Thursday, November 10, 2011

Exam Review #16


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


In the late 1800s, experiments were performed where electic currents were passed through various gases at low pressures. It was noticed that when current was passed through a cathode-ray tube, the surface directly opposite of the cathode in the tube glowed. They hypothesized that the glow was caused by a stream of particles, called cathode ray. The ray went from the cathode (a metal disk) to the anode (a metal disk).























In the Cathode Ray Tube Expirament JJ Thomson put a postitive and a negative magnet on opposite sides of the Cathode Ray, instead of going straight, the particles were deflected by the negative magnet. It was observed that:


1. Cathode rays were deflected by a magnetic field in the same manner as a wire carrying electic current, which was known to have a negative charge.


2. The rays were deflected away from a negatively charged objects.






These observations led to the hypothesis that the paricles that make cathode rays are negatively charged. J J Thomson had an investigation where he was able to measure the ratio of the charge of cathode-ray particles to their mass. The ratio was always the same, no matter what metal he used to make the cathode or the nature of the gas inside the cathode-ray tube. He concluded that all cathode rays are made up of indentical negatively charged paricles, which were named electrons.