1. Current and Resistance

2. Voltage and Current
Electrical potential energy – potential energy of a charged object due to its position in an electric field
Potential difference – the change in the electrical potential energy per unit charge
Voltage
Measured in volts (V)
Measured with a voltmeter, which has high resistance and is set parallel to a circuit element

3. Current and Charge Movement
Electric Current – the rate at which electric charges move through a given area
Movement of electric charge
Symbolized by I and measured in amperes (A)
1 A = 1 C/s
Electric current = (charge passing through a given area) / (time interval)
I=ΔQ/Δt

4. Drift Velocity
Drift velocity (vdrift)– the net velocity of a charge carrier moving in an electric field
Charge carriers are set in motion by electric forces generated by electric fields
This motion creates current
Motion is random due to particle collisions, but overall in the opposite direction of the electric field
Drift velocities are very small

5. 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

6. Behaviors of Resistors
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

7. 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

8. 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

9. Behaviors of Resistors

10. 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
Used in stress tests and some lie detector tests
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

11. 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

12. Electrical Resistance
Conductors have low resistance
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

13. Electrical Resistance
Meissner effect – the interaction between current in a superconductor and a magnetic field causes the magnet to levitate
Insulators have high resistance
Grounding – providing a pathway for current to leave a charged object
Semiconductors have intermediate resistance

14. Electrical Resistance

15. Electrical Resistance
Potentiometers – a special resistor that has a variable resistance
Volume controls
Adjustable lights
Joysticks on video game controlers