Chapter 18
Direct Current Circuits
1
Chapter 18
1.
2.
3.
4.
emf
Resistors in Circuits
Kirchhoff’s Rules
RC circuits
2
Sources of emf
What is emf?
What is the SI
units?
What is terminal
voltage?
3
Internal Resistance
What is internal
resistance?
How do I calculate
terminal voltage?
What about the
circuit?
What is an open
circuit?
What is a load
resistance?
The case for R>>r
4
Example
A battery having an emf of 9 V delivers
117 mA when connected to a 72
ohms load. Determine the internal
resistance of the battery.
5
Resistors in Series
What is a series
connection?
What happens to
current?
What about
voltage?
What about
equivalent
resistance?
6
Equivalent Resistance –
Series: An Example
Four resistors are replaced with their
equivalent resistance
7
Resistors in Parallel
What is parallel
connection?
What happens to
current?
What about
voltage?
How do I
calculate
equivalent
resistance?
8
Equivalent Resistance –
Parallel Example
9
Equivalent
Resistance –
Complex
Circuit
10
Parallel Circuits
What is the advantage?
11
Gustav Kirchhoff
1824 – 1887
Invented
spectroscopy with
Robert Bunsen
Formulated rules
about radiation
Formulated the
Kirchhoff’s rules
12
Kirchhoff’s Rules
Why do you need kirchhoff’s rules?
13
Statement of Kirchhoff’s
Rules
Junction Rule
The sum of the currents entering any
junction must equal the sum of the currents
leaving that junction
A statement of Conservation of Charge
Loop Rule
The sum of the potential differences across
all the elements around any closed circuit
loop must be zero
A statement of Conservation of Energy
14
More About the Junction
Rule
15
Setting Up Kirchhoff’s
Rules
Assign symbols and directions to the
currents in all branches of the circuit
If a direction is chosen incorrectly, the
resulting answer will be negative, but the
magnitude will be correct
When applying the loop rule, choose a
direction for transversing the loop
Record voltage drops and rises as they
occur
16
Convention for loop rule
17
Examples
18
RC Circuits
What is an RC circuit?
What is a time constant?
19
Charging Capacitor in an
RC Circuit
20
Discharging Capacitor in
an RC Circuit
Charge
21
Example
22
Electrical Safety
Electric shock can result in fatal burns
Electric shock can cause the muscles of
vital organs (such as the heart) to
malfunction
The degree of damage depends on
the magnitude of the current
the length of time it acts
the part of the body through which it passes
23
Effects of Various Currents
5 mA or less
10 mA
Can cause a sensation of shock
Generally little or no damage
Hand muscles contract
May be unable to let go a of live wire
100 mA
If passes through the body for just a few
seconds, can be fatal
24
Ground Wire
Electrical
equipment
manufacturers
use electrical
cords that have a
third wire, called
a case ground
Prevents shocks
25
Ground Fault Interrupts
(GFI)
Special power outlets
Used in hazardous areas
Designed to protect people from
electrical shock
Senses currents (of about 5 mA or
greater) leaking to ground
Shuts off the current when above
this level
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