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Chapter 1 Quiz — Electric Charge and Basic Circuit Quantities

Test your understanding of electric charge, current, voltage, power, resistance, and the fundamental quantities of circuit analysis. Each question includes a detailed explanation.

Question 1

A charge of 45 millicoulombs passes through a cross-section of wire in 9 seconds. What is the current?

  1. 5 A
  2. 5 mA
  3. 0.2 A
  4. 405 mA
Show Answer

Correct Answer: B

Using \(I = Q/t\): \(I = 45 \times 10^{-3} \text{ C} \div 9 \text{ s} = 5 \times 10^{-3} \text{ A} = 5 \text{ mA}\). Don't forget to keep units consistent — the charge is in millicoulombs, so the current is in milliamps.

Concept Tested: Electric Current Definition \(I = Q/t\)

Question 2

Voltage is formally defined as which of the following?

  1. The rate of charge flow past a point
  2. Energy per unit time delivered to a circuit element
  3. Work done per unit charge moved between two points
  4. The opposition to current flow in a material
Show Answer

Correct Answer: C

Voltage is defined as \(V = W/Q\), the energy (work) in joules required to move one coulomb of charge between two points. Choice A defines current, choice B defines power, and choice D defines resistance. Voltage is always a potential difference between two points, never an absolute value at a single point.

Concept Tested: Voltage Definition \(V = W/Q\)

Question 3

A 12 V source is connected to a 4 kΩ resistor. Using Ohm's Law, how much power is dissipated in the resistor?

  1. 48 mW
  2. 36 mW
  3. 3 mW
  4. 48 W
Show Answer

Correct Answer: B

Using \(P = V^2/R\): \(P = (12)^2 / 4000 = 144/4000 = 0.036 \text{ W} = 36 \text{ mW}\). You can verify with current first: \(I = 12/4000 = 3 \text{ mA}\), then \(P = VI = 12 \times 0.003 = 36 \text{ mW}\). The \(P = V^2/R\) form is the most direct when you know V and R.

Concept Tested: Power Equations \(P = VI = I^2R = V^2/R\)

Question 4

Which SI prefix multiplier corresponds to nano (n)?

  1. \(10^{-6}\)
  2. \(10^{-9}\)
  3. \(10^{-12}\)
  4. \(10^{-3}\)
Show Answer

Correct Answer: B

Nano (n) = \(10^{-9}\). The common prefixes in order: milli (m) = \(10^{-3}\), micro (μ) = \(10^{-6}\), nano (n) = \(10^{-9}\), pico (p) = \(10^{-12}\). You will encounter these constantly — capacitor values in nanofarads, inductors in nanohenrys, and timing signals measured in nanoseconds.

Concept Tested: SI Units and Prefixes

Question 5

In a circuit, conventional current flows from positive to negative through the external circuit. What is the actual physical direction of electron flow?

  1. From positive to negative, same as conventional current
  2. From negative to positive, opposite to conventional current
  3. Electrons do not flow in wires — only protons move
  4. The direction depends on whether the source is AC or DC
Show Answer

Correct Answer: B

Electrons flow from negative to positive (opposite to conventional current). Conventional current direction was established before electrons were discovered — scientists guessed positive charges moved, and they guessed wrong. The convention stuck. In circuit analysis we always use conventional current, and the math works out correctly either way.

Concept Tested: Conventional Current vs. Electron Flow

Question 6

What happens to the current in a circuit if the voltage is doubled while the resistance is held constant?

  1. The current is halved
  2. The current is unchanged
  3. The current is doubled
  4. The current is quadrupled
Show Answer

Correct Answer: C

By Ohm's Law \(I = V/R\), current is directly proportional to voltage when resistance is constant. If \(V\) doubles, then \(I = 2V/R\) — the current doubles as well. This linear relationship is a cornerstone property that makes resistive circuit analysis tractable.

Concept Tested: Ohm's Law Proportionality

Question 7

What is the key defining difference between an open circuit and a short circuit?

  1. An open circuit has zero resistance; a short circuit has infinite resistance
  2. An open circuit has infinite resistance and zero current; a short circuit has zero resistance and zero voltage across it
  3. An open circuit allows maximum current; a short circuit blocks all current
  4. An open circuit occurs only in AC systems; a short circuit occurs only in DC systems
Show Answer

Correct Answer: B

An open circuit has infinite resistance: no current flows (\(I = 0\)), but a voltage can exist across the open terminals. A short circuit has zero resistance: the voltage across it is zero (\(V = 0\)), but current is limited only by the rest of the circuit (potentially dangerously high). These are the two extreme cases that bracket all normal circuit operation.

Concept Tested: Open Circuit and Short Circuit

Question 8

A 9 V battery powers a circuit. Charge of 10 C flows through the battery during operation. How much energy is supplied by the battery?

  1. 0.9 J
  2. 9 J
  3. 90 J
  4. 1.11 J
Show Answer

Correct Answer: C

From the definition of voltage \(V = W/Q\), we can rearrange to \(W = QV = 10 \text{ C} \times 9 \text{ V} = 90 \text{ J}\). Voltage literally measures how much energy is delivered per coulomb of charge — this calculation makes that concrete. Every coulomb of charge moved through 9 V carries 9 J of energy.

Concept Tested: Electrical Energy \(W = QV\)

Answers Summary

Question Answer Concept Tested
1 B Electric Current \(I = Q/t\)
2 C Voltage Definition \(V = W/Q\)
3 B Power Equations
4 B SI Prefixes
5 B Conventional Current vs. Electron Flow
6 C Ohm's Law Proportionality
7 B Open Circuit and Short Circuit
8 C Electrical Energy \(W = QV\)