AFCT Electronics: Circuits and Schematics 2 — Questions and Answers

Question 1: In a series circuit with three resistors of 10 ohms, 20 ohms, and 30 ohms, what is the total resistance?

5.45 ohms
20 ohms
60 ohms (Correct answer)
180 ohms

Correct answer: 60 ohms

In a series circuit, total resistance is the sum of all individual resistances. Therefore, 10 + 20 + 30 = 60 ohms.

Series circuits have only one path for current flow. Because all current must pass through each resistor in turn, the total resistance equals the sum of individual resistances: R_total = R1 + R2 + R3. In this case, R_total = 10 + 20 + 30 = 60 ohms. This contrasts with parallel circuits where total resistance is always less than the smallest individual resistor. In series circuits, the same current flows through every component, but voltage divides proportionally across each resistor according to Ohm's law (V = IR). The larger the resistance, the greater the voltage drop across that component.

Question 2: What does a dashed line represent on a standard electrical schematic?

A ground connection
A shielded or hidden wire
A mechanical linkage or connection between components (Correct answer)
A high-voltage power line

Correct answer: A mechanical linkage or connection between components

On electrical schematics, dashed lines typically represent mechanical linkages or connections between components, such as ganged switches or mechanically coupled variable resistors, rather than electrical connections.

Schematic diagrams use various line styles to convey different types of information. Solid lines represent electrical conductors (wires). Dashed or dotted lines typically indicate mechanical linkages, showing that two or more components are physically connected and operate together. Common examples include ganged potentiometers (multiple variable resistors on a single shaft), multi-pole switches that actuate simultaneously, and relay contacts linked to a common coil. Dashed lines can also indicate shielding (an enclosure around a group of components) or components that are physically housed together. Understanding these conventions is essential for correctly reading and troubleshooting circuits from their schematics.

Question 3: According to Kirchhoff's Current Law, what happens at a junction in an electrical circuit?

Voltage is equal across all branches
The sum of currents entering the junction equals the sum of currents leaving it (Correct answer)
Resistance increases at every junction
Power is equally distributed among all branches

Correct answer: The sum of currents entering the junction equals the sum of currents leaving it

Kirchhoff's Current Law (KCL) states that the total current entering a junction must equal the total current leaving it. This is based on the conservation of electric charge.

Kirchhoff's Current Law (KCL) is fundamental to circuit analysis and states that the algebraic sum of currents at any node (junction) is zero. In practical terms, all current flowing into a junction must flow out. For example, if 5 amps enters a junction and splits into two branches, those branches might carry 3 amps and 2 amps respectively (3 + 2 = 5). KCL is used alongside Kirchhoff's Voltage Law (KVL), which states that the sum of all voltage drops around any closed loop equals zero. Together, these laws form the basis for analyzing complex circuits with multiple loops and nodes, and are essential for understanding how current distributes in parallel circuits.

Question 4: On a schematic diagram, what does a zigzag line symbol typically represent?

An inductor
A capacitor
A resistor (Correct answer)
A diode

Correct answer: A resistor

In standard electrical schematics (particularly US conventions), a zigzag line represents a resistor. This is one of the most commonly encountered symbols in circuit diagrams.

Schematic symbols are standardized representations of electronic components. In US convention (IEEE/ANSI), a resistor is drawn as a zigzag line, while international convention (IEC) uses a rectangle. Other common symbols include: capacitor (two parallel lines, one may be curved for electrolytic), inductor (a series of loops or humps), diode (a triangle pointing to a line), transistor (circle with internal lines), and ground (three horizontal lines decreasing in size). Being able to quickly identify these symbols is crucial for reading and interpreting circuit diagrams. The resistor symbol's zigzag shape visually suggests the concept of impeding or resisting the flow of current.

Question 5: What is the total resistance of two 100-ohm resistors connected in parallel?

200 ohms
100 ohms
50 ohms (Correct answer)
25 ohms

Correct answer: 50 ohms

For two equal resistors in parallel, the total resistance is half the value of one resistor. Using the formula 1/R_total = 1/R1 + 1/R2: 1/R_total = 1/100 + 1/100 = 2/100, so R_total = 50 ohms.

Parallel resistance is calculated using the reciprocal formula: 1/R_total = 1/R1 + 1/R2 + ... + 1/Rn. For two resistors, this simplifies to R_total = (R1 × R2)/(R1 + R2). With two 100-ohm resistors: R_total = (100 × 100)/(100 + 100) = 10000/200 = 50 ohms. A useful shortcut for equal resistors: divide one resistor value by the number of resistors. The total resistance in a parallel circuit is always less than the smallest individual resistor because adding parallel paths gives current more routes to flow, effectively reducing overall opposition. In parallel circuits, voltage is the same across all branches, but current divides according to each branch's resistance.

Question 6: What is the purpose of a fuse in an electrical circuit?

To amplify current flow
To protect the circuit by melting and opening the circuit when current exceeds a safe level (Correct answer)
To store energy for later use
To convert AC to DC power

Correct answer: To protect the circuit by melting and opening the circuit when current exceeds a safe level

A fuse contains a thin wire or metal strip that melts when excessive current flows through it, breaking the circuit and protecting wiring and components from damage due to overcurrent or short circuits.

Fuses are overcurrent protection devices placed in series with the circuit they protect. The fuse element is designed to melt at a specific current rating. When a short circuit or overload draws current beyond the fuse's rating, the element heats up and melts, creating an open circuit that stops current flow. This protects wiring from overheating and components from damage. Fuses are rated by both current (amperage) and voltage. Common types include blade fuses (automotive), glass tube fuses, and high-rupture-capacity (HRC) fuses for industrial applications. Unlike circuit breakers, fuses must be replaced after they blow. Time-delay (slow-blow) fuses tolerate brief surge currents, while fast-blow fuses respond immediately to overcurrent conditions.

AFCT Electronics: Circuits and Schematics 2

In a series circuit with three resistors of 10 ohms, 20 ohms, and 30 ohms, what is the total resistance?