1.2.12 · D5Circuit Analysis Fundamentals
Question bank — Read multimeter measurements (V, I, R)
Before we start, one word we'll reuse: ==loading== means the meter itself changing the very quantity it is trying to read, by drawing (or blocking) current it shouldn't. Picture a thermometer so big it cools the coffee you dip it in — that's loading.
True or false — justify
Each item is a claim. Say true or false and give the reason — bare T/F earns nothing.
A voltmeter should have as HIGH an internal resistance as possible.
True. A high (≈) means almost no current is diverted through the meter, so the voltage you read is the one that was really there — minimal loading.
An ammeter should have as HIGH an internal resistance as possible.
False. An ammeter sits in series in the current path; high resistance would throttle the very current it measures. An ideal ammeter has ≈ (a tiny shunt).
You can safely measure current by touching both probes across a battery's terminals.
False. Current mode has near-zero resistance, so across a source it is a dead short — huge current, blown fuse. Current is measured through a broken path, never across a supply.
Measuring a resistor while it is still soldered in a live board gives its true value.
False. Power must be off (live current corrupts the ohmmeter's own test current) and parallel neighbours would read the combination, which is lower than the true value.
A reading of "0.47" always means 0.47 ohms.
False. The number is only digits; the range/prefix sets the unit. On a range, means — see Metric Prefixes and Engineering Notation.
An ideal voltmeter draws exactly zero current.
True in the ideal limit (). Real meters draw a tiny sliver (), which is why "ideal voltmeter" is a limiting model, not reality.
Resistance mode and continuity mode both push a test current from the meter's own battery.
True. The circuit under test must be unpowered so only the meter's known current flows; that's the shared assumption behind .
Voltage can be measured at a single point in a circuit.
False. Voltage is a difference between two points; a single probe has no reference. You always need both probes (the black one defines "zero" / reference).
Spot the error
Each line describes what someone did. Name the mistake and the correct move.
"I set DCV, put the meter in series to read the voltage across a resistor."
Error: voltage is measured across (in parallel), not in series. In series a high- voltmeter nearly blocks the current, so the branch goes almost dead and the reading is wrong.
"I moved to the mA jack and touched the two probes straight across the 9 V battery."
Error: that's the ammeter's low-resistance path placed across a source = short circuit. Blows the internal fuse. Current must be inserted into a broken series branch.
"The resistor reads 220 Ω but it's marked 1 kΩ, so it's faulty — I'll bin it."
Error: it was measured in-circuit, where parallel paths lower the reading. Lift one leg (isolate) and re-measure before condemning it.
"I read 5 off the screen and wrote down 5 volts, done."
Error: no prefix recorded. It could be , , or (on a resistance dial) . Always read digits with the range unit.
"Wall adapter output looked like 0 V, so the adapter is dead."
Error: likely measured AC output on the DCV setting. AC averages toward zero on a DC scale — switch to ACV before declaring it dead.
"I read a fresh AA at 1.48 V under no load, then said it must deliver 1.48 V while driving a motor."
Error: ignored internal resistance. Under load, current through the cell's internal resistance drops the terminal voltage below the open-circuit 1.48 V.
"To find the current, I measured voltage across the resistor and just wrote that down as the current."
Error: volts are not amps. You'd still need Ohm's law (and the resistor's value) to convert — the number alone is a voltage.
Why questions
Answer the reason, not just the fact.
Why must a resistance measurement be done with the circuit powered off?
The ohmmeter forces its own known test current; any external current adds to and corrupts — and can blow the meter's fuse.
Why does an ammeter use a small SHUNT resistor rather than measuring current directly?
A DMM fundamentally measures voltage, so it converts current into a measurable voltage across a known tiny , then applies from Ohm's Law.
Why is a voltmeter connected in parallel but an ammeter in series?
Voltage is a difference between two points (parallel taps both points); current is a flow through a single path (series inserts the meter into that path).
Why can measuring current disturb a circuit even though ammeters have low resistance?
"Low" isn't zero. The small adds a tiny series resistance, dropping a little voltage and slightly reducing the current — a mild loading effect.
Why does the same tool need three different modes instead of measuring everything at once?
Each quantity demands opposite meter properties: voltmeter wants huge , ammeter wants near-zero , ohmmeter must supply its own current with the circuit dead. One setting cannot satisfy all three.
Why do all three modes ultimately reduce to Ohm's law?
has three unknowns; each mode fixes two and solves for the third — read , force to get ; know , read to get ; and so on.
Why does an in-circuit resistor read lower than its marked value, never higher?
Any parallel path offers an additional route for the ohmmeter's test current, and parallel resistances combine to less than the smallest branch — see Series and Parallel Circuits.
Edge cases
The scenarios the naive rule forgets.
You put the ohmmeter across a perfect wire (zero resistance) — what should it read, and what does that confirm?
Near . It confirms continuity: charge can flow freely with no opposition, exactly what "connected" means.
You put the ohmmeter across an open (broken) connection — what happens?
It reads "OL" / overload / infinity: no path exists for the test current, so . This is how you detect a broken wire or blown fuse.
An ideal ammeter () is placed across a component instead of in series — what does that do to the circuit?
It short-circuits that component, diverting essentially all current around it. The component "disappears" from the circuit and the reading is meaningless (and dangerous).
An ideal voltmeter () is accidentally inserted in series — what current flows?
Essentially zero. Infinite series resistance blocks the loop, so the branch it's in goes dead and downstream parts stop working.
You measure a 1 kΩ resistor with a real voltmeter whose across it — is the loading error large or small, and why?
Small. in parallel with is dominated by the , so it barely changes; loading only bites when the meter's is comparable to the circuit resistance.
You measure voltage across a very high resistance (say several MΩ) with the same meter — now is loading a problem?
Yes. Now is comparable to the circuit resistance, so the meter steals a real fraction of the current and reads noticeably low — the loading effect becomes significant.
A perfectly discharged battery is measured on DCV — what reading distinguishes "dead" from "healthy but loaded"?
A dead cell reads near 0 V even with no load; a healthy cell reads its full open-circuit voltage unloaded and only sags under load due to internal resistance.
You read a resistor whose colour bands say brown-black-red but the meter shows OL on the range — trap or truth?
Trap: brown-black-red = , which exceeds the ... actually it fits, so OL means the range is wrong or a probe is open. Re-check contact and range before doubting the colour code.
Recall One-line survival summary
Voltage = across (high-R), Current = cut in series (low-R), Resistance = remove & power off (meter supplies current). Every trap on this page is a violation of one of those three, or of "read the prefix."