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Electrostatics MCQs — Physics (page 3)

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.

51. A metal sphere of radius 0.1 m carries 1 10^-6 C. The field strength 1 m from its centre is (k = 8.99 10^9 N m^2 C^-2):

  1. 4500 N/C
  2. 8990 N/C
  3. 18,000 N/C
  4. 11,100 N/C

Answer: B) 8990 N/C

Explanation: Treat the charge as a point at the centre: E = kQr^2 = 8990 N/C (measure r from the centre). SLO P-11-E-05 · Knowledge

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52. Electric field strength is a:

  1. scalar quantity
  2. dimensionless number
  3. vector quantity
  4. unit of energy

Answer: C) vector quantity

Explanation: It has direction (force on a positive charge). SLO P-11-E-01 · Knowledge

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53. An electron is accelerated from rest through a p.d. of 100 V. The kinetic energy it gains is:

  1. 3.2 10^-17 J
  2. 1.6 10^-18 J
  3. 1.6 10^-16 J
  4. 1.6 10^-17 J

Answer: D) 1.6 10^-17 J

Explanation: KE = eV = 1.6 10^-19 100 = 1.6 10^-17 J (= 100 eV). SLO P-11-E-04 · Understanding

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54. Where electric field lines are closer together, the field is:

  1. weaker
  2. reversed
  3. stronger
  4. zero

Answer: C) stronger

Explanation: Line density shows strength. SLO P-11-E-03 · Understanding

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55. A charged particle placed in an electric field:

  1. becomes neutral
  2. stays at rest always
  3. accelerates (if free to move)
  4. loses its charge

Answer: C) accelerates (if free to move)

Explanation: F = qE gives acceleration. SLO P-11-E-01 · Knowledge

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56. Doubling the p.d. across parallel plates with the same spacing makes the field strength:

  1. double
  2. unchanged
  3. four times
  4. half

Answer: A) double

Explanation: E = V/d. SLO P-11-E-02 · Application

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57. The electric field 0.2 m from a point charge is 900 N/C. At 0.6 m from the charge it is:

  1. 300 N/C
  2. 8100 N/C
  3. 100 N/C
  4. 2700 N/C

Answer: C) 100 N/C

Explanation: Distance × 3 ⇒ field ÷ 9. SLO P-11-E-08 · Application

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58. When an external field acts on a Faraday cage, induced charges appear:

  1. inside the cage
  2. on the outer surface of the cage
  3. nowhere
  4. at the centre

Answer: B) on the outer surface of the cage

Explanation: They cancel the field inside. SLO P-11-E-06 · Understanding

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59. Which is NOT affected by an electric field?

  1. an ion
  2. a neutron
  3. a proton
  4. an electron

Answer: B) a neutron

Explanation: Neutrons have no charge. SLO P-11-E-01 · Knowledge

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60. An electron released from rest in a uniform field moves:

  1. along the field direction
  2. in a circle
  3. opposite to the field direction
  4. nowhere

Answer: C) opposite to the field direction

Explanation: Negative charge. SLO P-11-E-04 · Understanding

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61. If both charges are doubled (same distance), the force becomes:

  1. four times as large
  2. unchanged
  3. half
  4. double

Answer: A) four times as large

Explanation: F propto Q1Q2. SLO P-11-E-07 · Application

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62. A proton and an electron in the same electric field experience forces that are:

  1. equal in size and opposite in direction
  2. equal in size and direction
  3. both zero
  4. different in size

Answer: A) equal in size and opposite in direction

Explanation: Equal and opposite charges. SLO P-11-E-01 · Knowledge

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63. A person in a car is safe from lightning mainly because:

  1. the rubber tyres insulate the car
  2. the metal body acts as a Faraday cage
  3. cars never get hit
  4. lightning avoids metal

Answer: B) the metal body acts as a Faraday cage

Explanation: Charge flows around the outside. SLO P-11-E-06 · Understanding

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64. The field just outside a charged sphere of radius R is:

  1. kQR^2
  2. 0
  3. kQR
  4. kQR^2

Answer: A) kQR^2

Explanation: Point charge formula with r = R. SLO P-11-E-05 · Knowledge

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65. The edges of parallel plates show curved field lines. This is called:

  1. induction
  2. polarization
  3. the edge effect (fringing)
  4. resonance

Answer: C) the edge effect (fringing)

Explanation: Field is non-uniform at the edges. SLO P-11-E-03 · Understanding

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66. A charge of 4 10^-6 C experiences a force of 0.02 N. The electric field strength is:

  1. 10,000 N/C
  2. 50,000 N/C
  3. 500 N/C
  4. 5000 N/C

Answer: D) 5000 N/C

Explanation: E = Fq = 5000 N/C. SLO P-11-E-02 · Application

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67. The field due to a point charge depends on:

  1. the mass of the charge
  2. the speed of light
  3. the test charge used
  4. the charge and the distance from it

Answer: D) the charge and the distance from it

Explanation: Not on the test charge. SLO P-11-E-08 · Application

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68. Two charges +Q and +Q are a distance d apart. The field at the midpoint is:

  1. kQd^2
  2. 2kQd^2
  3. zero
  4. 8kQd^2

Answer: C) zero

Explanation: The fields are equal and opposite. SLO P-11-E-08 · Application

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69. Two charges repel with 9 N. If one charge is tripled and the distance tripled, the force becomes:

  1. 3 N
  2. 9 N
  3. 27 N
  4. 1 N

Answer: A) 3 N

Explanation: F' = 9 39 = 3 N. SLO P-11-E-07 · Application

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70. A proton and an electron in the same uniform field have accelerations that:

  1. differ, the electron's being much larger
  2. are equal
  3. are both zero
  4. differ, the proton's being larger

Answer: A) differ, the electron's being much larger

Explanation: Same force, the electron is ~1836 times lighter. SLO P-11-E-04 · Understanding

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71. A charged conducting sphere of radius 5 cm has a field of 9000 N/C at 10 cm from its centre. At 20 cm from its centre, the field is:

  1. 36,000 N/C
  2. 9000 N/C
  3. 4500 N/C
  4. 2250 N/C

Answer: D) 2250 N/C

Explanation: E propto 1/r^2. SLO P-11-E-05 · Knowledge

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72. An electric field exists around:

  1. only magnets
  2. only conductors
  3. only moving charges
  4. any charged object

Answer: D) any charged object

Explanation: Static charges create electric fields. SLO P-11-E-01 · Knowledge

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73. Aircraft struck by lightning are usually safe because:

  1. they are high in the sky
  2. the metal fuselage acts as a Faraday cage
  3. fuel absorbs the charge
  4. lightning can't reach them

Answer: B) the metal fuselage acts as a Faraday cage

Explanation: Current flows on the outside. SLO P-11-E-06 · Understanding

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74. The direction of an electric field at a point is the direction of the force on:

  1. a small positive test charge
  2. a neutron
  3. a magnet's north pole
  4. a negative charge

Answer: A) a small positive test charge

Explanation: Convention. SLO P-11-E-01 · Knowledge

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75. Field lines are drawn with arrows to show:

  1. the direction electrons move
  2. the speed of the field
  3. where the field is zero
  4. the direction a positive charge would be pushed

Answer: D) the direction a positive charge would be pushed

Explanation: Convention. SLO P-11-E-03 · Understanding

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