IB Physics HL D.3 Motion in electromagnetic fields questions

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Type

Multiple choice Written

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Question 1

Paper 2Medium7 marks

A straight wire segment of length $0.150\ \mathrm{m}$ carrying a current of $2.0\ \mathrm{A}$ is placed in a uniform magnetic field. The angle $\theta$ between the current and the magnetic field is varied, and the magnitude of the force on the wire is measured.

(a)

Use the gradient of Figure 1 to determine the magnetic field strength.

[2]

(b)

A separate straight wire segment of length $0.12\ \mathrm{m}$ carries a current of $1.5\ \mathrm{A}$ due east in a uniform magnetic field directed vertically upwards. The wire is perpendicular to the magnetic field. Use your answer to (a) to calculate the magnitude of the force on the wire and state its direction.

[2]

(c)

Two long parallel wires carry currents of $4.0\ \mathrm{A}$ and $6.0\ \mathrm{A}$ in the same direction. Their separation is $0.050\ \mathrm{m}$ Calculate the force per unit length between the wires and state whether the force is attractive or repulsive.

Use $\mu_0=4\pi\times10^{-7}\ \mathrm{N\,A^{-2}}$

[3]

Question 2

Paper 2Medium12 marks

A beam of protons passes through a velocity selector and then enters a region containing a uniform magnetic field. Figure 1 shows the arrangement. The diagrams are schematic and not to scale. The protons pass undeflected through the selector.

(a)

Calculate the magnitude of the electric field between the plates in region 1.

[2]

(b)

Consider the motion of the protons in region 1.

(i)

If the magnetic field in region 1 is switched off, state the direction of the acceleration of the protons while they are between the plates.

[1]

(ii)

Determine the speed of the protons that pass undeflected through the selector.

[2]

(c)

Use the path in region 2 to determine the charge-to-mass ratio of a proton.

[3]

(d)

Explain why the kinetic energy of a proton remains constant as it moves through region 2.

[2]

(e)

The speed of the protons entering region 2 is increased to $1.50$ times its original value. Using your result from (c), determine the new radius of their path in region 2.

[2]

Question 3

Multiple choiceHard1 mark

A straight horizontal conductor of length $0.50\ \mathrm{m}$ carries a current of $2.0\ \mathrm{A}$ northeast; its eastward:northward direction-component ratio is $3:4$ A uniform horizontal magnetic field of magnitude $0.50\ \mathrm{T}$ points northeast, with eastward:northward direction-component ratio $4:3$ What are the magnitude and direction of the force on the conductor?

Show answer and explanation

Answer: C. The current and field directions are $53.1^\circ$ and $36.9^\circ$ north of east, so the angle between them is $16.2^\circ$ with $\sin\theta=0.28$ Thus $F=BIL\sin\theta=(0.50)(2.0)(0.50)(0.28)=0.14\ \mathrm{N}$ Fleming’s left-hand rule gives a downward force.

A: gives the correct magnitude but reverses the force direction.
B: uses $\cos\theta$ giving $0.48\ \mathrm{N}$ and reverses the force direction.
D: uses $\cos\theta$ giving $0.48\ \mathrm{N}$ but gives the correct direction.
Remember: the force depends on the component of the current direction perpendicular to the magnetic field.

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