Electric current is the rate of flow of electric charge through a surface or conductor, I = dQ/dt, measured in amperes. One ampere is one coulomb of charge passing a point each second.
Learning objectives
- Define electric current and state the meaning of one ampere.
- Calculate current from charge and time.
- Explain the difference between conventional current and electron flow.
- Measure current correctly in series with a circuit.
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
For current to flow, there must be a complete path. Current does not get used up as it goes around a series circuit; the same current flows through every element in the loop. What gets used up is energy, which the load converts into heat, light or motion. In a parallel circuit, the total current splits between the branches.
Intermediate How it behaves in a circuit
Conventional current is drawn from the positive terminal through the external circuit to the negative terminal. In metals, electrons drift in the opposite direction. In electrolytes and plasmas, both positive and negative charge carriers can move. The direction convention matters for signs in Kirchhoff’s laws and for interpreting magnetic effects, but it does not change the magnitude of the current.
Advanced Limits, assumptions, engineering detail
Alternating current reverses periodically, so its instantaneous value changes sign; the RMS value is used for power calculations. A current-carrying conductor also creates a magnetic field, and changing current produces a changing field, which is the basis of inductance and transformer action. Current transformers allow large alternating currents to be measured safely by producing a proportional smaller current.
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Electric current | I | ampere (A) |
| Charge | Q | coulomb (C) |
| Current density | J | ampere per square metre (A/m²) |
| Time | t | second (s) |
Worked examples
Current from charge and time
Problem. A total charge of 240 C passes through a conductor in 4.0 minutes. What is the average current?
Solution. t = 4.0 × 60 = 240 s. I = Q / t = 240 C / 240 s = 1.0 A.
Current and current density
Problem. A 12 V supply drives 2.0 A through a 6 Ω load and a 1.5 mm² conductor. Find the current density in the conductor.
Solution. I = V/R = 12/6 = 2.0 A. Area = 1.5 mm² = 1.5 × 10⁻⁶ m². J = I/A = 2.0 / 1.5 × 10⁻⁶ ≈ 1.33 × 10⁶ A/m².
Practical applications
- Fuse and cable sizing
- Battery discharge and charging calculations
- Motor and load current checks
- Current sensing in control systems
Automotive applications
Automotive current loads range from a few milliamperes for a module in sleep mode to hundreds of amperes for the starter. Parasitic current draw is measured in series with the battery, or with a clamp meter around the negative cable, after all modules have gone to sleep. Because of the wide range, a clamp meter with a low-current range is more useful than a single fixed range.
Industrial applications
Industrial motors, contactors and busbars are rated by current. Protective devices are selected so that the normal load current is carried without tripping, while fault current is interrupted safely. Current transformers scale large alternating currents down to a measurable level and also isolate the measuring instrument from the power circuit.
Measurement procedure
- Break the circuit and connect the ammeter in series, observing the correct polarity for DC.
- Use a clamp meter for convenience, but clamp exactly one conductor; clamping the whole cable reads the net current.
- Select the highest range when the current is unknown, then reduce it for better resolution.
- Use a meter rated for the voltage of the circuit and for the expected fault current.
Common mistakes
- Believing current is used up in a series circuit: the same current flows through every element.
- Connecting an ammeter in parallel with a source, which creates a short circuit through the meter.
- Ignoring the direction convention when applying Kirchhoff’s laws.
- Assuming an AC clamp meter reads DC accurately; many clamp meters measure only AC unless explicitly rated for DC.
Safety considerations
- Never connect an ammeter across a voltage source; the low meter resistance can cause a violent short circuit.
- Use fused test leads and the correct meter range for the circuit being measured.
- High fault currents can cause arc flash and burns; work de-energised where practicable.