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In certain circuits when current reaches its maximum value after emf becomes maximum then current is said to lag behind emf
When current reaches its maximum value before emf reaches its maximum then current is said to lead the emf
Figure below shows the current lagging behind the emf by 90 0
A . C through pure resistor
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Figure below shows the circuit containing alternating voltage source V = V 0 sinω connected to a resistor of resistance R
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Let at any instant of time , i is the current in the circuit , then from Kirchhoff ’ s loop rule
V 0 sinωtRi or i =( V 0 / R ) sinωt = i 0 sinωt ---- ( 8 )
Where ,