CLASS 12 – PHYSICS -- Chapter 3: ELECTROMAGNETISM

CLASS 12 – PHYSICS
Chapter 3: ELECTROMAGNETISM
Short answer question:

Q1. A wire is covered with a carpet. How will you find whether any current is passing through the wire?
Ans. A current through the wire covered with the carpet can be detected by using a compass needle. As the flow of current through the wire produces magnetic field, which can be detected from the detection of the compass needle lying near the current carrying wire.

Q2. If a charged moves in a straight line through some region of space, can you say that the magnetic field in the region is zero?
Ans. No, we cannot say that there is no magnetic field in that region. If this be a magnetic field, but the charge particle in this field be moving either parallel or antiparallel still on such charged particle there will be no magnetic force.

Q3. Why does the picture on a TV screen become distorted when a magnet is brought near the screen?
Ans. The picture on the TV screen is formed by the beam of electrons. When a magnet is brought close to screen, the beam of electrons will be deflected due to magnetic field of magnet. As the result the picture of the TV screen will distorted.

Q4. Charged cosmic ray particles from outside the earth tend to strike the earth more frequently near the poles than at lower latitudes. Explain
Ans. At the poles the magnetic lines of force of earth’s magnetic field are normal to the earth surface where at the lower altitudes these magnetic lines of force are parallel to the surface of the earth. When the cosmic rays falls at the poles, there is no magnetic force on these cosmic rays particles and hence strike the earth. But at the lower altitude these cosmic rays when fall vertically in the magnetic field, the maximum magnetic force acts on them. This magnetic force deflects then cosmic rays particle back and the chances of striking earth becomes minimum.

Q5. A magnetic field accelerates a charge particle but does no work on the particle. Explain this statement.
Ans. The magnetic force acting on charged particle only changes the direction of linear velocity where the magnitude of linear velocity is constant. Therefore the acceleration produces in charge particle in magnetic field is centripetal acceleration. As this centripetal acceleration produces by the centripetal force which is directed towards the center of circle. As the displacement of charged particle in circular path and this centripetal force are acting at 90°.
W = f. d cos90
Cos90 = 0
W = 0

Q6. A beam of electrons is projected towards a horizontal wire in which current how’s left to right. In what direction is the beam deflected?
Ans. The current flow in the horizontal wire from left to right produces a magnetic field surrounding the wire. By right hand rule, the magnetic field in the upper region of the conductor is out of plane of paper where in the lower region is into the plane of paper. When the electron beam be directed into the magnetic field which is into plane of paper, these electron will deflect clockwise i.e. towards the right side.

Q7. A charged particle is moving in a circle under the influence of a uniform magnetic field. If an electric field is turned on the region of motion along the same direction as the magnetic field, what path will the charged particle take?
Ans. In the uniform magnetic field, the charged particle is acted upon by the magnetic force, which rotates the charged particle in a circle. When the electric field is provided in same direction, this electric field will accelerate this charged particle along the straight line. Under this condition the charged particle will move in path called helix.

Q8. A current –carrying wire is placed in a magnetic field. How must it be oriented so that the force on it
(a) Is zero (b) is maximum?
Ans. The magnitude of magnetic force acting on the current-carrying wire in magnetic field is given by
F = ILBsinθ
a) If θ = 0°
F= ILBsinθ
F = 0
When the current carrying wire be placed parallel to magnetic field, th force on it is zero.
b) When θ = 90°
F = ILBsin90 sin90 = 1
F = ILB
When the current-carrying wire be placed normal to the magnetic field, the magnetic force on it is maximum.

Q9. Why is the magnetic field strength greater inside a current-carrying loop of wire?
Ans. Consider a loop in which the current is flowing anticlockwise. By right hand rule the magnetic field in the side AB will anticlockwise where inside CD is clockwise. Inside the loop the magnetic field in these two sides are in same direction, which will support each other there by producing a strong magnetic field. But outside the loop the magnetic field is weak as compared to magnetic field inside.

Q10. A hollow copper tube carries a current. Why is B=0 inside the tube? Is B non-zero outside?
Ans. From Ampere circuit Law B*2πr = µ0I since there is no current by the inside amperian path so when I = 0 then B = 0.
Hence at point inside B will be zero. But in outside Amperian path the current is not zero hence B is not zero.

Q11. What is a shunt resistance? How is it used to convert galvanometer into an ammeter?
Ans. A shunt resistance is a low resistance which when connected in parallel to galvanometer converts the galvanometer into an ammeter.

Q12. How can you use a magnetic field to separate isotopes of chemical elements?
Ans. The magnetic force on the ions (isotopes) in magnetic field is given by
Fm = evB
This magnetic force provides the necessary centripetal force due to which the ions move in circle. This centripetal force
Fc = mv²/r
r = mv/eB if v, e and B are constant
r α m
The isotopes having different masses will be deflected at different radius from which the isotopes can be separated.

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