Circuits

RL circuit

Inductor current rises toward E/R (τ = L/R)

RL circuit — interactive Circuits simulation. Inductor current rises toward E/R (τ = L/R) Free browser-based virtual physics lab with live SI measurements and a guided tutorial.

RL current rise

Inductor current grows toward ε/R with time constant τ = L/R.

The inductor opposes changes in current via its back-EMF. After a long time current settles to ε/R.

• τ = L/R
• I(t) → ε/R

Investigation brief

Plan the question before you open the lab

The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.

Driving question

An inductor and resistor are connected to a 10 V battery. Does current jump instantly to its final value, or does it ramp up gradually like a charging capacitor's voltage?

Predictions to weigh

• Current ramps up gradually, following the same kind of exponential approach as an RC circuit's voltage.
• Current jumps instantly to E/R.
• Current oscillates before settling.

Variable roles

What you set:

• Time t (s)

What you measure:

• Time constant τ (s)
• Inductor current I (A)

How the investigation runs

1. Open the rl-circuit preset and press Reset. R = 10 Ω, L = 1 H, giving a time constant τ = L/R = 0.1 s; the battery supplies 10 V.
2. Enable the inductor-current readout.
3. Press Play and record the inductor's current at each listed time.

Governing equation

RL Time Constant — τ = L/R

The time constant τ = L/R sets the timescale of an RL circuit's exponential current rise (or decay). A larger inductance or smaller resistance means a longer τ — the inductor more strongly resists changes in current.

What the printable worksheet asks students to work out

• For one trial, compute I = (E/R)(1 − e^(−t/τ)) using E = 10 V, R = 10 Ω, τ = 0.1 s. Compare to the table.
• Explain why an inductor's current ramp-up (RL circuit) follows the same mathematical shape as a capacitor's voltage ramp-up (RC circuit), even though the two components store energy completely differently.

Where this shows up beyond the lab

• An inductor's defining behavior is opposing any CHANGE in current (not opposing current itself, the way a resistor does). Explain why, right at t = 0, the current must start at exactly 0 A even though the battery is fully connected.
• When a current-carrying inductor's circuit is suddenly broken (switch opened), the inductor tries to maintain its current, which can create a large voltage spike (this is how spark plug ignition coils work). Explain why interrupting an inductor's current path is more dramatic than interrupting a resistor's.

• AP Physics 2 — Unit 11: Electric Circuits
• AP Physics C: Electricity and Magnetism — Unit 11: Electric Circuits
• IB Physics — B.5 Current and circuits
• General High School Physics — Electricity & DC circuits
• NGSS High School Physics — Electric current and magnetic fields

1. Welcome to Rl Circuit
2. Press Play
3. Select the inductor
4. Current ramps up
5. Open the Properties panel
6. You did it!

Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.

Open Interactive Lab →

Download lab sheet →

Capacitor Charging & Time Constants