Vidaara.orgClass 12 · Physics
CodeVID-P12-02-FCC-01
Force on Charges & Conductors — Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Take $\mu_0=4\pi\times10^{-7}$ T m/A. Draw neat labelled diagrams wherever asked. For full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The magnetic force $\vec{F}=q(\vec{v}\times\vec{B})$ does work equal to:
- A.$qvB$
- B.zero
- C.$\frac{1}{2}mv^2$
- D.$qE$
2.
The cyclotron period $T=\frac{2\pi m}{qB}$ depends on:
- A.speed
- B.radius
- C.neither speed nor radius
- D.energy
3.
Force on a current wire is given by:
- A.$q\vec{v}\times\vec{B}$
- B.$I\vec{L}\times\vec{B}$
- C.$NIAB$
- D.$\mu_0 n I$
4.
Parallel currents in the same direction:
- A.repel
- B.attract
- C.do nothing
- D.rotate
5.
Fleming's left-hand rule gives the direction of:
- A.induced current
- B.force on a conductor
- C.field
- D.charge
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Why does a magnetic force do no work on a moving charge?
7.
Define the ampere using the force between two wires.
8.
Write the torque on a current loop and define magnetic moment.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
A proton moves at $4\times10^6$ m/s perpendicular to a 0.25 T field. Find the radius. (m = $1.67\times10^{-27}$ kg)
10.
Explain how a galvanometer is converted into an ammeter and a voltmeter.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Describe the principle, construction and working of a cyclotron. Derive the cyclotron frequency and the maximum energy gained by the ion.
Answer Key
Section A — Multiple Choice Questions
- (B) zero
- (C) neither speed nor radius
- (B) $I\vec{L}\times\vec{B}$
- (B) attract
- (B) force on a conductor
Section B — Short Answer (2 marks)
- It is always perpendicular to velocity, so $\vec{F}\cdot\vec{v}=0$.
- Currents of 1 A in two wires 1 m apart exert $2\times10^{-7}$ N/m on each other.
- $\tau=NIAB\sin\theta$; magnetic moment $m=NIA$.
Section C — Short Answer (3 marks)
- $r=\frac{mv}{qB}=\frac{(1.67\times10^{-27})(4\times10^6)}{(1.6\times10^{-19})(0.25)}=0.167$ m.
- Ammeter: low-resistance shunt in parallel; voltmeter: high resistance in series.
Section D — Long Answer (5 marks)
- Resonance gives $f=\frac{qB}{2\pi m}$; maximum energy $E_{max}=\frac{q^2B^2R^2}{2m}$.
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