A null point is found at 200 cm in potentiometer when cell in secondary circuit is shunted by 5Ω. When a resistance of 15Ω is used for shunting, null point moves to 300 cm. The internal resistance of the cell is _______ Ω.
Correct Answer: 4 - 5
Solution and Explanation
Step 1: From the first equation:
\(\frac{\epsilon}{r+5} \times 5 = 200x\)
\(\frac{\epsilon}{r+5} = \frac{200x}{5} = 40x\)
\(\epsilon = 40x(r+5)\)
Step 2: Now substitute \(\epsilon = 40x(r+5)\) into the second equation:
\(\frac{\epsilon \times 15}{r+15} = 300x\)
Substituting \(\epsilon = 40x(r+5)\) into the above equation:
\(\frac{40x(r+5) \times 15}{r+15} = 300x\)
Simplifying:
\(\frac{600x(r+5)}{r+15} = 300x\)
Step 3: Cancel out the common term of \(x\):
\(\frac{600(r+5)}{r+15} = 300\)
Simplify the equation:
\(600(r+5) = 300(r+15)\)
Step 4: Expand both sides:
\(600r + 3000 = 300r + 4500\)
Step 5: Solve for r:
\(600r - 300r = 4500 - 3000\)
\(300r = 1500\)
\(r = \frac{1500}{300} = 5 \, \Omega\)
Thus, the value of r is: 5 Ω
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Concepts Used:
Resistance
Resistance is the measure of opposition applied by any object to the flow of electric current. A resistor is an electronic constituent that is used in the circuit with the purpose of offering that specific amount of resistance.
R=V/I
In this case,
v = Voltage across its ends
I = Current flowing through it
All materials resist current flow to some degree. They fall into one of two broad categories:
- Conductors: Materials that offer very little resistance where electrons can move easily. Examples: silver, copper, gold and aluminum.
- Insulators: Materials that present high resistance and restrict the flow of electrons. Examples: Rubber, paper, glass, wood and plastic.
Resistance measurements are normally taken to indicate the condition of a component or a circuit.
- The higher the resistance, the lower the current flow. If abnormally high, one possible cause (among many) could be damaged conductors due to burning or corrosion. All conductors give off some degree of heat, so overheating is an issue often associated with resistance.
- The lower the resistance, the higher the current flow. Possible causes: insulators damaged by moisture or overheating.