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. The arrows indicate the rate of motion. a. What causes their rate of fall to vary from their rate in the absence of an electric field?

Verified step by step guidance

1. Understand the setup of the Millikan oil-drop experiment, which involves observing tiny oil drops between two charged plates.
2. Recognize that in the absence of an electric field, the oil drops fall solely due to gravity, influenced by their mass and the air resistance they encounter.
3. Consider the introduction of an electric field between the plates. The electric field exerts an upward force on the charged oil drops, which can counteract the downward gravitational force.
4. Analyze how the balance of forces (electrical force versus gravitational force) affects the motion of the oil drops. If the electrical force is strong enough, it can make the drops rise or remain stationary, opposing gravity.
5. Conclude that the variation in the rate of fall of the oil drops in the presence of an electric field compared to their rate in the absence of an electric field is due to the additional upward force exerted by the electric field on the charged particles of the oil drops.

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 drops. When the electric field is applied, it exerts a force on the charged oil droplets, altering their rate of fall compared to when no electric field is present.

Gravitational Force

Gravitational force is the attractive force between two masses, which in this experiment acts on the oil drops due to Earth's gravity. This force pulls the drops downward, and its magnitude depends on the mass of the drops. In the absence of an electric field, the drops fall at a rate determined solely by this gravitational force and the drag force from the air.

Drag Force

Drag force is the resistance experienced by an object moving through a fluid, such as air. In the Millikan experiment, as the oil drops fall, they encounter air resistance that opposes their motion. The balance between gravitational force and drag force determines the terminal velocity of the drops, which changes when an electric field is applied, as the electric force can counteract gravity and alter the drops' motion.