Circuit Builder
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flagWhat you'll discover
- arrow_forwardApply Ohm's law V = IR to predict current from voltage and resistance
- arrow_forwardCombine resistors in series (R₁ + R₂) and parallel (product over sum)
- arrow_forwardExplain why current is shared in parallel but voltage is shared in series
- arrow_forwardConnect electric current to the motion of charge carriers
Ohm's law
Current is the flow of electric charge, driven by a potential difference (voltage) and opposed by resistance. Georg Ohm found in 1827 that for metallic conductors at constant temperature the three are locked together: V = IR. Double the voltage and the current doubles; double the resistance and it halves.
In the simulation the ammeter reads I = V/R the instant you move a slider, and the drifting electron dots speed up or slow down to match. The bulb's brightness tracks the power delivered, P = VI = I²R, which is why dimming a light is really a resistance trick.
Series circuits
In a series circuit the components form a single loop, so the same current must pass through every element — there is simply nowhere else for the charge to go. The resistances add up: R_total = R₁ + R₂, and the battery voltage divides itself across the resistors in proportion to their resistance (V₁ = IR₁, V₂ = IR₂).
The weakness of series wiring is that one break stops everything, which is why old strings of fairy lights died entirely when a single bulb blew. Adding more resistors in series always decreases the current.
Parallel circuits
In parallel, both resistors connect directly across the battery, so each one feels the full battery voltage. The current divides between the branches — more current takes the easier (lower-resistance) path — and the branch currents add up to the total drawn from the battery.
The combined resistance is always less than the smallest branch: 1/R_total = 1/R₁ + 1/R₂. Two equal resistors in parallel halve the resistance and double the current. Homes are wired in parallel so every appliance gets the full mains voltage and each can be switched independently.
What is actually moving?
In a metal wire the moving charges are free electrons, drifting from the negative terminal towards the positive — opposite to the conventional current direction that circuit diagrams use by historical accident. Their average drift speed is astonishingly slow, often well under a millimetre per second.
Yet a lamp lights the instant you flip the switch, because the electric field that pushes the electrons is established along the whole wire at nearly the speed of light. Think of a pipe already full of water: push at one end and water emerges at the other immediately.