Electrical Energy and Current Current and Resistance

Current and Charge Movement 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

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

Current and Charge Movement Current is based on the movement of charge carriers Electrons (negative charge) and protons (positive charge) However, the general convention for current assumes motion of positive charge Because current was defined by Ben Franklin before the structure of the atom was known Materials that have free electrons are good electrical conductors and current easily passes through Metals Ions in solution (electrolytes)

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

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

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

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

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

Behaviors of Resistors

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

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

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

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

Electrical Resistance

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