AP® Physics 2: Algebra-Based review sheet from Aim for Five (aimforfive.com/physics-2/units/11/11-2)
Unit 11 · Topic 11.2
11.2 Simple Circuits
A circuit is a set of loops that charge can flow around. Charge flows only through a closed loop, a break makes an open circuit, and a short circuit is a path with no resistance that charge takes without losing any potential. You need to read and draw circuits with standard schematic symbols.
Key terms
- closed circuit
- open circuit
- short circuit
- schematic diagram
- circuit element
Loops and circuit elements
A circuit is built from loops containing elements such as wires, batteries, resistors, lightbulbs, capacitors, switches, ammeters and voltmeters. A closed loop is a complete path that charge could travel around and end up back where it started.
One element can belong to several loops at once. In a circuit with two parallel branches, the battery is part of both loops. That's why changing one branch can affect current in others.
Closed, open and short
Whether charge can flow, and where, depends on the paths available.
- Closed circuit: there's a complete path, so charge can flow.
- Open circuit: there's a break somewhere (an open switch, a burned-out bulb, a cut wire), so no charge flows in that loop.
- Short circuit: a path with essentially no resistance, so charge can cross it with no change in potential. Anything connected in parallel with a short gets zero potential difference and zero current; the charge takes the wire instead.
Schematic symbols
A schematic is a simplified drawing of a circuit. It shows how elements are connected, not what they look like or where they sit on the table. Wires can be stretched, bent or shortened in a drawing without changing the circuit, as long as the connections stay the same.
| Element | How the symbol looks |
|---|---|
| Battery | a long line (positive terminal) next to a short line (negative terminal) |
| Resistor | a zigzag line |
| Lightbulb | a circle with a loop or an X inside |
| Switch | a break in the wire with a short line hinged at one end |
| Capacitor | two parallel lines of equal length with a gap |
| Ammeter | a circle with the letter A |
| Voltmeter | a circle with the letter V |
| Variable element | the usual symbol with a diagonal arrow drawn through it |
Reading a circuit
How a circuit behaves depends on how its elements are arranged. Before calculating anything, trace each loop with your finger. Ask which elements a coulomb of charge must pass through on each possible path, where the paths split and rejoin, and whether any switch or short changes which paths are available.
A useful trick: any two points connected only by ideal wire are at the same potential. Label those points with the same letter. Elements whose ends share the same pair of letters are in parallel.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Shorting out a bulb
Two identical bulbs, A and B, are in series with a battery. A switch is connected directly across bulb B (from one side of B to the other), and it starts open. What happens to each bulb when the switch is closed?
Show the solutionHide the solution
- Step 1: Switch open: the current must go through both bulbs, and the battery's potential difference is shared equally. Both glow at the same brightness.
- Step 2: Switch closed: the switch is a wire with no resistance across B, so it's a short. Charge takes that path with no change in potential, so B has zero potential difference and zero current. B goes out.
- Step 3: Now A has the battery's whole potential difference across it instead of half. With twice the potential difference, A carries twice the current, so it uses (2)(2) = 4 times the power (11.4). A gets much brighter.
Answer: B goes out; A gets brighter (four times the power)
- Example 2
Open or closed?
A battery is connected to bulb A. Bulbs B and C are in series with each other, and that pair is connected in parallel with A. Bulb C burns out (its filament breaks). Which bulbs still light?
Show the solutionHide the solution
- Step 1: A burned-out bulb is an open circuit: no charge can pass through it.
- Step 2: B and C are in the same branch, so the break in C stops the current in B too. B goes dark.
- Step 3: A's branch still forms a closed loop with the battery, so A stays lit. With an ideal battery, A's potential difference is unchanged, so its brightness is unchanged.
Answer: Only A stays lit, at the same brightness
Common mistakes
- Thinking a short circuit means a broken circuit. A short is the opposite: a path with almost no resistance, not a gap.
- Judging series or parallel by how the drawing looks. Trace where the charge can go; elements are in parallel if their ends connect to the same two points.
- Mixing up symbols, such as a capacitor (two equal lines) with a battery (one long line, one short line).
On the exam
- Expect to redraw a described circuit as a schematic with correct symbols, or to predict what happens when a switch opens or closes.
- When a question asks what happens to a bulb, justify with the path the charge takes and the potential difference across that bulb.
Connected topics
Videos
Check yourself
4 questions on 11.2 Simple Circuits. Pick an answer to see if you got it, and why.
Three identical bulbs are connected in series with a battery, and all three glow. One bulb is unscrewed from its socket. What happens to the other two bulbs?
A battery is connected to two bulbs that are in parallel with each other. How many different closed loops include the battery?
On a circuit schematic, what does a resistor symbol with a diagonal arrow drawn across it represent?
Which best describes a short circuit?
0 of 4 answered