7.5.7

# Alternating Currents

Test yourself

## Alternating Current

Just like voltage, current can alternate (i.e. switch directions).

### Alternating current

• Current flow can change over time just like voltage.
• Alternating current goes like a sine wave.
• This means that current oscillates from one positive value to the same negative value.

### Measurement

• The peak to peak value is double the maximum positive value.
• The current has a root mean squared value that is calculated as:
• ${I_{rms}}=I/\sqrt{2}$

## Oscilloscopes

The Mains supply is alternating current (a.c). An oscilloscope is a convenient way of measuring the amplitude and period of a.c. supplies.

### Screen

• The screen on an oscilloscope displays the p.d. across the input terminals as a function of time (i.e. a sine wave for mains a.c).

### Time-base

• This is the control to change the x-axis scale of the waveform.
• The setting changes the amount of time per division across the screen.
• A time base of 10 ms per division displaying 50 Hz mains would record a complete cycle of the waveform every 2 divisions across the time-axis. (Since the period of the mains p.d. is 1/50 = 0.020 s.)

### Y-gain or y-scale

• This sets how much p.d. is displayed for each vertical division.
• A y-gain of 5V per division would record a d.c p.d. of +20V as a horizontal line that was 4 divisions above the time axis of the display.

### Example

• In this oscilloscope trace, the period of the wave is 4 squares across.
• The time base setting is 5 ms per division, which means that the period of the wave is 5 × 4 = 20 ms.

### Example cont.

• The peak-to-peak voltage is 6 squares. This is the vertical measurement from the top of the wave to the bottom of the wave.
• The y-gain setting is 100 V per division, so the peak to peak voltage is 600 V.
• The peak voltage is the vertical measurement from the 0V line to the peak. In this case, the peak voltage is 300 V.