A positive charge is moving towards south in a space where magnetic field is pointing in the north direction. The moving charge will experience :
- A.a deflecting force towards north direction.
- B.a deflecting force towards east direction.
- C.a deflecting force towards west direction.
- D.no deflecting force.
Correct answer
Explanation
The correct answer is D, no deflecting force. The charge moves exactly opposite to the field, and a magnetic field exerts no force on a charge moving along its line.
The magnetic (Lorentz) force on a moving charge is F = qvB sin θ, where θ is the angle between the velocity and the field. Moving south in a field pointing north means θ = 180°, and sin 180° = 0, so the force is zero and the charge carries on in a straight line. The force is largest when the charge moves at right angles to the field.
A is wrong because a magnetic force can never act along the direction of motion or the field; it is always perpendicular to both.
B is wrong because an eastward force would need the charge to have some motion across the field.
C is wrong for the same reason: with motion parallel or antiparallel to the field there is no sideways push at all.
Exam tip: F = qvB sin θ; zero when the charge moves along or against the field, maximum at 90°.
Practice Questions
View allWhich of the following statements about a fluid at rest in a cup is/are correct? 1. Pressure is same at all the points in the fluid 2. Pressure is exerted on the walls. 3. Pressure exists everywhere in the fluid. Select the correct answer using the code given below :
- A.1 and 2 only
- B.2 and 3 only
- C.1 only
- D.1, 2 and 3
Show answer
A fluid at rest presses on the walls and has pressure at every point, but that pressure is not the same at all depths.
Read the full explanation →The absolute zero temperature is 0 Kelvin. In °C unit, which one of the following is the absolute zero temperature?
- A.0 °C
- B.-100 °C
- C.-273.15 °C
- D.-173.15 °C
Show answer
Zero on the Kelvin scale lies 273.15 degrees below the freezing point of water. The Kelvin and Celsius scales have degrees of the same size, so T(K) = t(°C) + 273.15.
Read the full explanation →Dinitrogen (N₂) and dioxygen (O₂) are the main constituents of air but they do not react with each other to form oxides of nitrogen because
- A.the reaction requires initiation by a catalyst
- B.oxides of nitrogen are unstable
- C.the reaction is endothermic and requires very high temperature
- D.the stoichiometry of N₂ and O₂ in air is not ideal for the reaction to take place
Show answer
Nitrogen and oxygen combine only when a lot of energy is supplied. The nitrogen molecule is held by a very strong triple bond, so breaking it takes much energy, and the reaction N₂ + O₂ → 2NO absorbs heat.
Read the full explanation →Hydrogenation of vegetable oils using nickel catalyst is an example of
- A.Substitution reaction.
- B.Elimination reaction.
- C.Addition reaction.
- D.Free-radical polymerization.
Show answer
Hydrogen adds across the carbon–carbon double bonds of the oil. Vegetable oils are unsaturated: their fatty acid chains contain C=C double bonds, which keep them liquid.
Read the full explanation →Which one of the following about different frictional forces is correct?
- A.Kinetic friction > Static friction > Rolling friction
- B.Static friction > Rolling friction > Kinetic friction
- C.Static friction > Kinetic friction > Rolling friction
- D.Static friction > Kinetic friction = Rolling friction
Show answer
It takes most force to start an object moving, less to keep it sliding and least to keep it rolling.
Read the full explanation →