Alternating currents (A-level only) Flashcards – AQA A-level Physics (7408)

What makes a voltage or current alternating?

It changes direction periodically.

How does a sinusoidal waveform vary about zero?

It follows a sine wave between equal positive and negative extremes.

What is the period $T$ of a waveform?

The time for one complete cycle.

How is frequency $f$ related to period $T$?

$f = \frac{1}{T}$

What is the peak value of a voltage or current, and how is it denoted?

The maximum magnitude measured from zero: $V_0$ for voltage or $I_0$ for current.

What is the peak-to-peak value of a sinusoidal voltage or current?

The difference between its positive and negative extremes: $2V_0$ for voltage or $2I_0$ for current.

What is the RMS value of an alternating voltage or current?

The steady DC voltage or current that produces the same average power in the same resistor.

For a sinusoidal waveform, how do RMS current and voltage relate to their peak values?

$I_{\mathrm{rms}}=\frac{I_0}{\sqrt{2}},\qquad V_{\mathrm{rms}}=\frac{V_0}{\sqrt{2}}$

How is the peak-to-peak voltage of a sinusoidal waveform related to its peak voltage $V_0$?

It is $2V_0$.

What quantities are displayed on the vertical and horizontal axes of an oscilloscope?

Vertical: voltage; horizontal: time.

What does an oscilloscope’s volts-per-division control set?

The voltage represented by each vertical division.

What does an oscilloscope’s time-base control set?

The time represented by each horizontal division.

How do position controls and a zero-voltage reference help with oscilloscope measurements?

They allow displacements to be measured from the appropriate reference level.

How do the oscilloscope traces of steady DC and AC voltages differ?

Steady DC produces a horizontal trace displaced from zero; AC produces a time-varying trace.

How is a steady DC voltage calculated from its oscilloscope trace?

Multiply its vertical displacement from zero, in divisions, by the volts-per-division setting.

How are peak and peak-to-peak voltages found from an AC oscilloscope trace?

Multiply each corresponding vertical distance, in divisions, by the volts-per-division setting.

How do DC and AC coupling differ on an oscilloscope?

DC coupling displays the voltage including any steady offset; AC coupling suppresses the steady component.

How is a time interval calculated from an oscilloscope trace?

Multiply the horizontal distance between the two features, in divisions, by the time-base setting.

What interval on a periodic oscilloscope trace gives the period?

The time for one complete cycle.

How is frequency calculated from the period $T$ of a waveform?

$f = \frac{1}{T}$

What is the purpose of an oscilloscope’s trigger control for a repeating waveform?

It can be adjusted to obtain a stable display.