AQA A Level Physics

Topic Questions

A LevelPhysicsAQATopic Questions7. Fields & Their Consequences7.8 Magnetic Fields

7.8 Magnetic Fields

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11 mark

Which of the following statements about magnetic fields is incorrect?

  • The strength of a magnetic field is also known as the magnetic flux density

  • There is a force on a conductor carrying a current at 90º to magnetic field lines

  • The magnetic flux density is measured in units of Tesla, T 

  • There is a force on a conductor carrying a current parallel to magnetic field lines

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21 mark

A current of 2.0 mA flows in a wire of length 1.5 m. It is placed in a magnetic field of flux density 5 T such that the angle between the current and the field is 90­º.

What is the magnitude of the magnetic force on the wire?

  • 5 N

  • 0.015 N

  • 15 N

  • 3 N

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31 mark

Fleming’s Left-Hand Rule is a way for students to work out the direction of the force exerted on a current-carrying conductor in a magnetic field.

When used correctly, the direction of the thumb indicates the direction of which quantity?

  • The direction of the force

  • The direction of the magnetic field

  • The direction of conventional current 

  • The magnitude of the force

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41 mark

A proton moves normally across a magnetic field of flux density 0.5 mT with a speed of 2.0 × 105 m s–1.

What is the magnitude of force exerted on the particle?

  • 100 N

  • 1.6 × 10–14 N

  • 0 N

  • 1.6 × 10–17 N

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51 mark

Which of the following statements about cyclotrons is correct?

  • A direct voltage is applied to the semi-circular dees inside a cyclotron

  • Charged particles follow parabolic paths inside a cyclotron

  • The radius of each orbit of a charged particle increases inside a cyclotron

  • The electric field in the gap between semi-circular dees slows charged particles down

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11 mark

A straight wire PQ carrying a constant current I is placed perpendicularly to a uniform magnetic field, as represented by the dotted line in the diagram below.

qu11-intro

The current-carrying wire is then rotated anti-clockwise through an angle θ about an axis perpendicular to the plane of the diagram.

Which of the following graphs shows how the magnitude of the magnetic force F on the wire varies with θ in the range 0 less or equal than space theta space less or equal than 1 half pi radians?

qu11abcd

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21 mark

A particle P with charge q and mass m is shot with kinetic energy K into a region between two metal plates, as shown in the image below:

qu12-

If the magnetic flux density between the plates is B, what is the minimum magnitude of B which ensures the particle does not collide with the opposite plate?

  • fraction numerator square root of m K end root over denominator q d end fraction

  • fraction numerator m K over denominator q d end fraction

  • fraction numerator square root of 2 m K end root over denominator q d end fraction

  • fraction numerator 2 m K over denominator q d end fraction

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31 mark

A current I = 200 mA is passed through a pivoted square wire frame, of side length 25 cm, which is initially balanced.

A bar magnet is placed near one of the edges of the frame, which causes it to tilt. A rider of mass m = 10 g and length 10 cm is then placed at the midpoint of the opposite edge on the wire frame, such that the frame regains its balance, as shown in the diagram below.

qu13

What is the magnitude of the magnetic flux density near the magnet?

  • 4.0 T

  • 3.2 T

  • 2.7 T

  • 2.0 T

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41 mark

Three long parallel straight wires, A, B and C, are placed in the same plane in a vacuum. They carry a current of 1 A, 2 A and 3 A respectively, as shown in the image below:

qu14

The magnetic flux density B, due to a long straight wire at distance r, and carrying a current I, is given by the equation:

 B = fraction numerator mu subscript 0 I over denominator 2 pi r end fraction

where mu subscript 0 is the permeability of free space, 4π × 10–7 H m–1.

What is the force per unit of length acting on A?

  • 0 N m–1

  • 1.3 × 10–5 N m–1

  • 8.0 × 10–5 N m–1

  • 1.6 × 10–6 N m–1

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51 mark

A plotting compass is placed next to a vertical wire AB. When there is no current in the wire, the compass points North as shown in the image below:

qu15-intro

Which diagram shows a possible direction for the compass to point when a current passes from A to B?

qu15abcd

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11 mark

A horizontal straight wire of mass 15 g and length 0.3 m is placed on a surface. It carries a current perpendicular to a magnetic field of flux density 2 T as shown in the diagram below.

qu-6

(Electrical connections to the wire are not shown).

What size of current does the wire need to carry in order for it to ‘float’ above the surface when in the magnetic field? 

  • 0.025 A

  • 2.5 A

  • 25 A

  • 0.25 A

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21 mark

A beam of electrons enters a uniform magnetic field which causes it to change direction, as shown in the image below. 

qu7

What is the direction of the magnetic field?

  • Out of the page and perpendicular to it 

  • Into the page and perpendicular to it

  • In the direction indicated by ‘north’

  • In the direction indicated by ‘south’

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31 mark

Which line, A to D in the table, correctly describes the relationship between the directions of force and velocity for moving charged particles in magnetic and electric fields?

 

Magnetic force and velocity

Electric force and velocity

A

B

C

D

perpendicular

parallel

perpendicular

parallel

parallel

unrelated

unrelated

perpendicular

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41 mark

Two electrons, A and B, travel normally to a uniform magnetic field. They both experience a magnetic force, FA and FB respectively.

If the kinetic energy of A is a third of that of B, what is the ratio F subscript A over F subscript B?

  • 1 third

  • fraction numerator 1 over denominator square root of 3 end fraction

  • square root of 3

  • 3

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51 mark

A beam of electrons enters a region in which there are uniform magnetic and electric fields directed at right angles to each other. The field strengths are adjusted such that the beam remains undeflected, as shown in the diagram below.

qu10

Which of the following statements is incorrect?

  • The electric force on the electron beam is in the direction indicated ‘north’ 

  • The velocity of the electron beam v is equal to the ratio of the magnetic flux density B to the electric field strength E

  • The magnetic force on the electron beam is in the direction indicated ‘south’

  • Conventional current flows from right to left in this diagram

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