Electric Currents

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Transcript Electric Currents

Electric Currents
The Electric Battery
Electric Cell
Electric Current
When a continuous conducting path is connected between
the terminals of a battery, we have an electric circuit.
When such a circuit is formed, charge can flow through
the wires of the circuit, from one terminal of the battery to
the other. A flow of charge such as this is called as electric
current.
Q
I
t
Units of Current
1 Ampere (A) = 1 C /1 s
Conceptual Example: How to Connect a Battery.
What’s wrong with each of circuits shown below?
Conventional Current
When we speak of the current flowing in a
circuit, we mean the direction positive charge
would flow.
Important: Current is not a vector, it’s a
scalar!!!!
Important : In any single circuit, the current
at any instant is the same at one point as at
any other point!!! This follows from the
conservation of electric charge.
Ohm’s Law: Resistance and
Resistors
I~V
Definition of Resistance
I=V/R
Ohm’s Law
The current through a metal conductor is
proportional to the applied voltage.
Resistivity
L
R
A
Conceptual Example: Stretching
Changes Resistance
A wire of resistance R is stretched
uniformly until it is twice its original
length. What happens to its resistance?
Effect of Temperature
T  0 (1   (T  T0 ))
 – temperature coefficient of
resistivity is positive for metals
and negative for semiconductors.
Superconductivity
1. When you stack three flashlight batteries in the same direction,
you get a voltage of 3x1 ½ volts = 4 ½ volts. What voltage do
you get if one of the batteries is turned to face in the opposite
direction?
2. What is the difference between a bulb burning out and
removing the bulb from its socket?
3. If the resistance connected to a battery is cut in half, what
happens to the current through the battery?