Current and Resistance

# Current and Resistance

## Current and Resistance

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##### Presentation Transcript

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