Question

In Thompson’s experiment to measure e/m of the electron, the electric and the magnetic fields are:
A. In the same direction
B. In the opposite direction
C. At an angle of with each other
D. Perpendicular to each other

Answer 1

In 1897 J.J. Thomson first measured the charge-to-mass ratio (e/m) of the electron. His experiment involved analyzing the electron’s motion in an electric and magnetic field. Sometime later R.A. Millikan determined the charge of the electron and resolved this ratio using an electric field alone. It was Thompson’s hypothesis of the electron’s existence, and its charge-to-mass ratio, which leads to the concept of the first atomic particle.

Complete step by step solution:
J.J. Thompson performed experiments to show that atoms consisted of subatomic particles that had positive and negative charges.

He determined that the negatively charged particles (electrons) were much lighter than the positively charged particles.

The electron charge-to-mass ratio was measured by accelerating the electrons through a voltage $Vx$ towards a positively charged plate. Some of the electrons pass through a small hole in the plate and form an electron beam that travels to a region where an electric field and a magnetic field was present.

The electrons were accelerated in the $x$-direction, the electric field was in the $y$-direction, and the magnetic field was in the $z$-direction.
And since all the three $x - y - z$are mutually perpendicular to each other so the electric and magnetic fields must be perpendicular to each other.

Note: Be clear that all the three $x - y - z$are mutually perpendicular to each other. But sometimes confusion might be created by using symbols like $\otimes$and$\odot$. The symbol $\otimes$signifies that something is projected into the plane of the paper. The symbol $\odot$ signifies that something is projected out of the plane of the paper.