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Science / Electronics basics / Grade 8 / se8r1

Parallel Resistance: More Than One Path

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 6 V, the 200-ohm branch carries 30 mA and the 200-ohm branch carries 30 mA. Together they draw 60 mA = 0.06 A. The source sees 6 / 0.06 = 100 ohms equivalent resistance. A single 100-ohm resistor gives the same source current, not the same branch layout.

Worked parallel example: 200 and 200 ohms across 6 V are equivalent to 100 ohms, drawing 60 mA total. Internal readings need not match.
Worked model comparison. The drawings show resistor connections, not physical wiring instructions.
Question 1 Two complete 300-ohm branches are in parallel across 9 V. What is their equivalent resistance?
Question 2 For those two 300-ohm branches at 9 V, which A/B/total current readings are correct?
Question 3 At 6 V, complete 200-ohm and 300-ohm parallel branches draw 30 mA and 20 mA. What is their equivalent resistance?
Question 4 Why is 120 ohms below both 200 and 300 ohms in that parallel example?
Question 5 Open both parallel paths while the source remains 6 V. Which description fits?