DIRECT CURRENT (DC) — flows in ONE direction only; pd is constant.
Sources: batteries, solar cells, dc power supplies.
On an oscilloscope:
DC appears as a HORIZONTAL LINE — constant pd, no oscillation.
Higher line = higher pd. Line at zero = no pd.
Alternating Current (AC)
ALTERNATING CURRENT (AC) — current and pd REVERSE DIRECTION repeatedly.
Sources: mains electricity, generators.
On an oscilloscope:
AC appears as a WAVE (sinusoidal) — rises above and below zero.
FREQUENCY = complete cycles per second (Hz).
PEAK VOLTAGE = maximum pd from zero.
UK MAINS:
Frequency: 50 Hz
Voltage: ~230 V (rms value)
Mains is AC because generators naturally produce AC, and AC is easily transformed to different voltages for efficient transmission.
Reading Oscilloscope Traces
Y-axis: voltage. X-axis: time.
FREQUENCY:
Measure the time for ONE complete cycle (period T).
f = 1 ÷ T
PEAK VOLTAGE:
Measure from zero line to peak.
Peak pd = divisions × volts per division setting.
EXAMPLE:
Period T = 0.02 s:
f = 1 ÷ 0.02 = 50 Hz — UK mains ✓
DC trace: flat horizontal line.
AC trace: regular sinusoidal wave.
⚠️ Common Mistake
UK mains is 50 Hz — not 60 Hz (USA). The quoted 230 V is an rms value — the peak voltage is higher (~325 V). A DC trace is a flat line; an AC trace is a wave.
📐 Variables
fFrequency (f) is measured in hertz (Hz)
TPeriod (T) is measured in seconds (s)
VPotential difference (V) is measured in volts (V)
📐 Key Equations
f = 1 ÷ T
📌 Key Note
DC: constant direction, flat oscilloscope line. AC: reversing, sinusoidal trace. UK mains: 50 Hz, ~230 V rms. f = 1/T. Mains AC because generators produce AC and it can be transformed easily.
🎯 Matching Activity — DC vs AC
Match each description to DC or AC. — drag the symbols on the right to match the component names on the left.
DC
Drop here
AC
Drop here
DC
Drop here
AC
Drop here
AC
Drop here
Reverses direction repeatedly — sinusoidal oscilloscope trace
Produced by generators in power stations
Produced by batteries and solar cells
Flows in one direction only — flat horizontal oscilloscope trace
UK mains — 230 V, 50 Hz
⚽ FIFA Worked Examples
Frequency from Period
An oscilloscope trace shows one complete cycle takes 0.02 s. Calculate the frequency.
F
f = 1 ÷ T
I
T = 0.02 s
F
f = 1 ÷ 0.02
A
f = 50 Hz — UK mains frequency ✓
🎯 Test Yourself
Question 1 of 2
1. An oscilloscope shows a horizontal line above the zero axis. What does this represent?
2. What are the frequency and voltage of the UK mains supply?
⭐ How Well Do You Understand This Topic?
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🤖 Ask Mr Badmus AI
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