Ch 17 · 4.6 Uses of an Oscilloscope · O Level Physics 5054

Topic 17 of 17 in Electricity and Magnetism — O Level Physics 5054 — 49 lessons.

Introduction

An oscilloscope draws a graph of voltage against time, live, on a screen. It lets you see the shape of an a.c. supply, the sound wave from a microphone or the pulses in a circuit, and measure voltages and times far too quick for a stopwatch. (Syllabus section 4.6.)

Key ideas — What the screen shows

Displaying waveforms:

🧑‍🏫 Teacher's explanation

Imagine an oscilloscope as a high-speed camera that doesn't take pictures, but instead draws a continuous line on a screen. Instead of a still photo, think of it like a pen moving across a piece of paper. If you move the pen up and down while it moves right, you get a wavy pattern. In an oscilloscope, the 'pen' is an electron beam, and the 'paper' is the screen.

📖 Definitions

🔍 Worked example — Easy

A waveform on an oscilloscope shows a peak-to-peak voltage of 4 divisions. If the Y-gain is set to 2.0 V/div, calculate the peak-to-peak voltage in volts.

🔍 Worked example — Standard

A signal completes 5 full cycles in 10 divisions. If the timebase is set to 0.5 ms/div, calculate the period (T) of the waveform in milliseconds.

🔍 Worked example — Exam-style

An oscilloscope shows a sine wave. The vertical height from the center line to the peak is 3 divisions. The timebase is 2.0 ms/div. The peak-to-peak voltage is 12 V. Calculate the Y-gain setting.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Calculate the peak-to-peak voltage if the wave reaches 3 divisions above the center line. (b) Calculate the period of the waveform in milliseconds. [4 marks]

Key ideas — The two main controls

Y-gain (volts per division, V/div): how many volts each vertical division represents. For example, 2 V/div means a trace 3 divisions high represents 6 V.

🧑‍🏫 Teacher's explanation

Imagine you are looking at a graph drawn on a piece of grid paper. An oscilloscope is essentially a high-speed graphing machine. Instead of drawing with a pen, it uses a beam of light to trace a line that moves across the screen. The 'waveforms' you see are just visual representations of how voltage changes over time.

📖 Definitions

🔍 Worked example — Easy

A waveform on an oscilloscope has a peak-to-peak height of 4 divisions. The Y-gain is set to 2 V/div. Calculate the peak-to-peak voltage.

🔍 Worked example — Standard

A single cycle of a sine wave occupies 5 horizontal divisions. The timebase is set to 10 ms/div. Calculate the period (T) of the wave.

🔍 Worked example — Exam-style

An oscilloscope displays a signal. The peak voltage (from the center line to the top) is 3 divisions. The Y-gain is 5 V/div. The time for one full wave is 4 divisions, and the timebase is 2 ms/div. Calculate the maximum voltage and the frequency (f) of the wave.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Measuring p.d.

For a.c., this gives the peak voltage. (Measuring from peak to trough and halving it is often more accurate.)

🧑‍🏫 Teacher's explanation

Imagine you are watching a wave in a swimming pool. If you want to know how high the wave is, you need a ruler to measure the height from the bottom to the top. An oscilloscope does exactly this, but for electricity. Instead of measuring height in centimeters, it measures voltage (potential difference) on a vertical axis.

📖 Definitions

🔍 Worked example — Easy

A waveform is displayed on an oscilloscope. The vertical height from the center line to the peak of the wave is 4 divisions. The Y-gain is set to 5 ext V/div. Calculate the peak voltage.

🔍 Worked example — Standard

An oscilloscope shows an a.c. waveform. The vertical distance from the peak to the trough is 6 divisions. The Y-gain is set to 2 ext V/div. Calculate the peak voltage.

🔍 Worked example — Exam-style

A student measures a waveform. The peak-to-peak height is 8 divisions and the Y-gain is 10 V/div. The time for one full cycle (period) is 5 divisions. The timebase is set to 2 ms/div. Calculate the peak voltage and the period of the wave.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Measuring time

The frequency of a wave is then f = 1/period.

🧑‍🏫 Teacher's explanation

Imagine you are watching a movie on a screen. If you want to know how long a single scene lasts, you don't use a stopwatch; you look at how many frames pass by. An oscilloscope does the same thing for electricity. Instead of frames, it uses little squares on a grid. One axis shows how much voltage (pressure) there is, and the other shows how much time has passed.

📖 Definitions

🔍 Worked example — Easy

A waveform is displayed on an oscilloscope. One complete wave occupies 4 horizontal divisions. The timebase is set to 5 ext ms/div. Calculate the period T of the wave.

🔍 Worked example — Standard

A student observes a sine wave on an oscilloscope. The timebase is 0.5 ms/div. The wave spans 6 divisions from peak to peak. Calculate the frequency f of the wave in Hz.

🔍 Worked example — Exam-style

An oscilloscope is used to measure the time delay between two pulses. The first pulse is at 0 divisions and the second pulse is at 7.5 divisions. The timebase is set to $2.0\text{ $\mus/div. Calculate the time interval Δ t in microseconds.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

An a.c. trace has a height of 3.0 divisions from the centre line to the peak, and one complete wave spans 4.0 divisions. The Y-gain is 5.0 V/div and the timebase is 5.0 ms/div. Find the peak voltage, the period and the frequency.

🔍 Worked example — Easy

An oscilloscope shows a sine wave. The peak height from the center line is 2.5 divisions. The Y-gain is set to 4.0 V/div. Calculate the peak voltage.

🔍 Worked example — Standard

A waveform spans 5.0 divisions for one full cycle. The timebase is set to 2.0 ms/div. Calculate the period T in seconds.

🔍 Worked example — Exam-style

⚠️ Common errors

Common mistakes

Practice

Answers: (1) The number of volts represented by each vertical division. (2) 2.5 × 2 = 5 V. (3) Period = 5 × 2 = 10 ms = 0.010 s; f = 1/0.010 = 100 Hz. (4) (a) a straight horizontal line above or below the centre; (b) a regular wave above and below the centre line.

Ch 17 · 4.6 Uses of an Oscilloscope · O Level Physics 5054

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