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Ohm's law, series and parallel — in the bay, not the classroom
The value of Ohm's law is not arithmetic. It is that you can predict what the meter should say before you probe, so a wrong reading tells you something instead of just being a number.
One formula, three arrangements
Voltage equals current times resistance. Rearranged: current is voltage over resistance, resistance is voltage over current. That is the whole thing.
Its use in diagnosis is prediction. A 55 W headlight on 12 V should pull about 4.6 A and measure about 2.6 Ω cold. If your meter says 12 Ω, you are not looking at a good bulb, and you knew that before you pulled it.
Series: current is shared, voltage divides
In series there is one path, so the same current flows through every part. The supply voltage splits across the loads in proportion to their resistance.
This is why a corroded connection in series with a load steals voltage: it is a resistor you did not intend, and it takes its share. It is also why one open component kills the whole string.
Parallel: voltage is shared, current adds
In parallel every branch sees the full supply voltage, and each draws its own current. Total current is the sum, so total resistance falls as you add branches.
This is how vehicle circuits are wired — every lamp gets full voltage and one burned-out bulb does not take the rest with it. It also means adding accessories raises total draw, which is how a circuit that was fine for years starts cooking a relay.
How to test it
- Know the load's ratingWatts and volts are usually printed. Current is watts divided by volts.
- Predict the resistanceVolts divided by amps. Write the number down before you measure.
- Measure and compareA reading far above expected means added resistance — corrosion or a partial open. Far below means a short or a wrong part.
- Check your totalFor parallel loads, add branch currents and confirm the feed wire and fuse are rated for the sum.
Numbers to work to
| Ohm's law | V = I × R | I = V ÷ R, R = V ÷ I |
| Power | P = V × I | watts; how you get current from a bulb's rating |
| Series resistance | R total = R1 + R2 + … | same current everywhere, voltage divides |
| Parallel resistance | always less than the smallest branch | same voltage everywhere, currents add |
| 55 W bulb on 12 V | about 4.6 A, about 2.6 Ω | cold filament reads lower than hot |
Common mistakes
- Measuring a cold filament or heater and calling the resistance wrong — many loads change resistance dramatically when hot.
- Measuring resistance in a live circuit. Ohmmeters supply their own current and read nonsense with power applied.
- Forgetting parallel paths. A meter reads every path between your probes, not just the one you are thinking about.
- Adding accessories to an existing circuit without adding up the current the feed now has to carry.
Drill this until it's automatic
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