charge of the particle, Q/m
43
Q

Why might the speed of a charged particle in a magnetic field decrease in practice?

A

If the charged particle is not in an evacuated magnetic field (a vacuum)

44
Q

How is an electron beam produced in an ‘electron gun’?

A
  • A filament is heated electrically causing thermionic emission of electrons
  • The filament is placed near a metal anode which attracts the electrons emitted from the filament
  • The electrons pass through a small hole in the anode to form the beam
  • A larger potential difference between the anode and the filament will produce a beam with faster moving electrons
45
Q

What is a real life application of a cyclotron?

A

Used to produce high energy beams for radiation therapy

46
Q

Describe briefly the set up of a cyclotron

A
  • Consists of two hollow D-shaped electrodes (referred to as dees) in a vacuum chamber
  • A uniform magnetic field is applied perpendicular to the plane of the dees and a high frequency alternating voltage is applied between the dees
  • When the charged particles reach the desired speed they leave the cyclotron
47
Q

Describe briefly how a cyclotron works

A
  • A charged particle is released from the centre and directed towards a dee
  • Whenever the particle is within a dee it is accelerated on a semi circular path by the magnetic field and leaves the dee half way round
  • The electric field in the space between the dees is alternated at a frequency that means it always accelerates the particle across to the other dee which increases its velocity and so the particle follows the path of a circle with a greater radius next time
  • The particle thus follows a series of semi-circular paths of increasing radius and increasing velocity
48
Q

Describe briefly what a Mass Spectrometer is

A

A device used to analyse the different atoms present in a sample

49
Q

Describe briefly how a Mass Spectrometer works

A

The atoms of the sample are ionised and directed in a narrow beam at the same velocity into a uniform magnetic field. Each ion is deflected in a semi - circular path (in an evacuated chamber) by the magnetic field onto a photographic plate or a detector. The radius of curvature of the path of each ion depends on the specific charge of the ion, Q/m, in accordance with the equation r = mv/BQ (but B and v are constants)

50
Q

Describe briefly how a velocity selector works so that all ions in the mass spectrometer have the same velocity

A

A velocity selector consists of a magnet and a pair of parallel plates with opposite charges. The magnet is aligned so that the force felt by the charged particle due to the magnetic field acts in the opposite direction to the force felt due to the electric field. For the charged particles to go through the collimator plate (the small gap at the end that feeds into the evacuated chamber) the sum of the forces on an individual charged particle have to = 0N.

Therefore,
BQv = EQ
Bv = E
v = E/B

Therefore, there is only one specific velocity at which the ions will go through the gap and into the evacuated chamber

Physics A - Level (23 decks)

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