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Science / Electronics basics / Grade 8 / se8r2

Parallel Resistance: Same Total, Different Branches

Explain the resistance seen by a source, then check what the total does and does not tell you.

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For complete parallel branches, each resistor has the same source voltage. Find each current using I = V / R, add the branch currents, then calculate equivalent resistance with R = V / total current in amperes. Do not add parallel resistor values. An open branch contributes no current. These are ideal paper models, not physical wiring instructions.

Worked example

At 12 V, the 300-ohm branch carries 40 mA and the 600-ohm branch carries 20 mA. Together they draw 60 mA = 0.06 A. The source sees 12 / 0.06 = 200 ohms equivalent resistance. A single 200-ohm resistor gives the same source current, not the same branch layout.

Worked parallel example: 300 and 600 ohms across 12 V are equivalent to 200 ohms, drawing 60 mA total. Internal readings need not match.
Worked model comparison. The drawings show resistor connections, not physical wiring instructions.
Question 1 At 6 V, complete 150-ohm and 300-ohm parallel branches carry 40 mA and 20 mA. What is their equivalent resistance?
Question 2 Open the 150-ohm branch but keep the 300-ohm branch connected across 6 V. What remains?
Question 3 Compare one 300-ohm path with two complete 600-ohm parallel paths, both at 12 V. Which statement is correct?
Question 4 Add a second complete positive-resistance branch in parallel without changing the nonzero ideal source voltage or first resistor. What happens?
Question 5 Two 300-ohm resistors are first connected in series, then in parallel, both complete at 6 V. Which totals are correct?