Series Resistance: Check the Source Load

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 7 V, complete series resistors of 200 and 150 ohms add to 350 ohms. Current is 7 / 350 = 0.02 A = 20 mA through each. A single 350-ohm resistor draws the same total current, but does not preserve both internal resistor voltages.

Worked series example: 200 and 150 ohms across 7 V are equivalent to 350 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 has 300-ohm and 500-ohm resistors. What equivalent resistance does its source see? 400 ohms 500 ohms 800 ohms

Question 2 An 8 V ideal source supplies that 800-ohm path. What current passes through the 500-ohm resistor? 16 mA 10 mA 26 mA

Question 3 Keep 8 V and the 300-ohm resistor, but replace the 500-ohm resistor with 100 ohms. What is the new total current? 20 mA 10 mA 80 mA

Question 4 A different complete resistor network draws 25 mA from an 8 V ideal source. What is its equivalent resistance? 0.32 ohms 200 ohms 320 ohms

Question 5 Two different complete networks each have 400 ohms equivalent resistance at 8 V. What can you conclude from that alone? Every resistor inside each network has the same voltage Each draws 20 mA total; internal readings need not match Both networks must contain the same number of resistors

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