MCQs / Physics / Magnetism and Electromagnetic Induction

Magnetism and Electromagnetic Induction MCQs — Physics (page 7)

255 multiple-choice questions on Magnetism and Electromagnetic Induction from Physics, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

151. In Fleming's left-hand rule, the thumb points along the:

  1. current
  2. induced e.m.f.
  3. field
  4. force (motion)

Answer: D) force (motion)

Explanation: Thumb = Thrust. SLO P-09-E-30 · Application

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152. A seismometer works on the same principle as:

  1. a light bulb
  2. a solar cell
  3. a thermocouple
  4. a moving-coil microphone

Answer: D) a moving-coil microphone

Explanation: Vibrations → induced e.m.f. SLO P-09-E-43 · Application

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153. A flux density of 0.05 T passes perpendicularly through an area of 0.6 m^2. The magnetic flux is:

  1. 0.3 Wb
  2. 0.03 Wb
  3. 0.06 Wb
  4. 0.003 Wb

Answer: B) 0.03 Wb

Explanation: Phi = BA = 0.05 0.6 = 0.03 Wb. SLO P-09-E-39 · Application

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154. The flux through each of 500 turns of a coil changes by 0.02 Wb in 0.5 s. The average induced e.m.f. is:

  1. 20 V
  2. 40 V
  3. 10 V
  4. 0.04 V

Answer: A) 20 V

Explanation: mathcalE = N DeltaPhiDelta t = 500 0.020.5 = 20 V. SLO P-09-E-42 · Application

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155. An electron (charge 1.6 10^-19 C) moves at 2 10^6 m/s at 90^circ to a field of 0.5 T. The magnetic force on it is:

  1. 8 10^-14 N
  2. 1.6 10^-12 N
  3. 3.2 10^-13 N
  4. 1.6 10^-13 N

Answer: D) 1.6 10^-13 N

Explanation: F = Bqvsintheta = 1.6 10^-13 N. SLO P-09-E-32 · Application

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156. A coil's flux linkage can be increased by:

  1. turning it parallel to the field
  2. reducing turns
  3. adding turns, increasing area, or using a stronger field
  4. using thinner wire only

Answer: C) adding turns, increasing area, or using a stronger field

Explanation: NBAcostheta. SLO P-09-E-40 · Understanding

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157. Lenz's law is a consequence of conservation of:

  1. charge
  2. mass
  3. momentum
  4. energy

Answer: D) energy

Explanation: Work must be done to induce current. SLO P-09-E-41 · Understanding

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158. The field near the centre of a flat coil is stronger than that of a straight wire with the same current because:

  1. the current is larger in a coil
  2. the coil has more resistance
  3. coils are magnetic
  4. the fields of all parts of the loop add at the centre

Answer: D) the fields of all parts of the loop add at the centre

Explanation: Contributions reinforce. SLO P-09-E-35 · Application

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159. The flux through a coil changes when:

  1. the field, the area or the orientation changes
  2. the coil is heated only
  3. never
  4. the coil's colour changes

Answer: A) the field, the area or the orientation changes

Explanation: Phi = BAcostheta. SLO P-09-E-38 · Knowledge

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160. Magnetic flux density B is defined as:

  1. the force per unit current per unit length on a wire at right angles to the field
  2. flux times area
  3. current per unit area
  4. the force per unit charge

Answer: A) the force per unit current per unit length on a wire at right angles to the field

Explanation: B = FIL. SLO P-09-E-31 · Knowledge

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161. Very large currents in power-station busbars require strong supports because:

  1. the bars are heavy only
  2. they are magnetic materials
  3. the forces between the conductors can be large
  4. of thermal expansion only

Answer: C) the forces between the conductors can be large

Explanation: Mutual forces. SLO P-09-E-37 · Understanding

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162. A coiled spring carrying a large current tends to:

  1. stretch
  2. rotate
  3. contract
  4. stay the same

Answer: C) contract

Explanation: Turns attract each other. SLO P-09-E-37 · Understanding

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163. A circular loop of radius 10 cm is perpendicular to a 0.05 T field. The flux through it is:

  1. 5 10^-4 Wb
  2. 7.85 10^-4 Wb
  3. 0.00314 Wb
  4. 0.00157 Wb

Answer: D) 0.00157 Wb

Explanation: A = pi r^2 = 0.0314 m^2; Phi = BA = 0.00157 Wb. SLO P-09-E-39 · Application

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164. Pulling the magnet out of the coil gives a deflection:

  1. in the same direction
  2. in the opposite direction to pushing it in
  3. twice as large always
  4. of zero

Answer: B) in the opposite direction to pushing it in

Explanation: Flux decreases instead of increases. SLO P-09-E-41 · Understanding

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165. A coil of area 0.02 m^2 is in a field of 0.5 T perpendicular to its plane. The flux is:

  1. 25 Wb
  2. 0.1 Wb
  3. 0.01 Wb
  4. 0.04 Wb

Answer: C) 0.01 Wb

Explanation: Phi = 0.5 0.02. SLO P-09-E-38 · Knowledge

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166. Lenz's law states that the induced current flows in a direction that:

  1. is random
  2. opposes the change producing it
  3. is always clockwise
  4. helps the change

Answer: B) opposes the change producing it

Explanation: Consequence of energy conservation. SLO P-09-E-41 · Understanding

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167. Which change does NOT increase the strength of an electromagnet?

  1. adding more turns
  2. using a soft-iron core
  3. replacing the iron core with a wooden core
  4. increasing the current

Answer: C) replacing the iron core with a wooden core

Explanation: Wood isn't magnetic. SLO P-09-E-36 · Knowledge

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168. Adjacent turns of a solenoid carrying current:

  1. attract each other
  2. repel each other
  3. exert no force
  4. push the solenoid apart

Answer: A) attract each other

Explanation: Currents in the same direction. SLO P-09-E-37 · Understanding

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169. The SI unit of magnetic flux is:

  1. weber (Wb)
  2. henry (H)
  3. tesla (T)
  4. volt (V)

Answer: A) weber (Wb)

Explanation: 1 Wb = 1 T m^2. SLO P-09-E-38 · Knowledge

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170. Magnetic flux Phi through an area is:

  1. force per unit current
  2. the product of flux density and the area perpendicular to the field
  3. current times area
  4. flux density divided by area

Answer: B) the product of flux density and the area perpendicular to the field

Explanation: Phi = BAperp. SLO P-09-E-38 · Knowledge

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171. The motor effect is used in:

  1. electric motors and loudspeakers
  2. thermometers
  3. transformers only
  4. light bulbs

Answer: A) electric motors and loudspeakers

Explanation: Force on current in a field. SLO P-09-E-29 · Understanding

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172. A wire carries current from west to east in a field pointing vertically downward. The force on it is towards the:

  1. south
  2. north
  3. east
  4. up

Answer: B) north

Explanation: Left hand: field down, current east ⇒ thrust north. SLO P-09-E-30 · Application

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173. The direction of a magnetic field at a point is the direction in which:

  1. the south pole points
  2. a positive charge moves
  3. gravity acts
  4. the north pole of a small compass points

Answer: D) the north pole of a small compass points

Explanation: Convention. SLO P-09-E-28 · Knowledge

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174. The unit of flux linkage is:

  1. A
  2. Wb (often written Wb-turns)
  3. V
  4. T

Answer: B) Wb (often written Wb-turns)

Explanation: Turns are a pure number. SLO P-09-E-40 · Understanding

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175. 1 T is equivalent to:

  1. 1 N C^-1
  2. 1 Wb m
  3. 1 A m^-1
  4. 1 N A^-1 m^-1

Answer: D) 1 N A^-1 m^-1

Explanation: From B = F/IL. SLO P-09-E-31 · Knowledge

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