Question

A beam of cathode rays moving towards the south is under the action of a downward electric field. To prevent its deflection by the electric field, a magnetic field should be directed
A. Eastward
B. Westward
C. Upward
D. Downward

Answer 1

Electric field can be considered as an electric property associated with each point in the space where a charge is present in any form.

Complete step by step answer:
For equilibrium of cathode rays write the force equation:
$qE + q(v \times B) = 0$
$B \times v = E$
v is towards south and E is in downward direction, so B should be in east direction
An electric field is also described as the electric force per unit charge.
The formula of electric field is given as
$E = \dfrac{F}{Q}$
Where,
E is the electric field.
F is a force.
Q is the charge.
Electric fields are usually caused by varying magnetic fields or electric charges. Electric field strength is measured in the SI unit volt per meter (V/m).
The direction of the field is taken as the direction of the force which is exerted on the positive charge. The electric field is radially outwards from positive charge and radially in towards negative point charge.
The electric field is defined mathematically as a vector field which can be associated with each point in space, the force per unit charge exerted on a positive test charge at rest at that point.
A uniform field is one in which the electric field is constant at every point. It can be approximated by placing two conducting plates parallel to each other and maintaining a voltage (potential difference) between them; it is only an approximation because of boundary effects (near the edge of the planes, electric field is distorted because the plane does not continue).

So, the correct answer is “Option A”.

Note:
Electric fields are generated by the electric charge or by time-varying magnetic fields. In case of atomic scale, the electric field is responsible for the attractive forces between the atomic nucleus and electrons which hold them together.