Potentiometer

Question 1

What is a Potentiometer?

Answer 1

A device used to measure the electromotive force (EMF) of a cell, compare EMFs of two cells, or measure internal resistance of a cell

It is based on the principle of potential difference across a uniform wire.

Question 2

What is the basic principle of a potentiometer?

Answer 2

Potential difference (V) is directly proportional to the length (L) of the wire if the wire is uniform and the current is constant

V∝L or V=kL where k is the potential gradient.

Question 3

In the equation V=kL, what does V represent?

Answer 3

Potential difference

V is the potential difference measured along the length of the wire.

Question 4

What is the construction of a potentiometer?

Answer 4

A long uniform resistance wire (1 to 10 meters), connected in series with a battery and a rheostat, and includes a galvanometer and a jockey

Sometimes a secondary circuit is connected for measurements.

Question 5

How does a potentiometer work?

Answer 5

A constant current is passed through the wire, setting up a uniform potential gradient, and the jockey is moved to find the balancing point where the galvanometer shows zero deflection

This null point gives the potential difference equal to the EMF of the cell.

Question 6

What is the formula to measure the EMF of a cell using a potentiometer?

Answer 6

E = kL

E is the EMF, k is the potential gradient, and L is the length of the wire.

Question 7

How can you compare the EMFs of two cells using a potentiometer?

Answer 7

Using the lengths L1 and L2 of the balancing points for each cell: E1/E2 = L1/L2

This ratio allows comparison of the two EMFs.

Question 8

How to measure the internal resistance of a cell using a potentiometer?

Answer 8

r = R(L1 - L2)/L2

r is the internal resistance, R is the external resistor, L1 is the null point without load, and L2 is the null point with load.

Question 9

What are the advantages of using a potentiometer?

Answer 9

• Very accurate (more than a voltmeter)
• No current flows through the cell at balance point
• Can measure very small voltages

These factors contribute to the precision of measurements.

Question 10

What are the limitations of a potentiometer?

Answer 10

• Needs a very stable current
• Bulky and not portable
• Sensitive to temperature and wire quality

These limitations can affect the usability of the potentiometer in certain settings.

Question 11

Fill in the blank: A potentiometer wire can be thought of as a ruler made of electricity, where the voltage _______.

Answer 11

increases gradually

This analogy helps understand how the potentiometer measures voltage.