Transcript Lecture 11 ppt version

Current, Resistance
Chapter 24
24.1
Current
24.2
Conductivity and Resistivity
24.3
Resistance and Ohm’s law
24.4
Electric power
Question: When you turn the ignition key in the car, you complete a
circuit from the negative battery terminal through the electric
starter and back to the positive terminal. This is a DC circuit and
electrons migrate through the circuit from negative to positive
terminal. About how long must the key be in the ON position for
electrons starting from the negative terminal to reach the positive
terminal?
A. A time shorter than the human reflex turning a switch on or off
B. ¼ second
C. 4 seconds
D. 4 minutes
E. 4 hours
Answer: about 4 hours
Without electric field
With electric field
Why does the light come on
at once if vd is so small?
24.1 Current, drift velocity and current density
0.28 mm/s - but they all start moving at once
24.2 Conductivity and Resistivity
The presence of electric fields in conductors results in electric
current
No longer have electrostatic equilibrium
Collisions oppose the electric force, and the electric field sustains a
steady current
Answer: No
No voltage
difference across
its body, no current
flows
24.3 Resistance and Ohm’s Law
V
R
I
defines R for any conductor
Now will
the bird
get….
?
24.4 Electric Power
Answer: D
It consumes the
most energy per
second (power)
Answers: B
An electric motor
consumes energy,
not electricity.
A generator
generates electric
energy