Difference between Alternating Current and Direct Current

So far we have kept limited our discussion to Direct Current (DC) circuits. In contrast to the DC circuit, Alternating Current (AC) circuit have elements that vary its magnitude and direction with time. In the case of the Direct current circuit, the direction and magnitude of current and voltage in the circuit don’t change with time except for transients in inductors and capacitors. The difference between alternating current and direct current can be highlighted with the following figure. It shows that the direction and magnitude of Direct Current are constant while they are changing for alternating current around the zero points with respect to time.

Difference between Alternating Current and Direct Current
the direction of current in an ac circuit
Difference between AC (left) and DC (right) waveform

The above graph shows that DC can be represented by only magnitude and direction while these terms are insufficient for AC. To introduce the concept of Alternating Current completely, we have to define some other terms.

The following animation shows the behavior of AC through the resistor, assuming that the magnitude is changed according to the waveform.

the direction of current in an ac circuit
AC change direction two times per cycle

Comparison of Alternating Current and Direct Current:

Waveform:

Comparing the alternating current and direct current, the trace of the quantity i.e. voltage or current against time, degrees, radian, etc. is called a waveform.

Instantaneous value:

The magnitude of any quantity like voltage or current against the specific value of time, degrees, radian, etc. is called instantaneous value.

Peak Amplitude:

The maximum value of the AC quantity like voltage and current reaches the waveform.

Peak-Peak value:

The difference between the maximum and minimum value of the waveform is called peak to peak value of an AC quantity.

Periodic waveform:

If a portion of a waveform repeats itself after a specific time, degree, radian, etc. The waveform is called a periodic waveform. And that portion of the waveform is called a cycle. It’s Alternating current quantity.

Time Period:

The time required for the waveform to complete itself is called the Time period. It is denoted by T in seconds. It denotes alternating current phenomenon.

Cycle:

The waveform of one Time Period is called a cycle.

Frequency:

In the comparison of alternating current and direct current, the direct current has zero frequency. The number of cycles in one Time Period is called frequency. Frequency is represented in cycles per second or Hertz (name after scientist).

$f=\frac{1}{T} $ (in Hertz)  OR

$\frac{cycle}{second}$

Sinewave:

The waveform of AC voltage is most of the time sinusoidal (sine wave or cosine wave) unless it is stated. It is mainly that resistance, capacitance, or inductance responses do not affect the waveform. The sinusoidal waves can be plotted against degrees or radians. Where degrees and radian has the following relationship

$360 \quad deg=2\pi \quad radians$

Where $\pi $ is the ratio between the circumference of the circle and the diameter of the circle and is equal to 22/7.

Angular Velocity:

If a radius vector of a circle rotates around its center at a constant speed, the vertical projection of the vector can be plotted as a sine wave. The speed at which the vector rotates is called angular velocity.

$Angular Velocity=\frac{Distance }{Time }$

$\omega =\frac{\alpha }{t}=\frac{2\pi }{T}=2\pi f$

The sine wave for voltage can be represented by the following formula.

$e=E_{m} Sin \omega t$

Where $E_{m}$ shows the peak voltage of the voltage waveform and $\omega $ is the angular velocity of the waveform.

Average Value of a waveform:

The average value of the sine wave is the ratio of the area under the sine wave to the length of the sinusoidal. Looking at the waveform, it is clear that for the first half it is positive and for the second half its is negative and both are the same in magnitude for each complete cycle. So the average of AC sinusoidal is zero. The formula for average calculation is

$V_{avg}=\frac{\int_{0}^{T}{V_{m} sin\alpha d\alpha }}{T-T_{0}}$

Where T-T$_{0}$ is the time period of the AC sine wave and $V_{m}sin \alpha $ is the AC signal.

Root-Mean-Square (RMS) Value:

The RMS value of the AC signal provides a relationship between the AC and DC. The RMS value of the AC sinusoidal gives the information of delivering the same power by the corresponding DC source. The RMS value of the AC signal is used generally unless it is stated. The formula for the RMS value of an AC signal is

$V_{rms}=\frac{V_{m}}{sqrt{2}}$

Where $V_{m}$ is the peak voltage of the AC sinusoidal. The relationship holds for current too.

Sinusoidal voltage and current format:

The basic mathematical format for sinusoidal voltage is

$v=V_{m}sin \alpha $

Where

$\alpha =\omega t$

We get

$v=V_{m}sin \omega t$

Where V$_{m}$ is the peak voltage of the waveform, ω is the angular velocity of the waveform and t is the time duration from the rotation beginning. The ω, angular velocity represents the position of the instantaneous value at the waveform where the time t, determine the numbers of rotations.

Remember that the lowercase letter indicates the instantaneous value where the uppercase letter with subscripts of ‘m’ means the peak value i.e. v and V­m.

Phase relation:

In simple words, the phase shows the relationship among multiple waveforms, how soon and late each one of them started, achieved the peak, and/or reach its zero and final value. Suppose we have the first waveform of $I_{m}sin (\alpha +0)$ and the second waveform of $V_{m}sin (\alpha +30^{o })$, their waveforms could be shown as follow.

phase difference in alternating current

The first waveform will start from the zero points at axes where the second will start 30o later.

The general form of the phase relation equation is

$v=V_{m}sin \alpha \pm \theta $

The phase relation of two or more waveforms can be found through an analog and digital oscilloscope. Where the magnitude of AC quantiles is measured in RMS value through digital multi-meter and voltmeter and ammeter. A special instrument for the measurement of frequency meters is used for the measurement of frequency.

Conclusion :

The difference between the alternating current and direct current is the changing direction and magnitude of the current multiple times in a second. Our home appliances are using AC where electronic devices like handheld devices are using DC power.   

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