Chapter 18 Notes College Physics by Giambattista et al. Basic DC ...

Chapter 18 Notes
College Physics by Giambattista et al.
Basic DC Circuit:
“V – C – R”
+
voltage LOAD
source
–
Voltage Sources:
1) Batteries: chemical electrical
2) Solar cells: light electrical
3) Generators: magnetism electrical
4) Thermocouples: heat electrical
5) Piezoelectric crystal: pressure electrical
Emf E – “electromotive force”
theoretical, ideal voltage difference
voltage difference when source is not in use
Electric Current I
net flow of charge
conventional vs. electron flow
ac vs. dc
I
q
t
SI unit: ampere (A) = coulomb/sec
Resistance R
measure of how well a device conducts electricity, e.g. high R low I
depends on:
1) material 3) length
2) temperature 4) width (thickness)
For a wire,
R
L
A
SI unit: ohm (Ω)
ρ is the resistivity in Ω-m, Table 18.1, p.646
Resistors
have a fixed resistance
may be purchased commercially
regulate voltage and current, essential to every circuit
John B. Ross, Ph.D.
IUPUI Physics Dept.

Chapter 18 Notes
College Physics by Giambattista et al.
Ohm’s Law: ΔV = IR for resistors only!
Terminal Voltage V t
voltage difference between the terminals of a power supply when it’s in use
V t = E
V t = E – Ir
V t = E + Ir
when not in use
when energy leaves battery (to power circuit)
when energy enters battery (“charging”)
Electric Power
rate of electrical energy transfer
U
E qV
P P = IΔV in general Note: watt = amp*volt!
t
t
For resistors, ΔV = IR
P = I 2 R
or
2
V
P
R
Joule heat formulas
Question:
What are you being billed for by the electric company??
Energy measured in kilowatt-hours = (kilowatts)(hours) = PΔt
In the SI,
joules = (watts)(sec)
Resistor Combinations
I. Series – like a train, same current
R 1 R 2 R 3 R eq = Σ R i
E
When one goes out, they all go out – “open circuit”
II. Parallel – like a ladder, same voltage difference
1 1
R eq
R i
E R 1 R 2 R 3
John B. Ross, Ph.D.
IUPUI Physics Dept.

Chapter 18 Notes
College Physics by Giambattista et al.
When one goes out, the rest stays on.
More paths, more current to flow potential hazard!
discuss fuses, circuit-breakers
Short circuit – illegal parallel connection, path of least resistance
Solving DC Circuits: Two Methods
Method#1 – Resistor Combinations
Method #2 – Kirchhoff’s Rules
Junction Rule: Σ I in – Σ I out = 0 at any single junction
Loop Rule: Σ V rise – Σ V drop = 0 around any closed loop
Solving Multiloop Circuits Using KR:
1) Draw and label currents I 1 , I 2 , I 3 in each branch.
2) Write terminals on each device:
3) Pick a junction and apply the Junction Rule.
4) Apply the Loop Rule to two different loops.
5) You have 3 equations, 3 unknowns – solve for I 1 , I 2 , I 3 .
RC Circuits
Have you ever used an expensive camera with a flashbulb that needs to charge before you
snap the picture?
I. Charging Circuit
S V C (t) = E(1 – e –t/τ ) τ = RC
I(t) = (E/R)e –t/τ
E C
max
R 0 t
II. Discharging Circuit
S V C (t) = Ee –t/τ τ = RC
I(t) = (E/R)e –t/τ
R C
max
0 t
John B. Ross, Ph.D.
IUPUI Physics Dept.