Chapter 1 · Current & Ohm's Law for a closed circuit: practice questions
Unit 1 — Dynamic Electricity and Electromagnetism · Physics. These are 10 questions from the Magmo3 bank on this lesson, from easy to hard. Solve each one, then open "Show answer and explanation". Magmo3 has 67 questions on this lesson, each with an explanation.
The average drift velocity of the free electrons responsible for a steady current in a metal wire is...
Azero as long as a steady current flows
Bvery small compared with their random thermal speeds
Cmuch greater than their random thermal speeds
Dequal to the speed of light in the wire
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Correct answer: (B) very small compared with their random thermal speeds
Explanation: The drift speed is only of the order of fractions of a millimetre per second, far below the random thermal speeds.
Question 2Easy
The maximum current that a battery of e.m.f. (VB) and internal resistance (r) can deliver, when its terminals are short-circuited, is given by...
AVB − r
Br / VB
CVB / r
DVB × r
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Correct answer: (C) VB / r
Explanation: On short circuit R = 0, so I = VB/(R + r) = VB / r.
Question 3Easy
The 'lost volts' of a cell that delivers a current I through an internal resistance r is equal to...
AVB / r
BI / r
CI r
DVB − I
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Correct answer: (C) I r
Explanation: The potential difference lost inside the cell equals I r.
Question 4Medium
A copper wire of cross-sectional area 2 × 10⁻⁶ m² carries a current of 5 A. If the free-electron density is 8 × 10²⁸ electrons/m³, the drift velocity of the electrons is approximately (e = 1.6 × 10⁻¹⁹ C)...
A battery of e.m.f. 12 V and internal resistance 0.5 Ω delivers a current of 4 A to an external circuit. The terminal voltage of the battery equals...
A2 V
B10 V
C8 V
D14 V
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Correct answer: (B) 10 V
Explanation: V = VB − I r = 12 − 4×0.5 = 10 V.
Question 6Medium
A current of 0.32 A flows through a conductor for 10 s. The number of electrons that pass through it during this time is (e = 1.6 × 10⁻¹⁹ C)...
A2 × 10²⁰
B2 × 10¹⁸
C3.2 × 10¹⁹
D2 × 10¹⁹
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Correct answer: (D) 2 × 10¹⁹
Explanation: Number = Q/e = (0.32×10)/(1.6×10⁻¹⁹) = 3.2/1.6×10⁻¹⁹ = 2×10¹⁹.
Question 7Medium
A charge of 4 C acquires 48 J of energy as it is driven through an electric source. The e.m.f. (VB) of the source equals...
A48 V
B12 V
C192 V
D4 V
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Correct answer: (B) 12 V
Explanation: VB = W/Q = 48/4 = 12 V.
Question 8Hard
A source gives a terminal voltage of 8 V across a 2 Ω resistor and 10 V across a 5 Ω resistor. The e.m.f. (VB) of the source equals...
A10 V
B11 V
C14 V
D12 V
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Correct answer: (D) 12 V
Explanation: From 8 = VB·2/(2 + r) and 10 = VB·5/(5 + r): r = 1 Ω and VB = 12 V.
Question 9Hard
A battery delivers exactly the same electric power to an external resistor whether that resistor is 2 Ω or 8 Ω. The internal resistance (r) of the battery equals…
A6 Ω
B4 Ω
C5 Ω
D3 Ω
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Correct answer: (B) 4 Ω
Explanation: Equal power means VB²·2/(2 + r)² = VB²·8/(8 + r)², so (8 + r)/(2 + r) = 2, giving 8 + r = 4 + 2r and r = 4 Ω (r² = R₁R₂ = 16).
Question 10Hard
A source of e.m.f. 6 V drives a total charge of 20 C around a complete circuit. The total electrical energy delivered by the source is...