One route or two?
In series, every charge path passes through both loads, so the same current flows through them. In parallel, the path branches. Each branch sees the source voltage, and the source supplies the sum of the branch currents.
What does it take to turn on a light?
Close a switch. Follow a complete path. Give current a second route and discover what changes.
Enable JavaScript to change the conditions and run the interactive experiment.
Make a discovery
Electric current is the rate at which charge passes a point. A source transfers energy to the circuit; a lamp transfers energy to light and heat. Charge keeps circulating—it is not used up by the lamp.
Make a prediction
Two equal loads are in series. Is there less current after the first one?
- Yes, some current was used up
- No, the current is the same
Read the explanation
Both are on the same unbranched path. The loads transfer energy, while the current remains the same. Switch to parallel and compare branch currents instead.
Understand it
Inside a flashlight
The cell, contacts, switch and light source form a circuit. Our cutaway follows an older incandescent flashlight with a return conductor through its body. The constant-resistance lamp is an idealization; modern LED flashlights use a different load and often a driver circuit.
Energy, not disappearing charge
Voltage is energy transferred per unit charge. Across a resistor, electrical energy becomes thermal energy. The moving marks indicate conventional current from positive to negative outside the battery; electrons in metal drift the other way.
Look closer at the science
Ohm’s law, with a scope
For these ideal fixed resistors, I = V/R. Series: Rₑ = R₁ + R₂. Parallel: 1/Rₑ = 1/R₁ + 1/R₂. An open switch gives I = 0. We omit switching transients and use the settled DC state.
An accounting check
P = VI = I²R. In series, V₁ + V₂ = Vsource. At a parallel junction, I₁ + I₂ = Isource. Adding the load powers gives the source power. That is an energy balance, not a calibrated brightness scale.
Why real lamps differ
A filament heats up and changes resistance, so doubling voltage need not double its current. The battery also has internal resistance. The virtual source stays at its selected voltage; the glow shows relative modeled power, not lumens or temperature.
Try it yourself: Be a flashlight detective
Supplies
- 1 intact battery-powered flashlight
- Paper and a pencil
- A nearby wall or sheet of paper
- Predict the path
Keep the flashlight assembled. Sketch a loop containing a battery, switch and light source. Predict where the loop is interrupted when the switch is off.
- Make and break
Point the light at the paper, away from eyes. Turn the normal switch on and off. Compare the result with the open and closed switch here.
- Compare the model
Read any visible LED or bulb label without dismantling the light. Record which parts the outside view hides. An LED lamp needs a different electrical model from our ideal resistor.
Which part opens the path when you turn the light off?
Use only the flashlight’s normal controls and manufacturer instructions. No disassembly or new wiring. This observation cannot measure current, resistance or the speed of electricity.
Sources and model limits
- Two ideal ohmic loads, an ideal voltage source, ideal wires and a master switch. No shorts, AC, capacitors, internal resistance or thermal feedback.
- The flashlight view appears only for the single-load circuit; series and parallel arrangements are separate circuit comparisons.
- Current markers are explanatory and slowed down. Their spacing and speed are not electron trajectories, charge density or signal propagation.
- Lamp glow is a qualitative indication of load power. Home observation uses an intact flashlight, not a wiring or battery-building procedure.
Current is charge per unit time; its conventional direction is defined for positive charge.
University Physics Volume 2 §9.1, Eqs. 9.1–9.3 and direction-of-current discussion. Supports the current definition and marker convention.
OpenStax · Electric currentIdeal resistors obey V = IR; series and parallel circuits have different current and voltage constraints.
University Physics Volume 2 §10.2, Eqs. 10.2–10.3 and junction/loop derivation. Supports solver and two-load schematics.
OpenStax · Series & parallelElectrical power is the rate of energy transfer.
University Physics Volume 2 §9.6. P = VI and resistor-power relationships support numerical outputs, not the brightness rendering.
OpenStax · Electrical powerResistance can depend on temperature.
University Physics Volume 2 §9.3, resistivity versus temperature; scopes the fixed-resistance filament analogy.
OpenStax · ResistanceIndependent subject review is pending.
Read the sources and model assumptions