Question

Animation—The Millikan Oil Drop Experiment
Watch the animation to study the Millikan oil drop experiment.
Click to launch the animation
The Millikan oil drop experiment was a direct and convincing experiment for the measurement of the charge on a single electron. Millikan performed an experiment on a charged oil droplet. By balancing the forces acting on the oil droplet, the charge on a single electron can be calculated. Millikan found that the charge of an oil droplet is always a whole-number multiple of −1.60×10−19 C. Thus, the charge on a single electron is −1.60×10−19 C.
Various types of forces that are observed in nature
You can observe the following types of forces in nature:
The attraction or repulsion between objects or particles that have electrical charge is known as the electric force.
The force with which the Earth, the Moon, or any other massively large object attracts another object toward itself is known as the gravitational force.
The force exerted upon an object that is in contact with another stationary object is known as a normal force.
The special type of frictional force that acts upon objects as they travel through the air is known as an air resistive force.
The force exerted by a compressed or stretched spring upon any object that is attached to it is known as a spring force.
The upward force caused by the air pressure that keeps things afloat is known as a buoyant force.
Part A - The forces acting on an oil droplet
Watch the animation. Identify the types of forces acting on an oil droplet while the voltage is turned on and the droplet is either falling or rising.
Check all that apply.
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Check all that apply.
Gravitational force
Electric force
Spring force
Air resistive force
Buoyant force
Normal force
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Part B - The effect of an electric field (voltage) on a negatively charged oil droplet
In the Millikan oil droplet experiment, the oil is sprayed from an atomizer into a chamber. The droplets are allowed to pass through the hole into the chamber so that their fall can be observed. The top and bottom of the chamber consist of electrically charged plates. The upper plate is positively charged, and the lower plate is negatively charged. X rays are introduced into the chamber so that when they strike the oil droplets, the droplets will acquire one or more negative charges. The electric field (voltage) is applied to the metal plates.
Watch the animation and identify the effects of an electric field on the motion of a negatively charged oil droplet. Consider the gravitational force as Fg and the electric force as Fe. All the other forces acting on the oil droplet can be ignored as their effect on the motion of the oil droplet is negligible.
Check all that apply.
Hints
Check all that apply.
If Fe is increased until it is equal to Fg, the negatively charged oil droplet will remain stationary.
In the absence of an electric field, the oil droplet falls freely due to the gravitational force.
In the presence of an electric field, the negatively charged oil droplet moves freely toward the negatively charged plate.
If Fe is greater than Fg, the negatively charged oil droplet will move freely toward the negatively charged plate.
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Part C - The total charge on a negatively charged oil droplet
In Millikan’s experiment, the oil droplets acquire one or more negative charges by combining with the negative charges that are produced from the ionization of air by X rays. By measuring the charges on the oil droplets, he calculated the charge on a single electron as −1.60×10−19 C. The charge on any negatively charged oil droplet is always a whole-number multiple of the fundamental charge of a single electron.
If Millikan was measuring the charge on an oil droplet with 7 negatively charged electrons on it, what charge would he have measured on the droplet?
Express your answer to three significant figures and include the appropriate units.
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Answer 1