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A load that has a resistance of 10 ohms is to be connected to a supply that has a constant voltage of 120 volts. If it is desired that the currrent to the load be varied from 3 to 5 amperes, what are the resistance and the current rating of the series rheostat that permit this variation?
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- 30 ohms, 5A
- 10 ohms, 10A
- 20 ohms, 10A
- 20 ohms, 10A
Correct Option: A
The 10 ohms load when connected to 120 V supply will take a current
| = 12 A | ||
| 10 |
without rheostat in the circuit.
Since it is desired to restrict the current flow through the load at 3 to 5 amperes, therefore voltage drop in the load = 3 × 10 = 30 volts at 3 amperes, and
= 5 × 10 = 50 volts at 5 amperes.
When voltage drop across the load is 30 V, then voltage drop in the rheostat must be = 120 – 30 = 90 volts.
Since same curent flows through the rheostat, resistance of the rheostat must be
| = | = 30 ohms | |
| 3 |
When voltage drop across the load is 50 V, voltage drop in the rheostat must be = 120 – 50 = 70 volts.
Since same current flows through the rheostat, resistance of the rheostat must be
| = | = 14 ohms | |
| 5 |
Thus rating of rheostat must be 30 ohms and 5 amperes.
