Nature of the Atom: Charge to mass Ratio Studies of atoms from John Dalton's atmospheric studies indicated that properties were cyclic moving from group to group. This suggested some unit of atomic structure existed from group to group. Atoms of different elements differ in the amount and arrangement of these units. In 1855, Heinrich Geissler invented a powerful vacuum pump. Dalton's Contributions to Science 1
Cathode Rays: Julius Plucker connected an evacuated tube to a battery and noticed that current still flows at low pressure. With an anode and a cathode added to the tube, a glow was observed. Eugen Goldstein named these rays cathode rays. They were produced by a cathode and by passing current through a gas, the glow was observable to a better degree. 2
Crookes In 1875, Sir William Crookes used a bent tube and noticed that the glow did not follow the bend but travelled in a straight line like light. Crookes, in his experiments placed a Maltese cross at the end of the tube. He reasoned that a barrier would stop the rays and it did produce shadows. 3
Cathode Ray Properties: 1) Cathode rays travel in a straight line (rectilinear propagation) perpendicular to the surface of the cathode and therefore produce shadows. 2) Magnetic fields deflect cathode ray's path. 3) Charged objects deflect cathode ray's path. 4) They produce chemical reactions that are triggered by light (silver salts change colour in light or cathode rays.) 5) Changing the material of the cathode still produced cathode rays. 4
Magnetic and Electric Fields As magnetic fields affect them they behave like an electric current. The electric field tests verified that there were negative particles in cathode rays. In 1897, J.J. Thomson (1856-1940) proved finally that cathode rays are negatively charged particles (electrons). He passed them through electric and magnetic fields to get their charge - to mass ratio. He found that all cathode rays had a q/m = 1.76 x 10 11 C/kg 5
A magnetic field alone would cause perpendicular particles to travel in a circle: F c = F m A magnetic field can deflect cathode rays into circular paths! mv 2 = qvb r q = v m Br Need: v, B and r --> B is controlled by the coils generating the magnetic field (Number of turns of wire, type of metal, radius, current) and r could be measured. Need to find the speed of the charges! 6
The next part of the experiment: add an electric field to counteract the magnetic field deflection...cathode rays went into a straight line again. Fm = Fe qvb = q E v = E B Note: E is determined by applied voltage V and plate separation d: E = V/d Now the charge to mass ratio was found: for cathode rays derived from any metal, the ratio was the same. Therefore all these materials shared some common component - electrons! 7
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Thomson found that cathode ray particles are: 1) emitted by a wide variety of cathode materials. 2) smaller in mass than hydrogen (the smallest known particle in those days). 1. The electron is a fundamental particle of matter. 2. With Millikan's oil drop experiment, the charge of the electron was known so the mass of the electron was found. 3. The H ion is 1836 me 1.66 x 10-27 kg (atomic mass unit) and this was the smallest known particle at the time. 9
Example: An accelerating potential of 250 V is used on chlorine ions before entering a 1.00 T magnetic field. Chlorine gains an electron to become a stable ion. There are two isotopes of chlorine (Cl-35 and Cl-37) with masses of 5.80 x 10-26 kg and 6.10 x 10-26 kg respectively. Find the radii of the isotopes in the magnetic field. 1 10
Example 2: Determine the charge to mass ratio for the following situation: a) An electric field using 232 V applied to parallel plates 2.33 cm apart, straightens a beam of charged particles in a magnetic field of 23.7 T. b) In the 23.7 T magnetic field, the charged particles are deflected into a radius of 2.33 mm. 7.61 x 10 3 C/kg 11
IB Homework: Giancoli Page 799 #1, 2, Page 802 #93 and q/m worksheet. 4U Homework: q/m worksheet 12
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