Series Resistance: The Whole Path

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

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In a complete series path, resistances add and the same current passes through every resistor. Find current with I = source voltage / total resistance. Convert amperes to milliamperes by multiplying by 1,000. An open circuit is not zero resistance. These are ideal paper models, not physical wiring instructions.

Worked example

At 6 V, complete series resistors of 100 and 200 ohms add to 300 ohms. Current is 6 / 300 = 0.02 A = 20 mA through each. A single 300-ohm resistor draws the same total current, but does not preserve both internal resistor voltages.

Worked series example: 100 and 200 ohms across 6 V are equivalent to 300 ohms, drawing 20 mA total. Internal readings need not match. — Worked model comparison. The drawings show resistor connections, not physical wiring instructions.

Would you believe it?

Different resistor networks can draw the same current from the same voltage source. Equivalent resistance captures that source load, but cannot reveal every voltage or current inside the network.

OpenStax: equivalent resistance

Question 1 A complete series path contains 200-ohm and 400-ohm resistors. What is its equivalent resistance? 300 ohms 600 ohms 200 ohms

Question 2 A 12 V ideal source is connected to that 600-ohm series path. What total current flows? 20 mA 50 mA 7,200 mA

Question 3 Swap the positions of those 200-ohm and 400-ohm resistors. Keep the complete path and 12 V source. What changes? The first resistor now sets the total at 400 ohms Current doubles because the larger resistor comes first Neither the 600-ohm total nor the 20 mA current

Question 4 Now open the only series path. How should its zero current be described at 12 V? Zero ohms, because the current is zero Open circuit: no finite equivalent resistance Still 600 ohms, because the resistors are unchanged

Question 5 A complete series path must total 500 ohms. One resistor is 150 ohms. What must the other be? 350 ohms 650 ohms 500 ohms

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