P.3.7.5.5 Alternating currents

The mean power dissipated in a resistor is 47.5 μW when the root mean square (rms) voltage across the resistor is 150 mV.

What is the peak current in the resistor?

• 2.3 × 10⁻⁴ A

• 4.5 × 10⁻⁴ A

• 2.3 × 10³ A

• 4.5 × 10³ A

A transformer has 1150 turns on the primary coil and 500 turns on the secondary coil.

The primary coil draws a current of 0.26 A from a 230 V ac supply. The current in the secondary coil is 0.50 A. What is the efficiency of the transformer?

• 42%

• 50%

• 84%

• 100%

The above graph shows how the output emf, ε, varies with time, t, for a coil rotating at angular speed ω in a uniform magnetic field of flux density B. Which one of the following graphs shows how ε varies with t when the same coil is rotated at angular speed 2ω in a uniform magnetic field of flux density 0.5 B?

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A coil rotating in a magnetic field produces the following voltage waveform when connected to an oscilloscope.

With the same oscilloscope settings, which one of the following voltage waveforms would be produced if the coil were rotated at twice the original speed?

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The diagram below shows the waveform obtained when the output of an alternator is connected to a cathode ray oscilloscope.

Which one of the following best represents the output when the speed of rotation of the generator is doubled and no adjustment is made to the oscilloscope?

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