Voltage, Current and Resistance

Voltage, current and resistance are the three basic quantities used to describe an electric circuit. Voltage is the electrical push that moves charge. Current is the rate at which charge flows. Resistance is how strongly a part opposes that flow. They are measured in volts, amperes and ohms, and Ohm’s law connects them in one short equation.

Companion article to Episode 1 of Electronics Fundamentals · audio, about 15 minutes · no background needed

The water picture

Electric charge cannot be seen, so this series describes it with something that can: water in a hose. A tank sits at the top of a hill. A hose runs from the bottom of the tank, down the slope, to a garden. When the valve opens, water flows.

In a circuit, a battery plays the part of the tank, wires play the part of the hose, and electric charge plays the part of the water. Three questions can be asked about that water: how hard it is pushed, how much of it moves, and how much the hose holds it back. Each question has a name and a unit.

In the water picture	In a circuit	Quantity
The height of the hill	The push the battery gives the charge	Voltage
How much water passes a point each second	How much charge passes a point each second	Current
A narrow or kinked section of hose	A part that holds back the flow, such as a resistor	Resistance
The tank	The battery, which supplies the push	Source
The bottom of the hill, where height is zero	Ground, the point everything is measured from	Reference

Voltage

Voltage is the electrical push that makes charge move. It is measured in volts (V). Technically it is a potential difference: one volt is one joule of energy per coulomb of charge. In the water picture it is the height of the hill. A tall hill gives a hard push, and a small hill gives a gentle one.

Voltage is always between two points

Height only matters because there is a top and a bottom. Water at the top is pushed toward the bottom, and with no lower place to go there would be no push at all. A battery works the same way. It has two ends, and its voltage is the difference between them. That is why voltage is described as across something: across a battery, or across a part in a circuit.

Ground

Ground is the point in a circuit that is agreed to be zero volts. It is like sea level: a mountain is “two thousand metres tall” only because heights are measured from the same agreed zero. In a battery circuit, ground is often the battery’s negative terminal. Every other voltage in the circuit is then read as a height above ground.

Typical voltages

Current

Current is the rate at which electric charge flows past a point. It is measured in amperes, called amps for short (A). One amp is one coulomb of charge passing each second. In the water picture it is how much water moves through the hose each second, whether a trickle or a rush. The symbol is I, from the French intensité du courant.

Small currents and prefixes

Most small electronic parts use far less than one amp, so the milliamp (mA) is used constantly. The prefix milli means one thousandth, so 1,000 mA make 1 A. A few prefixes cover almost everything in electronics:

Prefix	Symbol	Means	Example
micro	µ	one millionth (0.000001)	250 µA = 0.00025 A
milli	m	one thousandth (0.001)	15 mA = 0.015 A
kilo	k	one thousand (1,000)	4.7 kΩ = 4,700 Ω
mega	M	one million (1,000,000)	2.2 MΩ = 2,200,000 Ω

Typical currents

Across and through

Voltage and current are different things. A tall hill with a very thin hose gives a hard push but a small flow. A wide hose on a gentle slope gives a big flow from a modest push. A simple way to keep them apart: voltage is measured across two points, and current flows through a path. For current to flow at all, the path must form a complete loop from one end of the battery back to the other.

Resistance

Resistance is how strongly a part opposes the flow of current. It is measured in ohms (Ω), and its symbol is R. In the water picture it is how narrow the hose is. The same push through a thin or kinked hose delivers much less flow. More ohms means less current, and fewer ohms means more current.

Resistors

A resistor is a part made to add a chosen amount of resistance. It is like a deliberately narrow stretch of hose. It does not make charge and it does not use charge up. It limits the flow. In a real resistor the energy that is held back turns into heat, which is why every resistor has a power rating that says how much heat it can safely handle. Large values are written with the prefixes above, so a ten-kilo-ohm resistor (10 kΩ) is 10,000 ohms.

Conductors and insulators

Material	Resistance to current	Role in a circuit
Copper and other metals	Very low	Wires, which carry current with almost no hold-back
Resistor material	Chosen on purpose, from a few ohms to millions	Limits current to a planned value
Plastic insulation	So high that practically no current gets through at low voltage	Coating on wires, keeping current in the metal

Even a plain wire has a tiny resistance. It is so small that circuit diagrams usually ignore it.

Ohm’s law

The three quantities are not independent. Ohm’s law, published by the German physicist Georg Ohm in 1827, states that the voltage across a resistor equals the current through it times its resistance.

The pattern to remember is that more resistance gives less current, less resistance gives more current, and more voltage gives more current. The explorer below draws that pattern. Move either slider and watch the current change.

Worked examples

Each example follows the same habit: write the formula, convert to base units, substitute, then convert the answer to a convenient unit.

Examples A to C show the pattern: doubling the resistance halves the current, and halving it doubles the current, exactly like making the hose twice as narrow or twice as wide.

Why it matters: limiting current

Almost every simple circuit leans on Ohm’s law for one job: setting a safe current. An LED, a small light, does not limit its own current. Connected straight to a battery, it can draw far more current than it can handle. It can burn out quickly or be damaged. A resistor placed in the same loop limits the flow, and Ohm’s law says which resistor to pick.

Common mix-ups

Limits of the water picture

The tank-and-hose picture is a tool for thinking, not the real physics. It works well for voltage, current and resistance in simple battery circuits. It starts to strain in a few places:

• Water can spill from a hose. Charge in a closed loop has nowhere to spill.
• Parts such as capacitors need a refined picture, which the series builds when it reaches them.
• Alternating current, where the push keeps reversing, does not match a tank on a hill.

Whenever the picture stops working, the series says so and adjusts it.

Safety

Glossary

Term	Meaning
Across	How voltage is measured: between two points, with one probe on each.
Ampere (amp, A)	The unit of current. One amp is one coulomb of charge passing a point each second.
Charge	The property of matter that makes electric effects happen. In the water picture, it is the water itself.
Circuit	A complete loop of conducting parts that current can flow around.
Conventional current	The convention of drawing current from the positive terminal, around the loop, to the negative terminal.
Current	The rate of flow of charge. Symbol I. Measured in amps.
Ground	The reference point in a circuit, agreed to be zero volts. Other voltages are measured from it.
Kilo- (k)	A prefix meaning one thousand. 1 kΩ is 1,000 Ω.
Milli- (m)	A prefix meaning one thousandth. 1 mA is 0.001 A.
Ohm (Ω)	The unit of resistance. One ohm lets one amp flow when one volt is across it.
Ohm’s law	V = I × R. The relationship between voltage, current and resistance in a resistor or wire.
Resistance	How strongly a part opposes current. Symbol R. Measured in ohms.
Resistor	A part made to provide a chosen amount of resistance, used to limit current.
Through	How current is described: it flows through a wire or a part.
Volt (V)	The unit of voltage. One volt is one joule of energy per coulomb of charge.
Voltage	The electrical push between two points. Symbol V. Measured in volts.

Quick reference

Quantity	Symbol	Unit	Water picture	Measured
Voltage	V	volt (V)	Height of the hill	Across two points
Current	I	ampere (A)	Water flowing per second	Through a wire or part
Resistance	R	ohm (Ω)	Narrowness of the hose	On a part, power off

To find	Formula	Rule of thumb
Voltage	V = I × R	More current or more resistance means more voltage across the part.
Current	I = V ÷ R	More resistance, less current. More voltage, more current.
Resistance	R = V ÷ I	For a given voltage, a smaller current means a larger resistance.

Unit habit: milli means divide by 1,000 and kilo means multiply by 1,000. Convert to base units (volts, amps, ohms) before calculating.

Try it

Read a label. Find a phone charger or a battery and read the numbers printed on it. You will usually find a voltage and a current. On a charger, the printed current is the most it can supply, not what is flowing right now. Say out loud what each number means, using the tank, the hose and the hill.

Practice problem 1. 5 V is across a 1,000 Ω resistor. What current flows?

Show the answer

I = 5 ÷ 1,000 = 0.005 A = 5 mA

Practice problem 2. A current of 20 mA flows through a 330 Ω resistor. What is the voltage across it?

Show the answer

V = 0.02 × 330 = 6.6 V

Practice problem 3. A part has 9 V across it and passes 15 mA. What is its resistance?

Show the answer

R = 9 ÷ 0.015 = 600 Ω

Written with AI assistance and narrated with an AI-generated voice.