What defines the ampere as the unit of electric current, and how is current calculated in amperes?

The ampere (A) is the SI unit of electric current, with two key definitions and calculation methods:  

1. Fundamental Definition   An ampere is formally defined as the constant current that, if maintained in two straight parallel conductors of infinite length, negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to \( 2 \times 10^{-7} \) newtons per meter of length.  

2. Practical Calculation Methods   Ohm’s Law: For most circuit applications, current (\( I \)) is calculated as:     \[ I = \frac{V}{R} \]     where \( V \) = voltage (volts) and \( R \) = resistance (ohms).   Charge Flow Rate: Current can also be defined as the rate of electric charge flow:     \[ I = \frac{Q}{t} \]     where \( Q \) = charge (coulombs, C) and \( t \) = time (seconds, s). For example, 1 A = 1 C/s.  

3. Specialized Contexts   For cable ampacity (current-handling capacity), calculations factor in thermal limits, conductor material, insulation type, and ambient temperature to ensure safe operation—all ultimately quantified in amperes.   These definitions bridge theoretical electromagnetism (via force between conductors) and practical circuit analysis (via Ohm’s Law or charge flow), making the ampere a foundational unit for electrical engineering.