PHY 2049 Chapter 26 Current and Resistance

Chapter 26 Current and Resistance In this chapter we will introduce the following new concepts: -Electric current ( symbol i ) -Electric current density vector (symbol ) -Drift speed (symbol v d ) -Resistance (symbol R ) and resistivity (symbol ρ ) of a conductor -Ohmic and non-Ohmic conductors We will also cover the following topics: -Ohm’s law -Power in electric circuits (26 - 1)

Physical Resistors

What Happens? “+” REMEMBER, THE ELECTRONS ARE ACTUALLY MOVING THE OTHER WAY!

What’s Moving?

What is making the charged move?? Battery

KEEP IN MIND A wire is a conductor We will assume that the conductor is essentially an equi-potential It really isn’t. Electrons are moving in a conductor if a current is flowing. This means that there must be an electric field in the conductor. This implies a difference in potential since E=  V/d We assume that the difference in potential is small and that it can often be neglected. In this chapter, we will consider this difference and what causes it.

DEFINITION Current is the motion of POSITIVE CHARGE through a circuit. Physically, it is electrons that move but … Conducting material  Q,  t

Conducting material  Q,  t

i + q conductor i - q conductor (26 - 3)

UNITS A current of one coulomb per second is defined as ONE AMPERE.

ANOTHER DEFINITION

(26 - 5)

+ - i V (26 - 6) R

Ohm’s Law

Graph

Ohm A particular object will resist the flow of current. It is found that for any conducting object, the current is proportional to the applied voltage. STATEMENT:  V=IR R is called the resistance of the object. An object that allows a current flow of one ampere when one volt is applied to it has a resistance of one OHM.

+ - i V (26 -7)

How can a current go through a resistor and generate heat (Power) without decreasing the current itself?

Loses Energy Gets it back Exit

Conductivity In metals, the bigger the electric field at a point, the bigger the current density.  is the conductivity of the material.  =(1/  ) is the resistivity of the material

Range of  and 

REMEMBER

Temperature Effect TT 

A closed circuit

Power

V ( )