Ampere (A)

The ampere (symbol: A) is the SI base unit of electric current, measuring the flow of electric charge through a conductor. One ampere is defined as the flow of one coulomb of electric charge per second.

Since the 2019 revision of the SI system, the ampere is no longer defined by electromagnetic forces. Instead, it is based on the exact charge of a single electron, which is fixed at 1.602176634 × 10⁻¹⁹ coulombs. This provides greater precision and universality in electrical measurements.

Mathematical Definition

In electrical circuits, Ohm’s Law expresses the relationship between current, voltage, and resistance:

I = V / R

Where:

• I = Current in amperes (A)

• V = Voltage in volts (V)

• R = Resistance in ohms (Ω)

Measurement and Practical Applications

Current is measured using an ammeter and is a key factor in electrical engineering.

Examples:

• A standard LED light bulb draws about 0.08 A at 120 V.

• A typical toaster draws about 8 A.

• Modern smartphones charge at 1–3 A, depending on charging technology.

• Industrial electric motors can require hundreds of amperes.

In AC (alternating current) systems, the current fluctuates periodically, whereas in DC (direct current) systems, such as batteries, the current flows steadily in one direction.

Electrical Safety Considerations

Excessive current can cause overheating and electrical fires. This is why circuit breakers and fuses are rated in amperes. Household protection devices are typically set between 10 and 16 A.