Every electrical circuit, from a phone charger to a factory motor, can be described with four basic quantities: voltage, current, resistance and power. If you understand what each one means and how they relate, wiring diagrams, motor nameplates and fault-finding all become much easier.
This guide explains each quantity in plain language, gives its unit and symbol, and works through simple real-world examples. It is the first article in our Electrical Basics series.
Quick summary table
| Quantity | Symbol | Unit | What it means |
|---|---|---|---|
| Voltage (potential difference) | V (or U) | volt (V) | The electrical "push": energy given to each unit of charge |
| Current | I | ampere (A) | The rate at which charge flows through a conductor |
| Resistance | R | ohm (Ω) | How strongly a material or component opposes current |
| Power | P | watt (W) | The rate at which electrical energy is converted (to heat, light, motion) |
What is electric charge?
Before voltage and current make sense, you need the idea of charge. Electrons carry a tiny negative charge. Charge is measured in coulombs (C). In a metal wire, some electrons are free to move between atoms; when they drift in one direction, charge is flowing and we call that an electric current.
What is current (I)?
Current is the rate of flow of charge. One ampere means one coulomb of charge passing a point every second.
Current only flows when there is a complete (closed) circuit. Open a switch anywhere in a simple series loop and the current stops everywhere in that loop.
Conventional current vs electron flow
Electrons actually move from the negative terminal towards the positive terminal. However, circuit diagrams and engineering formulas use conventional current, which is drawn flowing from + to −. Both describe the same thing; just stay consistent. All OhmForge diagrams use conventional current.
What is voltage (V)?
Voltage (also called potential difference or EMF for a source) is the energy given to each coulomb of charge. One volt means one joule of energy per coulomb.
Voltage is always measured between two points. When someone says "this wire is at 230 V", they mean 230 V relative to another point, usually neutral or earth. A battery, a solar panel or the mains supply provides the voltage that pushes current around a circuit.
The water-pipe analogy (and its limits)
A common comparison: voltage is like water pressure, current is like the flow rate, and resistance is like a narrow section of pipe. The analogy helps beginners, but don't push it too far. For example, a wire at high voltage can be dangerous even when no current is flowing yet, because current will flow the moment you provide a path, such as your body.
What is resistance (R)?
Resistance is the opposition to current. It is measured in ohms (Ω). Conductors such as copper have low resistance; insulators such as PVC have extremely high resistance. Components called resistors are made to have a specific resistance.
The resistance of a wire depends on four things:
- Material (its resistivity, ρ). Copper and aluminium are good conductors.
- Length (L): a longer wire has more resistance.
- Cross-sectional area (A): a thicker wire has less resistance.
- Temperature: for metals, resistance rises as they get hotter.
Worked example: resistance of a copper wire
Annealed copper has a resistivity of about 1.72 × 10⁻⁸ Ω·m at 20 °C. For a 10 m length of 1.5 mm² copper conductor (1.5 mm² = 1.5 × 10⁻⁶ m²):
A circuit uses two conductors (line and neutral), so a 10 m cable run has about 0.23 Ω of conductor resistance in total. That looks tiny, but at 10 A it causes a voltage drop of about 2.3 V. This is why long cable runs need thicker conductors. Real cable selection must follow your local wiring regulations and the cable manufacturer's data, because conductor temperature, installation method and grouping all matter.
What is power (P)?
Power is the rate at which energy is converted, measured in watts (W). One watt is one joule per second. In a DC circuit, or an AC circuit with a purely resistive load such as a kettle element:
Worked example: a 2000 W kettle
| Supply voltage | Current drawn (I = P ÷ V) | Element resistance (R = V² ÷ P) |
|---|---|---|
| 230 V (India, UK, Europe) | 2000 ÷ 230 ≈ 8.7 A | 230² ÷ 2000 ≈ 26.5 Ω |
| 120 V (USA, Canada) | 2000 ÷ 120 ≈ 16.7 A | 120² ÷ 2000 = 7.2 Ω |
The same power at a lower voltage needs more current. More current means thicker cables and higher-rated protective devices, which is one reason supply voltages differ between countries and appliances are made for a specific voltage.
Power vs energy (watts vs kWh)
Power is a rate. Energy is power multiplied by time. Electricity bills charge for energy in kilowatt-hours (kWh), often called "units".
- A 2 kW kettle running for 3 minutes (0.05 h) uses 2 × 0.05 = 0.1 kWh.
- A 1.5 kW heater running for 4 hours uses 1.5 × 4 = 6 kWh.
How each quantity is measured
| Quantity | Instrument | How it is connected |
|---|---|---|
| Voltage | Voltmeter (multimeter on V) | In parallel, across the two points |
| Current | Ammeter, or a clamp meter | Ammeter in series; a clamp meter goes around a single conductor |
| Resistance | Ohmmeter (multimeter on Ω) | Across the component, with the circuit switched off and isolated |
| Power | Power meter / energy meter | Measures voltage and current together |
Common beginner mistakes
- Thinking a "low current" device is safe to touch. Danger depends on the voltage and the path through the body, not on the appliance rating.
- Measuring resistance on a live circuit. This gives wrong readings and can damage the meter.
- Using P = V × I for motors. Motors need power factor and efficiency taken into account.
- Choosing cable size by current alone and ignoring length and voltage drop.
- Mixing up W (power) and Wh or kWh (energy).
Safety: why these basics matter
Mains voltage (for example 230 V or 120 V AC) can drive a lethal current through the human body. That is why installations use protective devices such as fuses, circuit breakers and RCDs; residual current devices for additional protection are commonly rated at 30 mA. Before working on any circuit, switch off, isolate, lock off and prove dead with a suitable voltage tester. If you are not trained and authorised for mains work, use a qualified electrician.
Frequently asked questions
Is it voltage or current that is dangerous?
Current through the body causes the harm, but voltage is what drives that current. A higher voltage can push a dangerous current through skin and body resistance, so both matter. Treat every mains conductor as potentially lethal.
Why do higher-voltage systems use thinner cables for the same power?
Because P = V × I. For the same power, a higher voltage needs less current, and cable size is mainly set by current (heating) and voltage drop.
What is the difference between EMF and voltage?
EMF (electromotive force) is the voltage a source generates internally. The voltage you measure at its terminals under load is usually slightly lower because of the source's internal resistance.
Does a thicker wire always mean less resistance?
For the same material, length and temperature, yes. Resistance falls as the cross-sectional area increases.
What to read next
- Ohm's Law explained with examples: how V, I and R are linked by one simple equation.
- AC vs DC: what's the difference?
- Types of electric motors
References
- BIPM, The International System of Units (SI Brochure), for the definitions of the ampere, volt, ohm and watt.
- IEC 60050 (International Electrotechnical Vocabulary), available at electropedia.org.
Technical note: values in worked examples are rounded and for learning only. Always follow equipment ratings, manufacturer instructions and your local electrical code.
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