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Convert between charge units and calculate charge from current and time. Essential for battery capacity and electronics applications.
Formula: Q (C) = I (A) × t (s)
The coulomb (C) is the SI unit of electric charge, named after French physicist Charles-Augustin de Coulomb. One coulomb represents the amount of charge transported by a constant current of one ampere in one second. Alternatively, one coulomb equals approximately 6.242 × 10¹⁸ elementary charges (electrons or protons). Electric charge is fundamental to all electrical phenomena, from static electricity to battery operation.
Ampere-hours (Ah) and milliampere-hours (mAh) are practical units for battery capacity. One ampere-hour represents the charge transferred by one ampere of current flowing for one hour, equal to 3,600 coulombs. Battery capacities are typically rated in mAh (for small batteries like phone batteries) or Ah (for larger batteries like car batteries). A 2000 mAh battery can theoretically deliver 2 A for 1 hour, or 1 A for 2 hours.
The fundamental relationship is Q = I × t:
Example: A 3000 mAh battery stores 3000 × 3.6 = 10,800 C of charge
One ampere-hour equals 3,600 coulombs. This comes from the definition: 1 A flowing for 1 hour (3,600 seconds) transfers 1 A × 3,600 s = 3,600 C. Similarly, 1 mAh = 3.6 C.
A 3000 mAh battery can theoretically deliver 3000 mA (3 A) for 1 hour, or 300 mA for 10 hours, or any equivalent combination. In coulombs, this is 3000 mAh × 3.6 = 10,800 C of stored charge. Actual runtime depends on the device's current draw and battery efficiency.
Use the formula Q = I × t, where Q is charge in coulombs, I is current in amperes, and t is time in seconds. For example, if 2 amperes flows for 5 seconds, the charge transferred is 2 × 5 = 10 coulombs. Make sure all units are consistent (use A and s, not mA and minutes).
One electron carries a charge of approximately 1.602 × 10⁻¹⁹ coulombs (also called the elementary charge, e). This means one coulomb represents about 6.242 × 10¹⁸ electrons. This fundamental constant is crucial in atomic physics and chemistry calculations.
Yes, electric charge can be positive or negative. Electrons carry negative charge (-e), while protons carry positive charge (+e). In circuit analysis, the sign indicates direction of charge flow. By convention, current flows from positive to negative, though electrons actually move in the opposite direction.
Battery capacity is typically measured in ampere-hours (Ah) or milliampere-hours (mAh) at a specified voltage. Manufacturers test batteries by discharging them at a constant current until the voltage drops to a cutoff level. The product of current and time gives the capacity. Energy capacity (Wh) is found by multiplying charge capacity (Ah) by voltage (V).