In this article
Circuit fundamentals
What is Ohm's law and when does it fail?
Ohm's law says the current through a conductor is proportional to the voltage across it, so voltage equals current times resistance. It holds for ohmic materials at steady temperature. It fails for diodes, transistors and lamp filaments, whose resistance changes with voltage or heat, so use their characteristic curves instead.
Why it is asked: it separates memorising a formula from knowing its limits. See how interview loops work for where a technical round fits.
What are Kirchhoff's rules?
Kirchhoff's rules are two conservation statements for circuits. The junction rule says the currents entering a junction equal the currents leaving it, and the loop rule says the changes in potential around any closed loop sum to zero (OpenStax, 2016). They are the basis of nodal and mesh analysis. State which conservation each reflects: charge and energy.
Why it is asked: nearly every circuit analysis method rests on them.
How do series and parallel resistors combine?
In series the same current flows through every resistor, the supply voltage divides between them, and resistances add. In parallel every branch sees the same voltage, the current divides between branches, and the reciprocals add (1/R = 1/R1 + 1/R2), so the combined resistance is always lower than the smallest branch. Illustrative figure: two 100 ohm resistors in parallel make 50 ohms. A quick check is that parallel must be less than the smallest value.
Why it is asked: it tests basic fluency and the habit of sanity checks.
How does a voltage divider work and what goes wrong when you load it?
A divider of two resistors outputs the input voltage times the lower resistor over the total. Illustrative figure: 12 volts across 1 kilohm and 2 kilohm gives 8 volts across the 2 kilohm resistor. A load in parallel with that resistor lowers the output, so keep the load much larger or buffer the divider with an op-amp.
Why it is asked: loading is a classic real-world surprise.
How do capacitors and inductors behave?
A capacitor passes changing voltage but blocks steady current: its current is proportional to how fast the voltage changes. An inductor does the opposite: it passes steady current and resists change, with voltage proportional to the rate of change of current. In a resistor-capacitor (RC) circuit the time constant is resistance times capacitance; while charging, the capacitor voltage reaches 1 − 1/e, about 63 percent, of its final value after one time constant (OpenStax, 2016, section 10.5).
Why it is asked: it tests intuition about energy storage.
Power and machines
What is the difference between AC and DC?
Direct current (DC) flows one way at a steady value; alternating current (AC) reverses direction periodically, usually as a sine wave. AC is used on the grid because transformers can change its voltage cheaply, which cuts transmission losses. DC is used inside electronics, batteries and increasingly in high-voltage links. Say why each is chosen.
Why it is asked: it opens the power-systems conversation.
What does RMS mean and why do we use it?
Root mean square (RMS) is the effective value of a varying signal: the DC value that would deliver the same heating in a resistor. For a sine wave it equals the peak divided by the square root of two. Illustrative calculation: a 230 volt RMS supply peaks at about 325 volts, which matters when you choose component voltage ratings.
Why it is asked: it tests whether you can compare AC and DC fairly.
What is power factor and why does it matter?
Power factor is the ratio of real power, which does work, to apparent power, the product of RMS voltage and current. For sinusoidal signals it equals the cosine of the phase angle between them. A low power factor means more current for the same useful power, which raises losses and cable size, so utilities and plants correct it with capacitors.
Why it is asked: it links theory to cost in real installations.
Why is three-phase power used for large loads?
Three-phase supply uses three voltages spaced one third of a cycle apart, so total power delivered is steady instead of pulsing, and a given power needs less conductor material than single phase. The phase offsets also create a rotating magnetic field that starts and turns motors smoothly.
Why it is asked: it separates power-systems knowledge from basic circuits.
How does a transformer work?
A transformer uses a changing magnetic flux in a shared core to induce voltage in a second winding. Ideally the voltage ratio equals the turns ratio and power is conserved, so the current ratio is the inverse. It works only with changing current, which is why it cannot step up steady DC. Real transformers have core and copper losses.
Why it is asked: it is a staple for power roles, and the follow-up probes losses.
How would you choose a motor for an application?
Start from the load: required speed, torque over time, duty cycle and control needs. Then compare types such as induction, brushless DC and stepper for efficiency, controllability and cost, check the supply, and leave margin. State assumptions aloud and ask the interviewer for the missing numbers.
Why it is asked: it tests structured engineering judgement.
Knowing it is half the job. Saying it is the other half.
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What are the ideal op-amp rules?
With negative feedback, an ideal op-amp draws no input current and drives its output until the two inputs are at the same voltage; this condition is called virtual short. From those two rules you derive the gain: the inverting amplifier has gain minus Rf over Rin, and the non-inverting amplifier has gain one plus Rf over Rg.
Why it is asked: it is the quickest test of analogue skill.
What limits a real op-amp?
Real op-amps have finite gain-bandwidth, so gain falls at high frequency, a limited output swing and slew rate, input offset voltage and bias current, and noise. Pick the amplifier from the signal's frequency, amplitude and accuracy needs, and check each limit against them.
Why it is asked: real designs fail at these limits.
What is a pull-up resistor?
A pull-up resistor connects a digital input to the supply so that it reads a defined high level when nothing else drives it. Without it, a floating input picks up noise and reads randomly. It is required on open-drain lines such as many bus signals, and its value balances speed against current.
Why it is asked: it tests practical digital design.
What is aliasing and how do you prevent it?
Aliasing is the false low-frequency signal that appears when a signal is sampled too slowly. To avoid it, sample at more than twice the highest frequency present, the Nyquist rate, and use an analogue low-pass filter before the converter so higher frequencies are removed first.
Why it is asked: every data-acquisition design depends on it.
Why do decoupling capacitors go close to IC pins?
A chip draws short bursts of current; wiring has inductance, so a distant supply cannot respond fast enough and the local voltage dips. A small capacitor placed right at the supply pin provides that burst. Keep the loop of capacitor, pin and ground short, and pair small and larger values for different frequencies.
Why it is asked: it shows layout awareness.
How do you reduce noise and interference?
Shorten loops, keep a solid ground path, separate noisy and sensitive circuits, filter supplies and signals, use twisted or shielded cables, and measure rather than guess. Find the coupling route first: conducted, capacitive, inductive or radiated, then break it.
Why it is asked: noise is the common reason prototypes misbehave.
Debugging and design
How do you debug a circuit that does not work?
Follow a fixed order: reproduce the fault, check power and ground, measure the signals, split the circuit in halves, and change one thing at a time while keeping notes. State what you expect before each measurement.
Why it is asked: method matters more than a lucky guess.
Text description of the diagram
- Follow a fixed order: reproduce the fault, check power and ground, measure the signals, split the circuit in halves.
- Change one thing at a time while keeping notes.
How do you decide between two designs?
List the requirements, weight them, compare the designs on each with numbers where possible, test the riskiest assumption first and record the decision with its reasons. Include cost, manufacturability and safety, not only performance.
Why it is asked: engineering is trade-offs.
How do you approach safety in your work?
Treat safety as a design input: follow the relevant standards and your employer's procedures, de-energise and lock out equipment before work, and speak up about any hazard. The BLS handbook notes that experienced engineers can hold a Professional Engineer (PE) license, which carries responsibility for sign-off (BLS, August 2026).
Why it is asked: public safety is central to the profession.
Career, licensing and behavior
Do you need a Professional Engineer (PE) license?
Not to start. The BLS handbook says licensure is not required for entry-level electrical and electronics engineer positions; experienced engineers may earn a license, which typically requires a degree, passing the Fundamentals of Engineering (FE) exam, several years of experience and the Principles and Practice of Engineering (PE) exam, which the National Council of Examiners for Engineering and Surveying (NCEES) sets (NCEES PE page, October 2026; BLS, August 2026). Say whether you plan to pursue it.
Why it is asked: it shows how you see your career path.
What is the FE exam?
The Fundamentals of Engineering exam is generally the first step toward the Professional Engineer license. NCEES describes it as computer-based with 110 questions and a six-hour appointment, and electrical candidates take the FE Electrical and Computer exam (NCEES, as of October 2026). Mention if you have passed it.
Why it is asked: it signals commitment to professional practice.
Tell me about a design that failed.
Use the situation, task, action, result (STAR) structure in the STAR method: what failed, how you found the cause, what you changed and what check you added. Own your part plainly and end with what you do differently. Interviewers value honest analysis over a spotless record.
Why it is asked: failure analysis is a daily engineering skill.
Tell me about yourself and what questions you have for us.
Give a short present, past, future summary tied to the role, as the page on telling your story shows. Then ask about the products, test equipment, review process and how new engineers are mentored; see questions to ask the interviewer.
Why it is asked: it opens and closes almost every interview.
How these interviews are run
Technical panels often use a structured format: the same questions for each candidate, rated on one scale. The Office of Personnel Management (OPM) describes it as designed to measure job-related competencies, asking about past behaviour, hypothetical situations or both (OPM). Expect a whiteboard circuit, a debugging scenario and a behavioral round. This page belongs to the wider trades and engineering interviews guide; also read how interview loops work, how to answer interview questions and the behavioral interview hub.
- Know the limits of each law: Ohm's law, ideal op-amps, ideal transformers.
- Check every answer with a sanity test, such as parallel resistance being below the smallest branch.
- Use RMS, power factor and three-phase to connect theory to real cost.
- Debug in a fixed order and change one thing at a time.
- Know the licensure path: degree, FE exam, experience, PE exam.
Sources
- Electrical and Electronics Engineers, Occupational Outlook Handbook — U.S. Bureau of Labor Statistics, 27 Aug 2026
- University Physics Volume 2, section 10.3 Kirchhoff's Rules — OpenStax, 6 Oct 2016
- University Physics Volume 2, section 10.5 RC Circuits — OpenStax, 6 Oct 2016
- FE exam — NCEES, 24 Oct 2026
- PE exam — NCEES, 24 Oct 2026
- Structured Interviews — U.S. Office of Personnel Management, 24 Oct 2026
