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1.1.14 · D5Electricity & Charge Basics

Question bank — Read and interpret circuit schematic symbols

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True or false — justify

Two wires drawn as a + with a dot at the centre are electrically connected.
True. The dot is the only mark that means "joined into one node"; with it, all four wire-ends share the same potential.
Two wires drawn as a + with no dot are connected.
False. No dot means they cross over as two separate nodes — like one road bridging another, no electrical contact.
The battery's long thin line is the negative terminal.
False. The long thin line is positive (+); the short thick line is negative (−). Conventional current leaves the long line.
A resistor and a battery can both be drawn as a plain rectangle, so they mean the same thing.
False. A battery has unequal parallel lines (long +, short −) and supplies EMF; a plain rectangle is a resistor that only drops voltage via (Ohm's Law).
In a series loop every component carries the same current.
True. Charge cannot pile up on a single wire path, so the current entering each component equals the current leaving it — the same everywhere.
In a series loop every component has the same voltage across it.
False. Series shares current, not voltage; each part drops whatever Ohm's Law or its device law demands, and the drops sum to the EMF (Kirchhoff's Voltage Law).
Two resistors sharing both end-nodes are in parallel.
True. Sharing both nodes means both endpoints sit at the same two potentials, so each resistor sees the same voltage — the definition of parallel (Series and Parallel Circuits).
Ground in a schematic is always a literal metal stake driven into the soil.
False. In a schematic, ground is the 0 V reference node; every voltage is measured against it. It is a bookkeeping choice, though it may also connect to earth (Grounding and Reference Voltages).
An open switch still lets a small current leak through its node.
False. An ideal open switch breaks the node entirely — the loop is cut, so current is exactly zero, no matter the voltage.
A capacitor passes steady DC current forever once connected.
False. A capacitor charges to and then blocks steady DC; no continuous current flows through it in a DC steady state.
An ideal wire has a voltage drop across it.
False. An ideal wire is a perfect conductor with drop ; it just extends a node, keeping every point on it at the same potential (Electric Potential and Voltage).
A backwards LED still lights up dimly.
False. A diode conducts essentially one way only; reversed, it blocks current, so it stays dark — not dim, off.
Every point on a single node is at the same potential.
True. A node is unbroken wire, and an ideal wire drops zero volts, so all its points share one potential — that is what makes a node a node.

Spot the error

"I traced the loop and the LED triangle points from − back toward +, so current flows fine."
The error is the LED orientation. The triangle must point with conventional current, i.e. from + toward −; pointing the other way means the diode blocks and the loop stays dark (Diodes and LEDs).
"These two resistors are end-to-end on one wire, so they're in parallel and is smaller."
End-to-end (sharing only one node between them) is series, so is larger. Parallel requires sharing both end-nodes.
"The long line on the cell is thick and heavy, so it must be the current sink — negative."
Length, not thickness of intuition, is the rule: the long line is positive regardless of how it "feels." The short line is negative.
"The wires cross in a plus shape, so they're one node — I'll compute the voltage there once."
Without a junction dot, a crossing is two separate nodes, each with its own potential. Treating them as one merges circuits that were meant to be independent.
"Ground is 0 V, so nothing interesting can ever happen at the ground node."
0 V is only the reference; large currents can still flow into and out of ground. It being 0 V is a labelling choice, not a claim that it is electrically idle.
"There's no resistor in the LED loop, but the battery is only 5 V so the LED is safe."
An LED has almost no internal resistance, so without a current-limiting resistor the current is set by nothing and can spike far past the LED's rating — voltage being "only 5 V" doesn't save it.
"Two branches share both nodes but have different resistances, so they must carry the same current."
Sharing both nodes forces the same voltage, not the same current. By , the smaller resistance carries the larger current (Series and Parallel Circuits).

Why questions

Why does a schematic deliberately throw away the physical shape and size of components?
Because what matters electrically is only which terminal connects to which (topology) and what each part does (function); shape and size don't affect the circuit's behaviour.
Why is a junction dot the only thing that means "connected"?
So that crossing wires can be drawn on flat paper without accidentally implying contact — the dot removes all ambiguity between "joined" and "just passing over."
Why must the current be identical everywhere in a series loop?
Charge cannot accumulate anywhere on a single unbranched path in steady state, so whatever flows in must flow out — the same at every point.
Why do the voltage drops around any closed loop sum to the source EMF?
Because potential is single-valued: walking a full loop back to your start must return you to the same potential, so the rises and drops cancel (Kirchhoff's Voltage Law).
Why does adding a second resistor in parallel lower the total resistance?
A parallel branch gives current an additional path, so more total current flows for the same voltage — and more current for the same voltage means a smaller effective resistance.
Why does the resistor in an LED loop set the current rather than the LED?
The LED holds a roughly fixed voltage drop and has negligible resistance, so the remaining voltage falls across the resistor, and fixes the current.
Why is choosing a ground node necessary before you can state any voltage?
Voltage is a difference between two points, so it is meaningless until you pick a reference; ground is that agreed 0 V baseline every other reading is compared to (Electric Potential and Voltage).
Why does an open switch produce zero current no matter how large the battery is?
An open switch breaks the only conducting path, so there is no complete loop; without a closed path current cannot flow regardless of the driving voltage.

Edge cases

What happens to current through a resistor whose value approaches (a dead short across a source)?
blows up toward infinity as ; the "wire" carries as much current as the source can deliver — this is a short circuit, the danger case.
What happens to current through a resistor whose value approaches infinity (an open)?
; an infinite resistance behaves like a broken wire, so effectively no current flows — the same as an open switch.
Two identical resistors in parallel: what is compared to one alone?
, exactly half of one, because two equal paths double the current for the same voltage (Series and Parallel Circuits).
If two nodes are joined by an ideal wire, how many distinct nodes remain?
One. An ideal wire drops zero volts, so joining two nodes merges them into a single node at one common potential.
An LED placed exactly reverse-biased with a huge voltage — is it still "one-way safe"?
No — beyond its reverse breakdown voltage the diode can conduct destructively; "one-way" holds only within its rated reverse voltage (Diodes and LEDs).
A capacitor at the very first instant of connection (before it charges) — how does it behave?
Momentarily like a wire (a short), because its voltage starts at 0 and current flows freely to charge it; only after charging does it block DC ().
A node that connects to nothing else (a dangling wire end) — what current flows into it?
Zero, because current needs a complete path; a dead end has nowhere for charge to go, so no steady current enters it.

Active recall

Recall Fast self-check
  • Dot vs no dot at a crossing — which means connected?
  • Long line on a battery — polarity?
  • Series shares ___; parallel shares ___.
  • Reverse-biased LED — lit or dark, and why?
  • Capacitor at behaves like a ___; at DC steady state like a ___.
Dot vs no dot — which means connected?
Dot = connected (one node); no dot = crossing over, two separate nodes.
Long line on a battery is which polarity?
Positive (+); the short thick line is negative (−).
Series shares what, parallel shares what?
Series shares the same current; parallel shares the same voltage.
Reverse-biased LED — lit or dark?
Dark — a diode conducts one way only, so reversed it blocks current.
Capacitor at vs DC steady state?
At it acts like a wire (short); at DC steady state it acts like an open (blocks current).

Connections