Water to Wires: Voltage, Current and Resistance

Every PLC lesson in this course uses three ideas: voltage, current and resistance. You cannot see electricity, but you can see water. So we learn with water first, then swap the water for electricity. Every step has a simulator: drag the sliders and watch what changes. All numbers on this page are teaching values.

+ WORDS USED ON THIS PAGE (OPEN THIS IF A WORD IS NEW)

voltage (V), unit: volt: The electrical push. Like water pressure.
current (I), unit: amp (A): How much electric charge flows past a point every second. Like the amount of water leaving a pipe every second.
resistance (R), unit: ohm (Ω): How much a part gets in the way of the flow. Like a narrow pipe.
circuit: A closed loop that the current can flow round: source, wires and a part that uses the energy.
PLC: Programmable logic controller: the industrial computer this series teaches.
conventional current: The direction engineers draw current: out of the + side of the battery, round the loop, back into the − side.

PART 1 The water tank and the battery

A battery and a tank on a stand look nothing alike, but they do the same job: they push something along a path.

01 The higher the tank, the bigger the push

Water in a tall tank is pushed out harder than water in a short tank. A battery with more volts pushes electric charge harder. Move the slider or tap a real-life voltage.

Try this: change the battery voltage and watch the stand under the tank grow. Height of the water is like the voltage.

Where you meet it	Voltage
AA battery	1.5 V
USB port	5 V
9 V battery	9 V
PLC inputs and outputs (most common)	24 V DC
Wall socket in India	230 V AC, dangerous. Never touch.

Key idea. Voltage (V) is the push. More volts means more push, like a taller tank.

02 Build the analogy step by step

Press Next to add one piece at a time. Back to return.

Key idea. Voltage is the height of the water. Resistance is the pipe. Current is the water flowing out. A wider pipe means low resistance and more flow, a narrow pipe means high resistance and less flow.

03 The translation table

The left side is a water tank, the right side is a circuit. The same two sliders control both. Tap a button to light up the matching parts.

Try this: make the pipe narrower (higher resistance) and see the water and the current both drop.

Key idea. The dots are charge flowing round the loop, drawn the conventional way: out of the + side, through the resistor, back to the − side. More current means faster dots.

PART 2 Two tanks, one race

Now we compare two tanks. They have the same height and the same plunger. Only the width of the output pipe is different: one is wide, one is narrow.

04 Which tank gives more water per second?

Press down with the same push on both. The narrow pipe resists the water more, so less water leaves every second. To get the same flow through the narrow pipe, you must push harder.

Try this: keep the push the same and make pipe B narrower. Then raise the push and see how much more is needed.

Key idea. More resistance means less flow for the same push. Less resistance means more flow.

PART 3 Ohm’s law

Voltage, current and resistance are linked by one short rule. Change one and the others change.

05 V = I × R, solve for any one

Try this: choose what to find. The greyed slider is the answer. Try the PLC preset: a 24 V output driving a 480 Ω relay coil.

Key idea. I = V ÷ R, V = I × R and R = V ÷ I are the same rule. Cover the letter you want in the triangle. A PLC engineer uses this to check how much current a load will draw from an output.

06 See how they are connected

Try this: raise R and watch the current fall along a curve. Raise V and the current climbs in a straight line.

Key idea. More resistance, less current. More voltage, more current. Higher resistance needs higher voltage for the same current.

07 Summary and quiz

Key takeaways

Voltage (V, volts) is the push: the height of the water.
Resistance (R, ohms Ω) is the pipe getting in the way.
Current (I, amps A) is the amount that flows every second.
V = I × R. Wide pipe: low R, high I. Narrow pipe: high R, low I.
Same voltage, double the resistance: half the current.
A PLC’s 24 V output across a 480 Ω coil carries 24 ÷ 480 = 0.05 A (50 mA).