Series, parallel, and a switch
Why does source current change when the same two resistors are connected differently?
Compare voltage, current, and equivalent resistance for two ideal resistors in series, parallel, and an open circuit.
Learning goals
- Apply Ohm's law, V = IR.
- Calculate equivalent resistance for two series or parallel resistors.
- Explain why current is zero in an ideal open circuit.
Before you begin
- Calculate with decimals and simple fractions.
- Recognise voltage (V), current (A), and resistance (Ω).
Ideal circuit experiment
- 1Series
- 2Change resistance
- 3Parallel
- 4Open switch
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
-
A series circuit has one path, so both resistors carry the same current; their voltage drops add to the source voltage.
Your task: Set R₁ = 10 Ω, R₂ = 20 Ω, V = 6 V and check I = 0.20 A.
-
Series equivalent resistance is R₁ + R₂. At fixed source voltage, increasing either resistance reduces current around the whole loop.
Your task: Increase R₂ and compare total resistance and current.
-
Each parallel branch has the source voltage. Total current is the sum of branch currents, and equivalent resistance is less than either branch resistance.
Your task: Keep the values and switch to parallel; verify Itotal = I₁ + I₂.
-
An open ideal switch breaks the conducting path, so steady current is zero; the model also reports zero voltage drop across each resistor.
Your task: Open the switch and confirm every current reads 0 A.
Build an explanation
Series: same current
With no branch, the same steady current passes through both components. Source voltage divides in proportion to resistance.
Parallel: same voltage
Both branches connect to the same two nodes, so each resistor has the same voltage. The lower-resistance branch carries more current.
Ideal low-voltage model only
This model omits wire and source resistance, temperature change, and transients. Never test it with mains outlets; practical classroom work should use teacher-approved safe low-voltage equipment.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Physics
Series, parallel, and a switch
Why does source current change when the same two resistors are connected differently?
- A 6 V source drives 10 Ω and 20 Ω in series. What is total current?
A. 0.20 A B. 0.60 A C. 1.80 A
- What is the equivalent resistance of 10 Ω and 20 Ω in parallel?
A. 6.67 Ω B. 15 Ω C. 30 Ω
- In an ideal parallel circuit, what is the voltage across each resistor?
A. Each equals the source voltage B. Always 0 V C. Always half, regardless of resistance
- What steady current does the model give when the ideal switch is open?
A. 0 A B. It depends on wire colour C. Infinite
Answer page
Series, parallel, and a switch
- 0.20 A
Rtotal = 30 Ω, so I = V/R = 6/30 = 0.20 A.
- 6.67 Ω
Req = (10 × 20)/(10 + 20) = 6.67 Ω, less than either branch.
- Each equals the source voltage
Parallel branches span the same two nodes, so they share the same potential difference.
- 0 A
The broken circuit has no complete conducting path, so source and branch currents are all zero.