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Current electricity looks simple in class: V = IR, series and parallel, a few rules. Then a question asks what happens to the brightness of bulb B when switch S closes, and confident answers fall apart. The problem is usually not the formula but a hidden picture of what current is. A circuit simulation is a fast, safe way to expose that picture and fix it. This post explains the common misconceptions research has found, then gives you five predictions to test.
What do students get wrong about current?
Physics education researchers have studied this for decades; McDermott and Shaffer's 1992 work on introductory electricity is one well-known example of using such research to design teaching (American Journal of Physics, 1992).
A later study by Engelhardt and Beichner built a 29-question diagnostic test on DC resistive circuits and studied how high school and university students reason about them (American Journal of Physics, 2004). They found:
Students tended to hold several misconceptions at once, even after instruction.
In interviews, the idea used most often was that a battery is a constant source of current, delivering the same current whatever is connected to it.
Students focused on current when solving problems and often confused terms, giving voltage or resistance properties that belong to current.
Another tempting idea is that current gets "used up" as it passes through a bulb, so less comes out than goes in. You can test that one directly below.
Why use a simulation for this?
Finkelstein and colleagues replaced the real-equipment DC circuits lab with a computer simulation for one group of students in a large introductory physics course (Physical Review Special Topics, Physics Education Research, 2005). The simulation showed electron flow explicitly. The simulation group outperformed the real-equipment group on a conceptual survey and even on assembling a real circuit and describing how it worked.
The free PhET Circuit Construction Kit: DC lets you build circuits with batteries, resistors, bulbs, switches, an ammeter and a voltmeter, and switch between a lifelike view and a schematic diagram (PhET). More on what simulation research shows is in what PhET research says simulations are good for.
Five predictions to test
Use a 9 V battery and 10 Ω resistors. If your simulation lets you set the battery's internal resistance, keep it at zero. Write each prediction and your reason before you build.
1. Is current used up?
One battery, one resistor. Predict the ammeter reading just before the resistor and just after it. Then measure both. They should be equal: 0.9 A. The resistor converts energy, but the charge flow is the same all the way round a series loop.
2. Is the battery a constant current source?
Add a second 10 Ω resistor in series. Predict the current. If the battery delivered fixed current, it would stay 0.9 A. It drops to 0.45 A, because total resistance doubled. The battery sets the voltage; the circuit decides the current.
3. What does a parallel branch do?
Go back to one resistor, then add a second 10 Ω resistor in parallel. Predict three things: current through the original resistor, current through the new one, and current from the battery. Each branch carries 0.9 A, and the battery now supplies 1.8 A. The original branch does not "share" a fixed amount.
4. Where is the voltage?
Use the voltmeter across each resistor, first in the series circuit from prediction 2, then in the parallel circuit from prediction 3. Series: 4.5 V across each. Parallel: 9 V across each. Voltage is measured across a component; current is measured through it.
5. The open switch puzzle
Put a switch in series with one resistor and open it. Predict the current, the voltage across the resistor, and the voltage across the open switch. Current is zero and the resistor has 0 V across it, yet the switch has the full 9 V across it. If that surprises you, you may have been assigning current's behaviour to voltage, the kind of confusion Engelhardt and Beichner described.
Turn the session into exam marks
Write one sentence per surprise in your notebook, such as "The battery fixes the voltage, not the current."
Solve three textbook problems on series-parallel combinations without the simulation, then check one by building it.
Redo predictions 2 and 5 from memory in a week.
If a prediction keeps going wrong, the gap may be in an earlier idea such as potential difference; the Fermi Concept Library covers every concept in physics with its own lesson and its place in the map. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read, where you can run the same predict-then-test routine. For a broader look at when simulations beat diagrams, see simulations or static diagrams.
