Lessons_93-96 Millikan Oil Drop Experiment Unit_3A_Charged Particles_AL Learning objectivities • show an understanding of the main principles of determination of e by Millikan’s experiment • summarise and interpret the experimental evidence for quantisation of charge Problem1 An oil drop, of mass 4.7×10-15 kg, is suspended between two parallel plates, as in figure below. a. Calculate the charge on the oil drop. b. Calculate the number of elementary charges required to make up this charge. c. Does the oil drop have a deficit or excess of electrons? Explain your answer. Let’s watch! And fill the gaps! J. J. Thomson’s charge-tomass ratio experiment • In 1897 J. J. Thomson demonstrated that cathode rays, a new phenomenon, were made up of small negatively charged particles, which were soon named electrons. • The electron was the first subatomic particle ever discovered. • Through his cathode ray experiments, Thomson also determined the electrical charge-to-mass ratio for the electron Millikan's oil-drop experiment Millikan's oil-drop experiment was performed by Robert Millikan and Harvey Fletcher in 1909. Actual apparatus used in the experiments Let’s think! 1. 2. 3. 4. 5. 6. 7. What is the idea of Millikan's oil drop method? Why Millikan used oil drops? How the oil drops were charged? What forces act on the oil drops? How have the charge & mass been determined? How was quantization of charge proved? What are the achievements of oil drop experiment? http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites /dl/free/0072512644/117354/02_Millikan_Oil_Drop.swf::Milikan%20Oil%20Drop • http://www.thephysicsaviary.com/Physics/Pro grams/Labs/MillikanOilDropLab/index.html Oil was sprayed out of an atomizer Oil drops fall through hole in top plate. The droplets enter the space between the plates and can be controlled by changing the voltage across the plates X rays place electrons on drops. Microscope is used for observing droplets. Idea of the experiment Millikan's experiment is based around observing charged oil droplets In free fall and in the presence of an electric field The oil drop experiment calculated the charge of an electron using charged oil droplets suspended in an electric field between the two plates . When drop is suspended, force due to gravity equals force due to electrical attraction to the positive plate. Idea of the experiment More details of the experiment • The charge on the plates is adjustable. By measuring the terminal velocity of the oil drops with the electric field off, Millikan could determine the mass of the drops. • Millikan and his graduate assistant were able to determine the force of the field on the drops when it was turned on by comparing the velocity of the drops with the field on to their velocity with the field off. • This is easily determined when the oil drop is stationary; namely, when the downward gravitational force exactly equals the upward electrical force on the drop. . Idea of the experiment When drop is suspended, force due to gravity equals force due to electrical attraction to the positive plate. The charge on this particular oil drop was 4.8×10−19 C. In doing his experiment, however, Millikan faced a problem. When the oil is sprayed through the atomizer, some oil drops are negatively charged, but we don’t know how many extra electrons the drops acquire. The charge on this oil drop could be the result of having one extra electron, or having five extra electrons. In order to determine the charge on one electron, the oil drop experiment was carried out many times, and the charges on many oil drops was determined. The smallest charge was found to be 1.6×10−19 C, and all the other charges on oil drops were found to be whole number multiples of 1.6×10−19 C. Major findings from the experiment Data from original Millikan’s 1911 paper q/C q/1.64 x 10-19 Round to whole number -1.970 x 10-18 -0.987 x 10-18 -2.773 x 10-18 -12.02 -6.02 -16.93 -12 -6 -17 The charges were found to all be integer multiples (n) of a single number, a fundamental electric charge (e). Therefore, the experiment confirmed that charge is quantised. What is meant by “ an electrical charge is quantized”? Quantization is the concept that a physical quantity can have only certain discrete values. The electrical charge is quantized, meaning that charge exists only in little "pieces" of definite size. The size of any electric charge is an integer multiple of this smallest possible "elementary charge." Anology Consider the following analogy; an electric charge is like a brick wall - the smallest possible size for a wall is one brick, a single brick is like the elementary charge. Just as a wall cannot exist that is smaller than one brick, an electric charge cannot exist that is smaller than the "elementary charge." Any large brick wall is made up of an integer number of bricks, just as any large electric charge is made up of an integer number of elementary charges. • The beauty of the oil drop experiment lies in its simple and elegant demonstration of the quantization of charge along with measuring the elementary charge on an electron . • Millikan found that the charge of a single electron was • e =1.6 x 10-19 C. Nobel Prize • Robert Millikan was awarded the Nobel Prize in 1911 for determining (through a now famous experiment) the charge on the electron. that is also known as the elementary charge or fundamental unit of charge. Among all physical constant there are two which will be universally admitted to be of predominant importance; the one is the velocity of light , which now appears in many fundamental equations of theoretical physics, and the other is the ultimate , or elementary, electrical charge • Robert Millikan • Millikan states in his paper that these results were a • ...direct and tangible demonstration...of the correctness of the view …that all electrical charges, however produced, are exact multiples of one definite, elementary electrical charge.. Controversy about the Oil Drop Experiment and Discovery • Robert Millikan was the sole recipient of the Nobel Prize in Physics in 1923 for both his work in this classic experiment and his research in the photoelectric effect. • Fletcher’s work on the oil drop project, however, was not recognized. Many years later, the writings of Fletcher revealed that Millikan wished to take the sole credit for the discovery in exchange for granting him a Ph.D. and helping him secure a job after his graduation. Controversy about the Oil Drop Experiment and Discovery • With the progress of time, considerable modifications have been made to the original setup resulting in obvious perfection in the results. • Still, no substantial deviation from the results of the classical experiment could yet be found. Success criteria • A students achieves the goal if he/she • Knows the values of charge and mass of electron • Is able to solve the problems Matching the questions 1. 2. 3. 4. 5. 6. 7. 8. 9. What was Millikan's experiment called? The spray bottle produced fine droplets of oil, under the influence of gravity, what would happen to some of the droplets? When some of the droplets needed extra electrons, how were they dislodged? How was the charge on the plates adjusted? How did Millikan determine the mass of the oil drops? How did Millikan determine the charge of each drop? Because atoms are electrically neutral, what must they contain? Because electrons are so much smaller in size than atoms, what must the atoms contain? What did Millikan conclude? Answers 1. 2. 3. 4. 5. 6. 7. 8. Millikan's Oil Drop Experiment They would fall through the hole By the gases in the air by x-rays To offset the gravitational force on the droplet Their radii and density Whole number multiple of the electron's charge A positive charge to balance the negative charge Additional particles that account for most of the mass 9. charge and mass of electrons • http://www.animatedscience.co.uk/blog/wpcontent/uploads/focus_fields/millikan__millik an-expt.html
