Calculating Frequency (OCR A Level Physics): Revision Note
Exam code: H556
Calculating Frequency
Frequency (f) is defined by the equation:
Where T is the time period, the time taken for one complete oscillation or cycle of the wave
Worked Example
Calculate the frequency of the following wave:
Answer:
Step 1: List the known quantities
Period, T = 0.28 ms = 0.28 × 10-3 s
Step 2: Write down the frequency equation
Step 3: Substitute in the values
f = 1 ÷ (0.28 × 10-3) = 3571.4 = 3.57 kHz
Examiner Tips and Tricks
When using the frequency equation, always make sure the time period is in seconds to get the value of the frequency in Hertz (Hz)
Determining Frequency from an Oscilloscope
A Cathode-Ray Oscilloscope (CRO) is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits
The x-axis is the time-base and the y-axis is the voltage (or y-gain)
The time-base is important for calculating the frequency of the signal
Diagram of Cathode-Ray Oscilloscope display showing wavelength and time-base setting
The frequency of a wave is determined from the time period of the wave
The period can be determined from the time-base
This is how many seconds each division represents measured commonly in s div-1 or s cm-1
Dividing the total time by the number of wavelengths will give a value for T
Use as many wavelengths shown on the screen as possible to reduce uncertainties
The frequency is then determined using the equation:
Worked Example
A cathode-ray oscilloscope (c.r.o.) is used to display the trace from a sound wave. The time-base is set at 7 µs mm-1.
What is the frequency of the sound wave?
A. 2.4 Hz
B. 24 Hz
C. 2.4 kHz
D. 24 kHz
Answer:
Remember that the time base is the scale of the horizontal c.r.o. axis, so one division represents 7 µs mm-1
The time base has units of µs mm-1 which means that the cm division needs to be converted to mm first.
Examiner Tips and Tricks
The time-base setting varies with units for seconds (commonly ms) and the unit length (commonly mm). Unit conversions are very important when calculating the time period and frequency.
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