Subelement A: — Topic :
Question 6A63
Element 6 (Radiotelegraph)If a vacuum tube with a filament rating of 0.25 A and 5 V is operated from a 6 volt battery, what value ofresistor is necessary?
Explanation
To operate a vacuum tube filament at its rated specifications from a higher voltage source, a series resistor is required to drop the excess voltage. The filament needs 5 V and draws 0.25 A. When powered by a 6 V battery, the series resistor must drop the difference in voltage: 6 V (source) - 5 V (filament) = 1 V.
Since the resistor is in series with the filament, the same current (0.25 A) flows through both. Using Ohm's Law, R = V/I, where R is the resistance, V is the voltage drop across the resistor, and I is the current.
Therefore, the necessary resistance is:
R = 1 V / 0.25 A = 4 ohms.
This value ensures the filament operates at its correct voltage and current, preventing damage from over-voltage. Options B, C, and D would result in different voltage drops, causing the filament to operate outside its specifications.
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