Inside a hand-cranked emergency radio, a coil is turned between the poles of a permanent magnet. Fig. 1 is an oscilloscope trace of the voltage from the coi...
Question 1 Report
Inside a hand-cranked emergency radio, a coil is turned between the poles of a permanent magnet. Fig. 1 is an oscilloscope trace of the voltage from the coil when the handle is rotated steadily. The time-base setting is 2.0 ms per division and the voltage scale is 0.50 V per division. The radio charges a small energy store before its loudspeaker is used.
(a) State which component produces the magnetic field in the generator. [1]
(b) Calculate the time period of one complete wave shown on the oscilloscope. [2]
(c) Calculate the frequency of the alternating voltage. [2]
(d) Calculate the peak voltage of the trace. [2]
(e) Explain why the voltage changes from positive to negative as the coil continues to rotate. [3]
Answer Details
(a) The permanent magnet produces the magnetic field in the generator. [1]
(b) One complete wave occupies 5 horizontal divisions. Therefore:
\[T=5\times2.0\ \mathrm{ms}=10\ \mathrm{ms}=0.010\ \mathrm{s}\]
The time period is \(10 ms\), or \(0.010 s\). [2]
(c) \[f=\frac{1}{T}=\frac{1}{0.010\ \mathrm{s}}=100\ \mathrm{Hz}\]
The frequency is \(100 Hz\). [2]
(d) The peak is 2 vertical divisions from the \(0\ \mathrm{V}\) line:
\[V_{\text{peak}}=2\times0.50\ \mathrm{V}=1.0\ \mathrm{V}\]
The peak voltage is \(1.0 V\). [2]
(e) Rotation changes the magnetic flux linkage of the coil. In the opposite half-turn, the coil cuts field lines in the opposite direction, so the induced voltage reverses. The output therefore alternates between positive and negative voltage. [3]
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