EI · 8 cards

AC and DC flashcards

8 cards with the formulas, rules and traps from this lesson. Answer in your head, then flip to check.

Term · 1/8
What distinguishes AC from DC?
AC periodically reverses direction (the signed value crosses zero to the opposite sign). DC flows one way only; it may be steady or vary in size, and still be DC.
Term · 2/8
What is pulsating DC?
Current that rises and falls (often touching zero) but never reverses, such as rectifier output. It changes size but keeps one direction, so it is DC.
Formula · 3/8
How are frequency and period related?
$f = \frac{1}{T}$ and $T = \frac{1}{f}$ ; also $f = N / t$ for N full cycles in t seconds. f is in hertz, T in seconds. A 4 ms period gives 250 Hz.
Formula · 4/8
Sine wave: how do RMS and peak voltage relate?
$V_{r m s} = \frac{V _{p}}{2} \approx 0.707 V_{p}$ and $V_{p} \approx 1.414 V_{r m s}$ . Peak-to-peak is $V_{pp} = 2 V_{p}$ . This 0.707 factor applies only to sine waves.
Fact · 5/8
What are standard U.S. household outlet voltage and frequency?
120 V RMS at 60 Hz (240 V circuits for large appliances). The voltage actually peaks near 170 V, and one cycle lasts about 16.7 ms.
Rule · 6/8
Rectifier, inverter and transformer: what does each do?
A rectifier (diodes) converts AC to DC. An inverter converts DC to AC. A transformer steps AC voltage up or down and does not work on steady DC.
Formula · 7/8
Average power in a purely resistive AC load?
Use RMS values: $P = V_{r m s} I_{r m s} = \frac{V _{r m s}^{2}}{R} = I_{r m s}^{2} R$ , in watts. 30 V RMS across 10 $Ω$ gives 90 W. Peak values overstate power.
Common trap · 8/8
Counting cycles on a waveform graph: what is the common mistake?
Counting every hump as a cycle. One full cycle has one positive and one negative half, so 8 humps (4 up, 4 down) is 4 cycles. Also convert ms to seconds before using f = 1/T.

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