Millikan Oil Drop Experiment

实验教学

外语教学

Millikan Oil Drop Experiment

时间:2009-11-02来源:南航物理实验教学中心点击:496次

Introduction

An experiment performed by Robert Millikan in 1909 determined the size of the charge on an electron. He also determined that electric charge exists as integral multiples of “e” which is a smallest 'unit' charge-1.60×10^-19C, or that charge is 'quantized'. He received the Nobel Prize for his work. Historically, this experiment ranks as one of the greatest experiments of modern physics. We're going to explain that experiment here, and show how Millikan was able to determine the size of a charge on a single electron.

Aims

The aims of this experiment are to demonstrate that electrical charge is quantised in discrete multiples of the electronic charge, e, and to measure the value of e.

Experimental Method

Considering an oil drop with charge q in two parallel charged plates, it can remain still when the gravitational force equals the electric force by adjusting the voltages between the plates(figure 1), i.e., , where d is the distance between the plates, U the potential difference applied to the plates. The small buoyant force from the air is neglected. The mass of oil drop is hard to be measured directly in the measurement. The setup of the equipment is shown below.

Without the electric field, forces acting on the drop when it is falling in air include gravitational force and friction of air. When the two forces have the same magnitude, terminal velocity is reached (a few milliseconds for the droplets used in this experiment after falling down). According to Stokes’ Law, we have , where η the coefficient of viscosity of air, r the radius of oil drop and v₀ the terminal velocity. Therefore the radius of oil drop can be expressed as . Since the velocities of the droplets used in this experiment will be in the range of 0.01 to 0.001 cm/s, the viscosity must be multiplied by a correction factor. The resulting effective viscosity is: , where b is a constant, p is the atmospheric pressure, and r is the radius of the drop as calculated by the uncorrected form of Stokes’ Law. Considering the above equations, the charge q is given as . In experiment the distance moving with the terminal speed is taken as 2mm. Considering all other constants, the equation used to calculate the charge is given by

Apparatus

Figure 2 Cross-section of experiment equipment

⑴oil drop box ⑵microscope lens ⑶Time measured

⑷time anew ⑸CCD camera ⑹focus-adjusting knob

⑺voltage option switch(Up, Balance and Down) ⑻balance-voltage-adjusting knob;

⑼voltage ⑽time ⑾display monitor

Procedure

1. Place the experiment equipment along a horizontal plane and put the oil box hole in a suitable position. Make sure the oil drops could enter the two charged plates through another hole in the middle of the upper plate, then open the power switch and wait a while.

2. Put the voltage option switch in ‘Down’ and spray some oil drops into the charged plates by squeezing the atomiser bulb. Then rotate focus-adjusting knob of the microscope to make the oil drops clear in the filed of view.

3. Put the voltage option switch in ‘Balance’ and make the value of balance voltage in the range of 100~300V by regulating balance-voltage-adjusting knob in order to search a suitable oil drop. Then change the voltage to make the oil drop motionless and write down the balance voltage U.

4. As soon as the ‘Up’ works, the oil drop will rise. When the oil drop gets above the filed of view, choose the switch ‘Down’. Then the oil drop will fall, and it can move a distance(about 2mm from the second line to the sixth line in the screen) at a constant speed. So the time t can be measured by controlling counter knob.

5. With U and t the charge q can be calculated.

Data Analysis

You should find that the charge on each drop is given approximately by: , where N is an integer , =1.60×10^-19C. In order to obtain ‘e’ , a method is introduced:

e (=q/N)

In the equation N means the nearest integer number to q/1.6*10^-19

Then finish the table below:

	1	2	3
t
U
e (=q/N)*