Ohm's Law – Two Resistors in Series
PhysicsElectricity & CircuitsAges 14–15
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Electricity & Circuits
A source U with two resistors R1 and R2 connected in series. The ammeter reads I = U/(R1+R2); the electrons move faster or slower depending on the current. The voltage across each resistor follows Ohm's law.
Lesson: Ohm's law, series circuits
What it shows
In a series circuit there is only one path, so the same current flows through every component: I = U/(R1 + R2), where R1 + R2 is the total resistance. Applying Ohm's law to each resistor gives the voltage across it, U1 = I·R1 and U2 = I·R2. The source voltage is shared in proportion to resistance, so the larger resistor takes the larger share, and U1 + U2 = U. The moving electrons show that charge is not used up by the first resistor: the current is the same all the way around the loop.
How to use
Start with Show readings turned off. Change Source voltage, Resistance R1 or Resistance R2, watch how fast the electrons move and ask students to predict whether the current has increased or decreased. Then turn Show readings on to check I, U1 and U2. Try making R2 twice R1, compare the two voltages, and confirm that U1 + U2 always equals the source voltage.
Parameters you can change
- Source voltage 1–24 V
- Resistance R1 1–100 Ω
- Resistance R2 1–100 Ω
- Show readings
Questions to explore
- When R2 is increased, what happens to the current and to the voltage across R1?
- With R1 = 10 Ω, R2 = 20 Ω and U = 12 V, what are U1 and U2?
- Why is the current through R1 exactly the same as the current through R2?
