23-07-2014, 10:12 AM
Electric Current and Resistance
Electric Current.ppt (Size: 1.4 MB / Downloads: 210)
Part I
Basic electric circuit and its diagram.
What causes the flow of electrons in a circuit.
Drift velocity.
Voltaic cell.
What occurs in a wire when the circuit switch is closed?
An electric field is established instantaneously (at almost the speed of light, 3x108 m/s).
Free electrons, while still randomly moving, immediately begin drifting due to the electric field, resulting in a net flow of charge.
Average drift velocity is about 0.01cm/s.
Question:
If the drift velocity is about 0.01cm/s, why do the lights turn on instantaneously when the circuit switch is closed?
Conventional Current
By tradition, direction in which “positive charges” would flow.
Direction is opposite of electron flow.
Resistors use up Energy
A resistor uses up energy.
When the current goes through the resistor it goes to a lower potential.
Temperature Dependence of Resistance
For metals: as temperature increases the resistance increases. At very low temperatures resistance can become zero: superconductivity.
For semiconductors: the opposite occurs.