1. Current and Resistance Resistance

2. Behaviors of Resistors Resistance – the opposition to the flow of current in a conductor –Symbolized by R –Measured in Ohms (Ω) = 1V/A Resistance = (potential difference) / current R =  V / I Ohm’s law - resistance is constant for a range of potential differences –Does not hold for all materials Materials that obey Ohm’s law are classified as ohmic –Current versus potential difference graph is linear Materials that do not obey Ohm’s law are classified nonohmic –Current versus potential difference graph is nonlinear –Diode – a common nonohmic material High resistance in one direction and low resistance in the other –Treat all resistors as ohmic unless specifically told otherwise

3. Behaviors of Resistors Resistance is based on length, cross- sectional area, material, and temperature Resistance is caused by electron collisions with the material they are traveling through and other electrons –With each collision, the electrons loose energy –Factors that contribute to electron collisions increase resistance

4. Behaviors of Resistors All else being equal, shorter wires have less resistance than longer wires All else being equal, wires with greater cross-sectional areas have less resistance than those with less cross-sectional area Colder temperatures are typically associated with reduced resistance

5. Behaviors of Resistors Resistors are used to control the amount of current in a conductor –Too much current can overload a circuit Moisture, particularly salt water (including sweat) decreases the resistance of the human body Normal dry resistance is around 500,000  Soaked with salt water, the body’s resistance decreases to about 100  Currents of less than.01A are felt as a slight tingling if they are perceived at all Currents above.15A through the chest cavity are fatal

6. Behaviors of Resistors Galvanic skin response (GSR) test (stress test) – uses perspiration as an indicator of stresses on body –Decreases resistance Certain devices have variable resistances –Carbon microphone in some telephones Compressions in the sound waves decrease resistance by causing a diaphragm to flex inward Rarefactions cause the reverse –This variable resistance converts sound waves into electrical impulses and back to sound waves

7. Behaviors of Resistors Superconductors – a material whose resistance is zero at or below some critical temperature –Varies with each material Usually very cold –Some new high temperature superconductors »Superconduct at 150K –Best conductors such as copper do not exhibit superconductivity Meissner effect – the interaction between current in a superconductor and a magnetic field causes the magnet to levitate