Fig. 1 shows part of a sound-recording box used by a student monitoring bats near a bridge. A transformer provides a suitable alternating voltage for the mi...
Question 1 Report
Fig. 1 shows part of a sound-recording box used by a student monitoring bats near a bridge. A transformer provides a suitable alternating voltage for the microphone circuit. The primary coil has 400 turns and is connected to a 12 V a.c. supply. The secondary coil has 1200 turns.
Table 1 shows measurements made while checking the transformer.
| primary voltage / V | secondary voltage / V |
|---|---|
| 10 | 30 |
| 12 | 36 |
| 14 | 42 |
(a) State the secondary voltage when the primary voltage is 12 V. [1]
(b) Calculate the number of volts produced by the secondary coil for each volt across the primary coil. [2]
(c) Explain why a direct-current supply would not provide a continuous output voltage from this transformer. [2]
(d) Calculate the output power when the microphone circuit takes a current of 0.12 A at 36 V. [2]
(e) State the input current if the transformer is ideal. [2]
Answer Details
(a) The secondary voltage is \(36 V\). [1]
(b) \[\frac{V_s}{V_p}=\frac{36}{12}=3\]
The transformer produces \(3 V per volt across the primary, a voltage ratio of \(3:1\). [2]
(c) A direct current produces a magnetic field that becomes constant after switching on. A constant magnetic field does not induce a continuous voltage in the secondary coil, so a.c. is needed. [2]
(d) \[P=VI=36\ \mathrm{V}\times0.12\ \mathrm{A}=4.32\ \mathrm{W}\]
The output power is \(4.32 W\). [2]
(e) For an ideal transformer, input power equals output power:
\[I_p=\frac{P_p}{V_p}=\frac{4.32\ \mathrm{W}}{12\ \mathrm{V}}=0.36\ \mathrm{A}\]
The input current is \(0.36 A\). [2]
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