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Current, resistance, and
electromotive force (emf):
Chapter 25 (sec. 1-5)
• Charges (electrons) moving in a conductor
• Ohm’s Law & resistance to flow of charge
• Energy and power in electrical circuits
C 2012 J. Becker
Learning Goals - we will learn:
• The meaning of electric current, and how
electric charges move in a conductor.
• What is meant by the resistivity of a
substance.
• How to calculate the resistance of a
conductor from its dimensions and its
resistivity.
• How an electromotive force (emf) makes it
possible for current to flow in a circuit.
• How to do calculations involving energy and
power in circuits.
ELECTRON MOTION IN A CONDUCTOR
WITH AND WITHOUT AN ELECTRIC FIELD
ANALOGY OF A CHARGE MOVING
IN A CONDUCTOR
12 Volts
0 Volts
CONDUCTOR WITH CONVENTIONAL
CURRENT MOVING FROM
HIGH ELECTRICAL POTENTIAL (VOLTS)
TO LOW POTENTIAL
“CONVENTIONAL” CHARGES
DRIFTING IN A CONDUCTOR
HIGHER POTENTIAL
LOWER POTENTIAL
LAMP
Which Box (A, B, or C) has the most resistance
to the flow of electric charge (current)?
Each lamp has the same amount of resistance
to the flow of charge. Current is the flow
of charge past a point in the circuit per unit
time interval.
C 1998 McDermott, et al., Prentice Hall
Which network has the most resistance to the
flow of charge?
Rank the networks according to decreasing
resistance.
C 1998 McDermott, et al., Prentice Hall
1. Rank the brightness of the
bulbs (bright to dim).
2. A wire is added as shown below.
a) Does the brightness of bulb C
increase, decrease, or remain
the same?
b) Does the brightness of bulb A
increase, decrease, or remain
the same?
c) Does the current through the
battery increase, decrease, or
remain the same?
C 1998 McDermott, et al., Prentice Hall
Resistance (R) is proportional to resistivity (r):
R = r L / A
The resistivity (r) depends on temperature and the
physical properties of the material, so it has a
different value for each material.
Temperature dependence of resistance (and
resistivity) is generally linear over limited
temperature ranges and is characterized by the
temperature coefficient of resistivity (a):
R(T) = R0 [ 1 + a (T-T0)]
r(T) = r0 [ 1 + a (T-T0)]
where R0 and T0 are the resistance and temperature
at a standard temperature, usually room temperature
or 20o C. (Measured in Lab #5)
Current – voltage relations
a) a resistor obeys
Ohm’s Law: I = V/R
with constant slope = 1/R
(or DV = I R)
b) A vacuum tube diode
c) A semiconductor diode
Electric potential (DV) rises and drops in a
circuit (from previous slide)
CIRCUIT ENERGY and POWER
R
P = Vab I = I2 R = Vab2/ R
e I = rate of conversion of nonelectrical (chemical) energy to
electrical energy within the
source
I2 r = rate of electrical energy
dissipation in the internal
resistance of the source
(battery)
e I - I2 r = the rate at which
the source delivers electrical
energy to the load (headlight)
You can view a 4-minute youtube video of a
smoke detector
and how it works,
from classmate Marjo Mallari
http://www.youtube.com/
watch?v=oFUUQcpGR3k