Physics Lesson Plan
Teacher
Howard
Unit Title
Length
Goal(s)/PLO(s):
Course
Grade Level
Block/Period
Phys 12
12
K1 apply Ohm’s law and Kirchhoff’s laws to
direct current circuits
 define resistance in terms of Ohm’s law
 solve problems involving
– electric potential difference
– current
– resistance
 calculate the total (equivalent) resistance for
resistors connected in parallel, series, or a
combination of both
19-01
 draw and interpret circuit diagrams
 construct circuits from schematic diagrams
 define electromotive force (emf), terminal
voltage, and internal resistance
 solve problems using
– terminal voltage
– electromotive force (emf)
– internal resistance
– current
– electric potential difference
Materials:
Timeline
Date
Class Size
Lesson #, of
Class Activities
Introduction
Body
Notes 19-01, 19-02
Closure
Questions 1-8, Problems 1-21 odd
Chapter 19 DC Circuits
All electric devices are essentially
circuits with power sources, wires,
switches and resistors combined in
various ways. We will now look at
how to analyze and understand simple
circuits
See table 19-1 for the symbols for
circuit elements
19-01 EMF and Terminal Voltage
All circuits need a device that transforms some form of energy into
electrical energy
• Battery, generator, solar cell…
These devices are sources of electromotive force (emf)
• Not a force measured in Newtons
• In equations the symbol ε is used
o Don’t confuse with E for electric field
EMF (ε)
The potential difference between the terminals of a source when no
current flows. Units are volts.
When you start a car with the lights on, the lights dim.
• The voltage difference across the terminals of the battery
momentarily dropped below the rated emf.
o The chemical reactions
in the battery couldn’t
keep up with the large
current drawn by the
starter
o The charge must move
within the battery and
there is some internal
resistance r.
The Terminal Voltage
The emf minus the voltage drop due to the current and the internal
resistance of the battery.
V terminal= ε - Ir
Example 19-1
In many questions we will assume that the internal resistance is
small and that V terminal ≅ ε
19-02 Resistors in Series and Parallel
When elements of a circuit are
connected end to end along a single
path they are said to be in series.
• The same current must pass
through each of the resistors
V is the potential difference supplied
by the battery.
V 1 , V 2 , V 3 are the potential differences across each resistor R 1 , R 2 ,
R3.
circuit-construction-kit-dc_en.jar
From Ohm’s Law V=IR so
V 1 =IR 1, V 2 =IR 2, V 3 =IR 3,
Since the resistors are connected end to end and energy is
conserved the total change in potential V is equal to the sum of the
changes across each resistor.
V = V 1 + V 2 + V 3 = IR 1 + IR 2 + IR 3,
Or
V = I (R 1 + R 2 + R 3 )
V= IR eq
For resistors in series, the equivalent single resistor has a resistance
equal to the sum of the individual resistors.
If resistors are connected in parallel, the current
from the source splits into separate paths
The current I, splits into 3 separate currents I 1 , I 2 , and I 3 . Which
pass through each resistor R 1 , R 2 , R 3 . Since electric charge is
conserved
I = I1 + I2 + I3
The full voltage is applied to each resistor and applying ohm’s law:
For the complete circuit the equivalent resistance from ohm’s law
is:
After combining these 3 equations we get:
The result is that the equivalent
resistance is less than any individual
resistor.
• With the water analogy it is like
adding an extra pipe to let water
flow down from a dam
• There is less resistance and more
current flows
Example 19-2, 19-3, 19-4, 19-5, 19-7
Questions 1-8, Problems 1-21 odd