The Nature of Resistance Original: Revision: 7 August 2008; 29 June 2010; 6 November 2013 Authors: Jim Overhiser and Julie Nucci Appropriate Level: Regents and AP Physics Abstract: Students perform a series of guided activities that illustrate how microscopic and macroscopic features of metals relate to resistance and current flow. Activities include: thermal considerations of resistance, electron under an applied voltage, the effect of defects on resistivity, and the effect of length and cross-sectional area on resistance. Time Required: Three to four 40 minute lab periods NY Standards Met: Energy may be stored in electric or magnetic fields. This energy may be transferred through conductors or spaces and may be converted to other forms of energy. (4.1j) All materials display a range of conductivity. At constant temperature, common metallic conductors obey Ohm’s Law. (4.1l) The factors affecting resistance in a conductor are length, cross-sectional area, temperature and resistivity. (4.1m) Special Notes: This activity assumes knowledge of atomic bonding, Ohm’s Law and conceptual definitions of current, resistance, and voltage. These materials are made available to Purdue University Science Express through a collaboration with the Center for Nanoscale Systems Institute for Physics Teachers (CIPT), Cornell University, Ithaca, NY.
3b. Thermal Motion + Voltage + Grain boundaries: (Effect of defects on an electron drifting at temperatures above absolute zero) Grain boundaries, the regions where differently oriented crystals in a metal meet, are the dark lines in the figure to the They separate the light regions in between them, called grains. Grain boundaries are defects since the arrangement of atoms is more disordered there than within the well ordered grains. Rules of this activity: 1. Begin at the black electron dot. 2. Roll the die. The number rolled indicates the next atom the electron will move toward. 3. Here are the rules for moving to the next location (read carefully – there are additional rules for defects!): (1) Use the rules for the voltage activity. (2) In addition, you can only move across a grain boundary if you are on one of the adjacent atoms (indicated with a *). (3) When you are at an atom along the grain boundary, you must roll the number of the atoms just on the other side of the boundary to cross it. If you do not roll this number, the electron must scatter back to the closest number you rolled. (4) Once you have moved passed a grain boundary, you can not cross back. 4. Draw a dot in the numbered circle to indicate the new position of the electron and an arrow from the 1 2 present electron position to the new location based on the number rolled. 5. Continue this process until you have reached the end of the wire or rolled the dice 15 times. 6. Draw a fat arrow from the original to the final electron position. This vector represents the net motion of the electron. 7. Answer questions on the student sheet. Wire connected in a circuit with an applied voltage indicated by + and - signs Grain boundaries Wire 1 2 4 6 5 *3 3 5 1 2 4 *6 2 4 6 3 1 5 2 1 4 2 6 3 4 *1 *5 3 1 2 4 5 3 6 4 3 2 6 6 1 4 3 5 2 4 2 1 6 4 * 1 5 *3 6 5 3 4 * 2 2 1 6 2 *1 3 3 5 3 5 6 1 4 2 1 4 3 2 2 3 1 *5 5 3 6 3 2 1 4 * 6