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CAIE A-Level Physics 20 Magnetic Fields

Practise analysing magnetic fields, forces on conductors and charges, Hall and velocity-selection applications, magnetic flux and electromagnetic induction.

Syllabus
2028–2030
Course
Physics 9702
Level
A2

Exam points

  • represent magnetic fields and interpret their strength and sources
  • calculate forces on conductors and moving charges, including Hall and velocity-selection applications
  • analyse field patterns due to currents and forces between conductors
  • calculate magnetic flux and flux linkage and apply electromagnetic-induction laws

Covered syllabus points

20.1 Concept of a magnetic field20.2 Force on a current-carrying conductor20.3 Force on a moving charge20.4 Magnetic fields due to currents20.5 Electromagnetic induction

20. Magnetic fields question 1

[Maximum number: 10]

Question (a)

(a)

State what is meant by a magnetic field.

[ 2 ]

Question (b)

(b)

A long, straight wire P carries a current into the page, as shown in Fig. 6.1.

Fig. 6.1

Fig. 6.1

On Fig. 6.1, draw four field lines to represent the magnetic field around wire P due to the current in the wire.

[ 3 ]

Question (c)

(c)

A second long, straight wire Q, carrying a current of 5.0 A out of the page, is placed parallel to wire P , as shown in Fig. 6.2.

Fig. 6.2

Fig. 6.2

The flux density of the magnetic field at wire Q due to the current in wire P is 2.6 mT .

[ 5 ]

Question (i)

(i)

Calculate the magnetic force per unit length exerted on wire Q by wire P.
force per unit length = Nm−1\mathrm{Nm}^{-1}

[ 2 ]

Question (ii)

(ii)

State the direction of the force exerted on wire Q by wire P.

[ 1 ]

Question (iii)

(iii)

The flux density of the magnetic field at wire P due to the current in wire Q is 1.5 mT . Determine the magnitude of the current in wire P. Explain your reasoning.
current = A

[ 2 ]

20. Magnetic fields question 2

[Maximum number: 10]

Question (a)

(a)

Define magnetic flux density.

[ 3 ]

Question (b)

(b)

An insulated rectangular coil of wire, consisting of 40 turns, is suspended in a cradle from a newton meter, as shown in Fig. 7.1.

Fig. 7.1

Fig. 7.1

The vertical sides of the coil have a length of 5.00 cm and the horizontal sides have a length of 3.00 cm . The initial reading on the newton meter is 0.563 N .

A U-shaped magnet rests on a top-pan balance that is set to a reading of 0.00 g . The lower edge of the coil is lowered into the region between the poles of the U-shaped magnet, as shown in the side view in Fig. 7.2.

Fig. 7.2

Fig. 7.2

The magnetic field in the region between the poles is uniform.
The lower edge of the coil is entirely within the uniform magnetic field.
A current of 3.94 A is now passed through the coil. This causes the reading on the top-pan balance to change to 2.16 g .

[ 7 ]

Question (i)

(i)

Explain why the current causes a vertical force to act on the coil.

[ 2 ]

Question (ii)

(ii)

Determine, to three significant figures, the flux density B of the uniform magnetic field.

B=
[ 3 ]

Question (iii)

(iii)

Determine what is now the reading on the newton meter. Explain your reasoning.

[ 2 ]

20. Magnetic fields question 3

[Maximum number: 5]

Question (a)

(a)

A uniform magnetic field is now applied in the region of the electric field in Fig. 5.1, so that the electron in (b)(iii) travels undeviated through the region.

[ 5 ]

Question (i)

(i)

Determine the direction of the uniform magnetic field.

[ 1 ]

Question (ii)

(ii)

Explain, with reference to the forces exerted by the two fields on the electron, why the path of the electron is undeviated.

[ 2 ]

Question (iii)

(iii)

Determine the flux density B of the uniform magnetic field. Give a unit with your answer.
B= unit

[ 2 ]
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