Question

Assertion (A): From Thomson's experiment and Millikan's oil drop experiment, mass as well as charge of an electron can be calculated.

Reason (R): Specific charge of an electron is obtained from Thomson's experiment, whereas the charge of an electron is obtained from Millikan's oil drop experiment.

• A. Both A and R are true and R is the correct explanation of A.
• B. Both A and R are true, but R is not the correct explanation of A.
• C. A is true and R is false.
• D. A is false and R is true.

Answer 1

The correct option is A

Both A and R are true and R is the correct explanation of A.

The explanation for the correct option:

1. Assertion(A):

Thompson's experiment- This experiment was done by J.J Thompson to determine the inner structure of an atom and discovered the negatively charged particles in the atom i.e. electrons. He assumed the presence of only protons and electrons in the atom and thus also gave the concept of charge to mass ratio.

Millikan's oil drop experiment- This experiment measured the charge of an electron using the electric field created between the two metal plates placed parallel to each other and held apart by an insulating material. When a charged oil droplet enters the electric field and fell at a constant rate, the gravitational and electric forces are equal on it. Using this relation, the charge of the electron is found to be$1.6\times {10}^{-19}\mathrm{C}$.

Hence, from these two experiments, mass, as well as the charge of an electron, can be calculated.

Thus, the Assertion is TRUE.

2. Reason(R): Specific charge is the ratio of charge to mass of the particle. J.J. Thompson estimated the mass to charge ratio using the amount by which the cathode ray was deflected by a magnetic field.

The charge of an electron ( $1.6\times {10}^{-19}\mathrm{C}$ ) is determined by Millikan's oil drop experiment by using the formula- $Q=\frac{m.g}{E}$

Hence R is TRUE and the correct explanation of A.

Conclusion: The correct option is a).