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Question

Two \(100\text{-}\Omega\) resistors are connected in parallel, and this combination is connected across a 40-V supply. Find the current supplied by the voltage source.

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is
0.8 A

Calculating Total Current for Parallel Resistors

The problem asks for the total current supplied by the voltage source when two \(100\text{-}\Omega\) resistors are connected in parallel across a \(40\text{-}\text{V}\) supply.

Step 1: Calculate Equivalent Resistance

When resistors are connected in parallel, the reciprocal of the equivalent resistance (\(R_{eq}\)) is the sum of the reciprocals of individual resistances. For two resistors, the formula simplifies to:

\( R_{eq} = \frac{R_1 \times R_2}{R_1 + R_2} \)

Given \(R_1 = 100\text{ }\Omega\) and \(R_2 = 100\text{ }\Omega\):

\( R_{eq} = \frac{100\text{ }\Omega \times 100\text{ }\Omega}{100\text{ }\Omega + 100\text{ }\Omega} = \frac{10000\text{ }\Omega^2}{200\text{ }\Omega} = 50\text{ }\Omega \)

Step 2: Apply Ohm's Law

Ohm's Law states that the voltage (\(V\)) across a circuit (or component) is equal to the product of the current (\(I\)) flowing through it and its resistance (\(R\)): \(V = I \times R\). To find the current supplied by the source, we rearrange the formula to \(I = \frac{V}{R}\).

Using the supply voltage (\(V = 40\text{ V}\)) and the calculated equivalent resistance (\(R_{eq} = 50\text{ }\Omega\)):

\( I = \frac{40\text{ V}}{50\text{ }\Omega} = \frac{4}{5}\text{ A} = 0.8\text{ A} \)

Therefore, the current supplied by the voltage source is \(0.8\text{ A}\).

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Important Questions from Combination of Resistors — Series and Parallel

  1. A cell of negligible resistance and e.m.f 2 volt is connected to series combination of 2 ohm, 3 ohm and 5 ohm. The potential difference across the 3 ohm resistance is:

  2. 3 resistors of 3 ohm each connected in series. What is the mean values of resistors?

  3. Ten cells, each of 2 volts emf and 1 ohm internal resistance are connected in series. What is the current flowing through a resistance of 10 ohms connected across this combination of cells?

  4. Three resistor, each equal to 3 Ω are connected so as to form a triangle. The equivalent resistance between any two vertices of the triangle is:

  5. Two resistors R 1 and R 2 arranged in parallel combination in an electrical closed circuit are made of the same material and of the same thickness. If the length of R 2 is twice the length of R 1, then the total resistance R satisfies
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