Tuesday, November 9, 2010

FINAL review question #16

How did Thomson determine that the cathode ray was negatively charged?

In 1897, the electron was discovered by an English physicist named J. J. Thomson. An electron is a negatively charged subatomic particle. To discover the electron, Thomson experimented by passing electric currents through gases at low pressures. The gases were sealed in glass tubes that were closed at the ends by two electrodes, or metal disks. The electrodes were connected to a source of electricity. One electrode was positively charged and one electrode was negatively charged. The positively charged electrode was called an anode and the negatively charged electrode was called a cathode. The result of the two electrodes was a cathode ray, or a glowing beam, that traveled from the cathode to the anode. A cathode ray is deflected by a magnet. It is also deflected by electrically charged metal plates. The cathode ray is attracted by the positively charged plats, and is repelled by the negatively charged plate. Thomson knew when conducting this experiment that opposite charges attract and negative charges are repelled from each other. Thomson hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at a high speed. Thomson originally referred to these particles as corpuscles, but they were later renamed as electrons. J. J. Thomson set of an experiment to measure the ratio of the charge of an electron to its mass so he could prove his hypothesis. It was discovered that the ratio was constant and the charge-to-mass ratio of electrons did not depend on the type of gas in the cathode-ray tube or the type of metal used for the electrodes. Thomson was able to conclude from his experiment that electrons must be parts of the atoms of all elements.