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Electrostatics MCQs — Physics (page 2)

120 multiple-choice questions on Electrostatics 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.

26. A Faraday cage shields its interior from external electric fields because:

  1. charges on the cage rearrange to produce a field that cancels the external field inside
  2. it is magnetic
  3. it absorbs all light
  4. it is made of rubber

Answer: A) charges on the cage rearrange to produce a field that cancels the external field inside

Explanation: Induced internal fields cancel. SLO P-11-E-06 · Understanding

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27. A charged oil drop is held stationary between horizontal plates. The electric force on it equals:

  1. zero
  2. twice its weight
  3. its weight
  4. its charge

Answer: C) its weight

Explanation: Millikan's experiment: qE = mg. SLO P-11-E-04 · Understanding

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28. Doubling the distance from the centre of a charged sphere (outside it) reduces the field to:

  1. double
  2. one half
  3. the same
  4. one quarter

Answer: D) one quarter

Explanation: Inverse square law. SLO P-11-E-05 · Knowledge

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29. The electric force between a proton and an electron compared with their gravitational force is:

  1. smaller
  2. about 10^39 times larger
  3. about the same
  4. about 10 times larger

Answer: B) about 10^39 times larger

Explanation: Electric forces are enormously stronger. SLO P-11-E-07 · Application

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30. Electric field lines never cross because:

  1. they move too fast
  2. they repel each other
  3. they are too thin
  4. the field can have only one direction at a point

Answer: D) the field can have only one direction at a point

Explanation: Two directions would be impossible. SLO P-11-E-03 · Understanding

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31. Two parallel plates 10 cm apart have a p.d. of 500 V between them. The field strength between them is:

  1. 50 V/m
  2. 50,000 V/m
  3. 500 V/m
  4. 5000 V/m

Answer: D) 5000 V/m

Explanation: E = Delta VDelta d = 5000.1 = 5000 V/m. SLO P-11-E-02 · Application

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32. Gravitational and electric fields are similar because both:

  1. are always attractive
  2. act only on charges
  3. act only on masses
  4. exert forces at a distance and follow inverse-square laws for point sources

Answer: D) exert forces at a distance and follow inverse-square laws for point sources

Explanation: Both are fields of force. SLO P-11-E-01 · Knowledge

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33. The electric field strength 2 m from a point charge of 5 10^-6 C in free space is:

  1. 11,200 N/C
  2. 5620 N/C
  3. 12,200 N/C
  4. 22,500 N/C

Answer: A) 11,200 N/C

Explanation: E = kQr^2 = 8.99 10^9 5 10^-6(2)^2 = 11,200 N/C. SLO P-11-E-08 · Application

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34. Inside a uniform field, the acceleration of a charged particle is:

  1. zero
  2. constant
  3. increasing
  4. decreasing

Answer: B) constant

Explanation: a = qE/m. SLO P-11-E-04 · Understanding

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35. A metal sphere of radius 0.1 m carries 2 10^-6 C. The field strength 0.5 m from its centre is (k = 8.99 10^9 N m^2 C^-2):

  1. 36,000 N/C
  2. 71,900 N/C
  3. 1.12 10^5 N/C
  4. 1.44 10^5 N/C

Answer: B) 71,900 N/C

Explanation: Treat the charge as a point at the centre: E = kQr^2 = 71,900 N/C (measure r from the centre). SLO P-11-E-05 · Knowledge

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36. An electron is accelerated from rest through a p.d. of 2000 V. The kinetic energy it gains is:

  1. 6.4 10^-16 J
  2. 3.2 10^-17 J
  3. 3.2 10^-15 J
  4. 3.2 10^-16 J

Answer: D) 3.2 10^-16 J

Explanation: KE = eV = 1.6 10^-19 2000 = 3.2 10^-16 J (= 2000 eV). SLO P-11-E-04 · Understanding

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37. Charges +4 μC and +1 μC are 3 m apart. The field is zero at a point between them, at a distance from the +4 μC charge of:

  1. 2 m
  2. 1.5 m
  3. 1 m
  4. 2.4 m

Answer: A) 2 m

Explanation: 4x^2 = 1(3 - x)^2 Rightarrow x = 2 m. SLO P-11-E-08 · Application

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38. An electron is between parallel plates 2 cm apart with 1000 V across them. Its acceleration is (e = 1.6 10^-19 C, me = 9.11 10^-31 kg):

  1. 8.78 10^16 m/s^2
  2. 8.78 10^14 m/s^2
  3. 8.78 10^15 m/s^2
  4. 4.78 10^12 m/s^2

Answer: C) 8.78 10^15 m/s^2

Explanation: E = 50,000 V/m; a = eEm = 8.78 10^15 m/s^2. SLO P-11-E-04 · Understanding

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39. Two identical charged spheres are 1 m apart (centre to centre). For Coulomb's law, the distance used is:

  1. zero
  2. the gap between surfaces
  3. the radius
  4. 1 m (between centres)

Answer: D) 1 m (between centres)

Explanation: Treat them as point charges at the centres. SLO P-11-E-05 · Knowledge

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40. Electric fields can act:

  1. through a vacuum
  2. only through metals
  3. only through air
  4. only through water

Answer: A) through a vacuum

Explanation: No medium needed. SLO P-11-E-01 · Knowledge

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41. Electric field lines meet the surface of a conductor:

  1. never
  2. parallel to the surface
  3. at 45^circ
  4. at right angles

Answer: D) at right angles

Explanation: No field component along the surface in equilibrium. SLO P-11-E-03 · Understanding

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42. Two parallel plates 0.4 cm apart have a p.d. of 12 V between them. The field strength between them is:

  1. 3000 V/m
  2. 30,000 V/m
  3. 300 V/m
  4. 30 V/m

Answer: A) 3000 V/m

Explanation: E = Delta VDelta d = 120.004 = 3000 V/m. SLO P-11-E-02 · Application

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43. A charge of 5 10^-9 C is placed in an electric field of 2 10^5 N/C. The force on it is:

  1. 0.002 N
  2. 0.01 N
  3. 1 10^-4 N
  4. 0.001 N

Answer: D) 0.001 N

Explanation: F = qE = 5 10^-9 2 10^5 = 0.001 N. SLO P-11-E-02 · Application

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44. Michael Faraday showed shielding by:

  1. sitting inside a charged metal-foil room and detecting no field
  2. heating a wire
  3. using a compass
  4. flying a kite in a storm

Answer: A) sitting inside a charged metal-foil room and detecting no field

Explanation: 1836 experiment. SLO P-11-E-06 · Understanding

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45. An alpha particle (charge +2e) accelerated through 1000 V gains:

  1. 1000 eV
  2. 4000 eV
  3. 2000 eV
  4. 500 eV

Answer: C) 2000 eV

Explanation: KE = qV = 2e 1000. SLO P-11-E-04 · Understanding

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46. The field pattern of two equal positive charges near each other shows:

  1. a neutral point midway between them
  2. uniform field
  3. no field anywhere
  4. lines joining the charges

Answer: A) a neutral point midway between them

Explanation: Fields cancel at the midpoint. SLO P-11-E-03 · Understanding

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47. Charges of 2 10^-6 C and 3 10^-6 C are 0.1 m apart in free space. The force between them is (k = 8.99 10^9 N m^2 C^-2):

  1. 0.539 N, repulsive
  2. 10.8 N, repulsive
  3. 5.39 N, repulsive
  4. 5.39 N, attractive

Answer: C) 5.39 N, repulsive

Explanation: F = kq1q2r^2 = 5.39 N; like charges repel. SLO P-11-E-07 · Application

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48. Electric field lines start on charges and end on charges.

  1. positive; negative
  2. positive; positive
  3. neutral; positive
  4. negative; positive

Answer: A) positive; negative

Explanation: Out of + and into −. SLO P-11-E-03 · Understanding

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49. Inside a hollow charged conductor, the electric field is:

  1. radial
  2. very strong
  3. uniform and non-zero
  4. zero

Answer: D) zero

Explanation: No field lines inside. SLO P-11-E-03 · Understanding

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50. Cathode-ray oscilloscopes use deflecting plates to:

  1. heat the screen
  2. make magnetic fields
  3. move the electron beam by electric fields
  4. slow the electrons to rest

Answer: C) move the electron beam by electric fields

Explanation: Electrostatic deflection. SLO P-11-E-04 · Understanding

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