Read Section 3.1 before viewing the slide show.

1. Read Section 3.1 before viewing the slide show.

2. Unit 9The Relationship Between Electricity and the Atom Electrolysis (3.1) Cathode Ray Tubes (3.1) Mass-to-Charge Ratio for Electron (3.1) Positive Particles (3.1) Determination of Electron Charge (3.1)

3. The First Battery (3.1) Alessandro Volta (1800) invents a cell similar to today’s battery This was around the time a bodyof evidence was beginning tosuggest that matter had electricalproperties The battery proved to be useful inexploring these properties and infurther understanding the atom Volta’s battery – alternating discs of zinc and copper with sulfuric acid

4. Experiments with Electricity (3.1) Humphry Davy (early 1800’s) used powerful batteries to break apart compounds and discover new elements Michael Faraday, a protegè of Davy, further developed a field called electrochemistry. Electrochemistry recognizes that charged particles must be present to conduct electricity

5. Cathode Ray Tubes (3.1) Apply voltage across here William Crookes (1875) developed a low pressure gas discharge tube as illustrated below. Screen coated with zinc sulfide – will fluoresce Metal electrodes Most of the air is removed. Application of a voltage across the electrodes leads to a fluorescence on the screen as seen in the right hand image (simulated beam). The beam is called a cathode ray, where the term cathode refers to one of the electrodes. The ray emanates from the cathode. Image from www.crtsite.com

6. The Nature of Cathode Rays (3.1) J.J. Thomson (1897) applied an electric field to the cathode ray in a Crooke’s tube with a result similar to that shown in the figure. The cathode ray was deflected awayfrom the negative plate suggesting thecathode ray has properties ofnegatively charged particles – namedelectrons by Thomson. Based on the curvature of thedeflection, Thomson could determine theratio of the mass of the negative particle to its electric charge. Characteristics of the electrons were independent of the metal used as electrodes – this suggests that the electron is common to atoms of all elements. Notice this is contrary to Dalton’s indivisible atom concept from nearly 100 years earlier.

7. Complementary Positive Particles Image from http://chemed.chem.purdue.edu/ Eugen Goldstein (1886) performedexperiments with a cathode ray tubewhich was perforated as in the image He found the usual electrons going away from the cathode toward the anode (right to left in the image) but also found positive particles moving left to right The positive particles had varying masses depending upon the metal used to make the electrodes in the cathode ray tube.

8. Millikan’s Oil Drop Experiment Oil droplets are sprayed into the upper chamber. As they leave the atomizer, they will acquire negative charges just like static electricity. Alternatively, X-rays or a similar source may be used to produce charges As the charged particles enter the chamber, they fall due to gravity The bottom electrical plate is negatively charged so it repels the falling charged particles. The charged particles may be madeto “hang” in the air by properlyadjusting the voltage. The chargemay be determined from the voltagerequired to suspend the charged particles. Coupled with Thomson’s mass to charge ratio, the charge may be used to find the mass of the electron: 9.109 × 10-31 kg. Image from http://library.thinkquest.org/