OCR AS Physics

Revision Notes

4.6.2 Calculating Frequency

Calculating Frequency

  • Frequency (f) is defined by the equation:

Frequency equation, downloadable AS & A Level Physics revision notes

  • 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:

WE - Wave equation question image, downloadable AS & A Level Physics revision notes

 

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

Exam Tip

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
    • It can therefore be used as an a.c and d.c voltmeter
  • An a.c voltage on an oscilloscope is represented as a transverse wave
    • Therefore you can determine its frequency, time period and peak voltage
  • A d.c voltage on an oscilloscope is represented as a horizontal line at the relevant voltage
  • The x-axis is the time and the y-axis is the voltage (or y-gain)

CRO diagram, downloadable AS & A Level Physics revision notes

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:

Frequency-period equation, downloadable AS & A Level Physics revision notes

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.

WE - CRO question image, downloadable AS & A Level Physics revision notes

What is the frequency of the sound wave?

A    2.4 Hz          B     24 Hz             2.4 kHz             24 kHz

WE - CRO answer image, downloadable AS & A Level Physics revision notes

Exam Tip

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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