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Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?
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For a Direct Current (DC), one ampere' is equal to one coulomb charge flowing per sec.
l ampere $=1$ coulomb $/ \mathrm{sec}$
So, an AC current changes direction with the source frequency and the attractive force would average to zero. So that the AC ampere must be defined in terms of some property that is independent of the direction of current. Joule's heating effect is such property and hence it is used to define rms value of AC. According to joule's heating effect, one ampere current in $\mathrm{AC}$ is the current which can produce same quantity of heat per second as the direct current can produce in one $\mathrm{ohm}$ resistance.
l ampere $=1$ coulomb $/ \mathrm{sec}$
So, an AC current changes direction with the source frequency and the attractive force would average to zero. So that the AC ampere must be defined in terms of some property that is independent of the direction of current. Joule's heating effect is such property and hence it is used to define rms value of AC. According to joule's heating effect, one ampere current in $\mathrm{AC}$ is the current which can produce same quantity of heat per second as the direct current can produce in one $\mathrm{ohm}$ resistance.
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