What is Direct Current ?

AC can come in a number of forms, as long as the voltage and current are alternating

Waveforms
AC can come in a number of forms, as long as the voltage and current are alternating.

. If we hook up an oscilloscope to a circuit with AC and plot its voltage over time, we might see a number of different waveforms

Direct current (DC) and alternating current (AC) are the are two types of electric current

Electric current can be either direct or alternating. Direct current (DC) flows in the same direction at all points in time

If we hook up an oscilloscope to a circuit with AC and plot its voltage over time, we might see a number of different waveforms.

The most common type of AC is the sine wave. The AC in most homes and offices have an oscillating voltage that produces a sine wave.

Triangle waves are found in sound synthesis and are useful for testing linear electronics like amplifiers.

The most common type of AC is the sine wave. The AC in most homes and offices have an oscillating voltage that produces a sine wave.

Direct current is produced by electrochemical and photovoltaic cells and batteries.

We often want to describe an AC waveform in mathematical terms. For this example, we will use the common sine wave. There are three parts to a sine wave: amplitude, frequency, and phase.

To generate AC in a set of water pipes, we connect a mechanical crank to a piston that moves water in the pipes back and forth (our “alternating” current).

V(t) is our voltage as a function of time, which means that our voltage changes as time changes. The equation to the right of the equals sign describes how the voltage changes over time.

In contrast, the electricity available from utility mains in most

VP is the amplitude. This describes the maximum voltage that our sine wave can reach in either direction, meaning that our voltage can be +VP volts, -VP volts, or somewhere in between.

Notice that the pinched section of pipe still provides resistance to the flow of water regardless of the direction of flow.

The sin() function indicates that our voltage will be in the form of a periodic sine wave, which is a smooth oscillation around 0V.

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2π is a constant that converts the freqency from cycles (in hertz) to angular frequnecy (radians per second).

f describes the frequency of the sine wave. This is given in the form of hertz or units per second. The frequency tells how many times a particular wave form (in this case, one cycle of our sine wave - a rise and a fall) occurs within one second.

t is our independent variable: time (measured in seconds). As time varies, our waveform varies.

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φ describes the phase of the sine wave. Phase is a measure of how shifted the waveform is with respect to time. It is often given as a number between 0 and 360 and measured in degrees. Because of the periodic nature of the sine wave, if the wave form is shifted by 360° it becomes the same waveform again, as if it was shifted by 0°. For simplicity, we sill assume that phase is 0° for the rest of this tutorial.

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We can turn to our trusty outlet for a good example of how an AC waveform works. In the United States, the power provided to our homes is AC with about 170V zero-to-peak (amplitude) and 60Hz (frequency). We can plug these numbers into our formula to get the equation (remember that we are assuming our phase is 0):