Current, Charge and Time — the Tap and the Bucket
Two taps filling two buckets against two stopwatches. Hold the clock fixed and doubling the charge doubles the current; hold the charge fixed and doubling the time halves it. The same run of tap B anchors both halves of I = Q/t.
How to use this simulation
- Press Same time, more charge to hold Δt fixed and vary the charge delivered
- Press Same charge, more time to hold Q fixed and vary the time taken
- Use Pause to freeze both buckets mid-fill and read them against each other
- Press Restart to run the comparison again from empty
What to look for
- At equal times, the bucket holding twice the charge corresponds to twice the current
- At equal charge, the tap taking twice as long corresponds to half the current
- Tap B appears in both comparisons, tying the two halves into one relationship
- Neither the charge alone nor the time alone tells you the current
- A steady tap fills at a constant rate, which is the constant-current case
The physics behind it
Current is charge per unit time, I = Q/t, so it is a ratio and neither quantity fixes it on its own. Comparing two runs at equal times isolates the numerator: I ∝ Q, and twice the charge in the same interval is twice the current. Comparing two runs carrying equal charge isolates the denominator: I ∝ 1/t, and taking twice as long to deliver the same charge halves the current. Keeping one run common to both comparisons makes the two proportionalities parts of a single relationship rather than separate rules.