Electric Charge Measurement
Date: Spring 2013
How do scientists actually measure and determine the electric charges of subatomic particles such as protons (positive charge), neutrons (zero charge), and electrons (negative charge)? For really inquisitive middle school students, providing an accurate and simplified explanation to this question becomes extremely difficult.
The original experiment that measured charge was the Millikan oil drop experiment (http://en.wikipedia.org/wiki/Oil_drop_experiment). Another way to measure charge is to accelerate particles to a known speed, and send them through a magnetic field of known strength. Charged particles are accelerated sideways when they move through a magnetic field (this is called the "Lorentz force"), so they take a curved path. If you know the mass and speed of the particles, and the strength of the magnetic field, and the path the particles took through the field, you can calculate their charge from an equation.
Various creative experiments have been developed for these measurements.
One of the first is the Millikan oil drop experiment. Tiny balls of mist are sprayed between two plates. Often a mist ball can lose an electron during spray. By observing the mist balls with a microscope lens as they fall, and then turning on a strong electric field to watch what happens, it is possible to find the charge of the charged balls. Some will have a charge of +e. Some will have charge +2e.
Another experiment uses magnetic field to turn a beam of electrons along a circular path. The launcher is a Cathode Ray Tube, a device similar to that which launched electron beams at the screen in old-style televisions. The launcher gives information about the electron speed and charge. The size of the loop gives information about the electron?s charge and mass, based on the strength of the magnetic field. When all the information is put together, the experiment gives a very reliable value for (e/m), the ratio of electron charge to electron mass.
In real science and engineering work, direct measurement is often impossible, or at least unreasonable. Indirect measurements based on how something behaves in various situations are often needed. Scientists and engineers must work hard to develop the creativity needed to design such experiments.
Dr. Ken Mellendorf
Illinois Central College
Thanks for the question. Scientists measure the charge on particles by measuring how they curve when the travel in a magnetic field. For instance, a proton curves the opposite way that an electron does. A neutron does not curve at all. All else being equal, the more charge a particle has, the more it curves (i.e., the tighter the curve). At the high school and college levels,
I hope this helps. Please let me know if you have more questions.
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Update: November 2011