In the Millikan oil-drop experiment (see Figure 2.5), the tiny oil drops are observed through the viewing lens as rising, stationary, or falling, as shown here. (a) What causes their rate of fall to vary from their rate in the absence of an electric field?

Key Concepts

Electric Field

An electric field is a region around a charged particle where other charged particles experience a force. In the context of the Millikan oil-drop experiment, the electric field is created between two charged plates, influencing the motion of the oil droplets. When the field is applied, it exerts a force on the charged droplets, altering their rate of fall compared to when no field is present.

Gravitational Force

Gravitational force is the attractive force between two masses, which in this experiment acts on the oil droplets due to Earth's gravity. This force causes the droplets to fall towards the ground. The balance between gravitational force and the electric force from the electric field determines whether the droplets rise, fall, or remain stationary.

Charge of the Droplets

The charge of the oil droplets is crucial in the Millikan experiment, as it determines how they interact with the electric field. Each droplet can acquire a charge through ionization or contact with charged surfaces. The magnitude and sign of this charge affect the force exerted by the electric field, thus influencing the droplets' motion and their observed rates of fall or rise.