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Wonders of Physics


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DC Circuit – Diode

An a.c. circuit is shown below.

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Which of the following diagrams show the voltage V across the resistor?

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Solutions: Option C

The power supply is a alternating supply, hence direction of current changes many times in 1 second. (alternating current = a.c.)

You may ask which direction of current should you first assume in order to represent the first ‘hump’?

That is not that important. Rather you must be able to know that the ‘humps’ should alternate as the V across alternates due to the diodes.

When current flows in anticlockwise, current will flow through the 6Ω and the 12Ω (in series). When the current flows in clockwise, current will flow through the 12Ω and the 12Ω (in series). Hence V across the 6Ω and the 12Ω will alternate. Hence B and D are out.

D is out as there is an continuous alternating current flowing through the circuit. Not rectification due to the diodes in the circuit.

The diodes in the 2 branches only stop current from flowing in either branch at any one time, current still continue to flow through the circuit.

Consider when current flows anticlockwise, the V is across the 6Ω. As 6Ω is smaller than the 12Ω in series, the p.d. across 6Ω is smaller. Hence a smaller ‘hump’.

On the other hand, when current flows clockwise, the V is across the 12Ω. As this 12Ω is equal to the 12Ω in series, the p.d. across will be higher than across 6Ω previously. Hence bigger ‘hump’. So C is the answer.

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