In an AC voltage changes polarity always from positive voltage to negative voltage and back once more over time and conjointly whose current with relevance the voltage oscillate back and forward. The periodical form of an AC source trails that of the scientific sort of a “sine wave” that is often referred to as a sinusoidal waveform. Thus, a sinusoidal voltage will be outlined as V(t) = V

_{max}sin Ï‰t.
While using pure resistances in AC circuits that have tiny values of capacitance or inductance, a similar principals of Ohm’s Law, circuit rules for power, voltage and current apply as they are doing for DC resistive circuit the sole distinction now is within the use of the instant “peak-to-peak” or “rms” quantities.

When operating with AC alternating voltage and current it's normal to use solely “rms” value to evade confusion. Conjointly the representation symbol used for outlining an AC voltage supply is that of a “wavy” line as critical a battery representation for DC which is shown below.

Resistors are “passive” devices, that's they are doing not turn out or consume any electric energy, however convert electric energy into high temperature. In DC circuit the linear magnitude relation of voltage to current in an exceedingly resistance is termed its resistor. But, in AC circuit this magnitude relation of voltage to current relied on the frequency and phase angle (Ï†) of the availability. Thus once use of resistor in AC circuit the term impedance, symbol Z is that the typically used and that we will say that AC impedance = DC resistance, R = Z.

It is vital to notice, that once employed in AC circuits, a resistance can invariably have a similar resistive value notwithstanding what the availability frequency from DC to terribly high frequencies, not like capacitors and inductors.

For resistance in AC circuit the movement of current passing through them has no impact on the behavior of the resistor thus can rise and fall because the voltage increases and drops. The voltage and current reach most, drop through zero and touch minimum at precisely the same time. i.e., they increase and drop at the same time and are same to be “in-phase” as shown below.

We can see that at any point on the horizontal axis that the instant voltage and current are in-phase as a result of this and therefore the voltage reach their most values at a similar time, that's their phase Î¸ is 0

^{o}. Then these instant values of voltage and current will be compared to relinquish the resistance unit value of the resistance just by using ohms law. Contemplate below the circuit consisting of an AC supply and a resistance.
The rapid (instantaneous) voltage across the resistance, V

_{R}is equivalent to the supply voltage, V_{t}and is given as:
The instant current flowing in the resistors will therefore be:

I

_{R}= V_{R}/R
I

_{R }= (V_{max}Sin Ï‰t)/R
I

_{R }= I_{max}Sin Ï‰t
For instance the voltage across a resistance is given as V

_{R}= I.R, the rapid voltage across the resistor above can be given as:
V

_{R}=I_{max}Sin Ï‰t
In strictly resistive series AC circuit, all the voltage falls across the resistor will be intercalary along to search out the whole circuit voltage in place of all the voltages are in-phase with one another. Likewise, in an exceedingly strictly resistive parallel AC circuit, all the specific branch currents will be intercalary along to search out the full circuit current as a result of all the branch currents are in-phase with one another.

Since for resistor in AC circuit the phase Ï† among the voltage and therefore the current is zero, then the PF issue of the circuit is assumed as cos0

^{o}=1. The facility within the circuit at any instant in time will be found with the product of the voltage and current at that instant.
The power (P), used by the electric circuit is given as P=V

_{rms}Î™cosÎ¦ in watt’s. However since cosÎ¦=1 in an exceedingly strictly resistive circuit, the power consumed is just given as, P=V_{rms}Î™ a similar as for Ohm’s Law.
This then provides us the “Power” undulation and that is shown below as a series of positive pulses as a result of once the voltage and current are each in their positive half of the cycles the resulting power is positive. Once the voltage and current are each negative, by multiplying the 2 negative values provides a positive power pulse.