Physics, visually · Electricity & Magnetism · Concept 11
Ohm's Law and Circuits
Why does one dead bulb put out a whole string of old fairy lights, while in your house the other lights stay on?
Voltage pushes, resistance resists, current is the result. Then see how connecting parts in series or parallel changes everything.
- Ohm tab. Keep V at 6 V and double the resistance. The current halves. Now raise V instead.
- Circuits tab, series. Add the two resistances. Both bulbs get the same current and glow dimmer than one alone.
- Circuits tab, parallel. Both bulbs get the full battery voltage. Switch to parallel and see total current jump.
V is the push, R is the squeeze, I is the flow: V = I R.
Ohm's law: current I = V / R. Series is one path, so the same current everywhere and resistances add. Parallel has many paths, so the same voltage across each and the currents add.
Wiring a control panel
24 V DC sensors are wired in parallel from the supply: each gets the full 24 V, and one failure does not stop the others.
Safety chains and e-stops
Emergency stop contacts are wired in series on purpose: any one opening breaks the whole circuit and stops the machine.
Cable sizing
A long thin cable has resistance. Voltage drop = I × R, so a motor far from the supply may get too little voltage. Thicker cable, smaller R.