Skip to content

Normal Circuit vs. Short Circuit

Run the Normal Circuit vs. Short Circuit MicroSim fullscreen

About This MicroSim

Two loops, the same 6 V battery, and one difference: the left loop has a 220 Ω resistor in it and the right loop is bare wire.

left:   6 V / 220 Ω        = 27 mA      warm at most
right:  6 V / ~0.001 Ω     = amps       hot in seconds

The right-hand figure is approximate on purpose. A short circuit's current is not limited by the wire, it is limited by what the battery can physically deliver and by the resistance of its own internals — which is why the honest answer is "as much as the battery has", and why the number varies by battery type rather than being a property of the circuit.

What actually goes wrong. Not a bang. The wire and the battery get hot, fast. A AA pack will get too hot to hold; a lithium pack can vent or catch fire. The damage is thermal, it takes seconds rather than an instant, and it is reversible if somebody disconnects it — which is exactly why "disconnect first, then look" is the rule.

The resistor is not optional decoration. It is the only thing standing between the battery and a very large current, and it is the component students most often leave out because the circuit "works without it" for a few seconds.

How to Use

  • Hover markers 1 and 2 to see the two components that make the left loop safe: a resistor in series, and a battery of a known voltage.
  • Hover marker 3 for the current the left loop actually draws.
  • Hover marker 4 for the right-hand loop, and note that the number is approximate for a reason.
  • Switch to Quiz to be asked which is which.

Lesson Plan

Bloom level: Understand (L2) -- explain

Learning objective: Given a circuit diagram, the learner explains why a low-resistance path produces a current spike compared to a normal, resistor-limited path.

Before (5 min). Write Ohm's law on the board and work the left loop together: 6 divided by 220. Then ask what happens when you divide 6 by something close to zero.

With the diagram (10 min). Compare the two loops marker by marker. The useful question is not "which is dangerous" — everyone can see that — but "what exactly is different", and the answer is one component.

After (10 min). Agree the club's rule out loud: check the loop before connecting power, and if something gets warm, disconnect first and look second.

Check for understanding. Ask: "Why can't we say exactly how many amps the short circuit draws?" Because nothing in the circuit limits it — the battery's own internal resistance does, so the answer depends on the battery rather than on the wiring. A student who gives a confident precise number has missed what makes a short dangerous.

Embedding This MicroSim

Paste this into any page of the book, adjusting the relative path to docs/sims/ for the page's depth:

<iframe src="../../sims/normal-circuit-vs-short-circuit/main.html" width="100%" height="520" scrolling="no"></iframe>

Specification

The full specification below is extracted from Chapter 16: Physical Computing and Electrical Safety Basics.

Type: infographic
**sim-id:** normal-circuit-vs-short-circuit<br/>
**Library:** Interactive Infographic Overlay (diagram.js, side-by-side comparison + numbered callouts)<br/>
**Status:** Specified

Purpose: Show why removing the resistor and LED from a circuit's current path causes a dangerous current spike, using the same numbers worked through in the prose above.

Bloom Taxonomy: Understand (L2)
Bloom Taxonomy Verb: explain

Learning objective: Given a circuit diagram, the learner explains why a low-resistance path produces a current spike compared to a normal, resistor-limited path.

Image style: Two side-by-side breadboard circuit illustrations — left labeled "Normal Circuit" (battery, resistor, LED, wire loop), right labeled "Short Circuit" (battery, bare wire loop, no resistor)

Image dimensions: 1200x600 (landscape)

Callouts (4 total): 1. "6V battery pack" on both illustrations; 2. "220-ohm resistor" on the normal circuit only, color #4A90D9; 3. "~27 mA (safe)" current readout on the normal circuit, color #2ECC71; 4. "~6 A (dangerous, gets hot)" current readout on the short circuit, color #E74C3C

Interactive features: Hover either current readout to reveal the Ohm's law calculation behind it; click either full illustration to toggle a heat-glow effect on the wire showing where the energy is being dissipated

Implementation: Interactive Infographic Overlay Guide (callout engine) — `diagram.js` + `style.css` render the two illustrations with hover-revealed calculation tooltips

References