Transcript Series and Parallel Circuits 2_ppt_RevW10

Physics 106 Lesson #12
Series & Parallel Circuits II
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
Review:
Series Resistors
• For resistors R1 & R2
connected in series
(sequentially), the
current i passing
through each resistor
must be the same
• The voltages across
R1 & R2 must add up
to V (Loop Rule)
V
+
Review: Parallel Resistors
• Resistors R1 & R2
connected in parallel
have the same potential
difference (voltage) V
across them
• Charge must be
conserved, so the
currents I1 & I2 flowing
through the two resistors
must add up to the total
current I leaving the
battery (continuity for
electric current!)
Junction Rule
V
+
I  I1  I 2
Review: Power
• Power has a precise definition in physics:
Power is the rate at which work is done or
how much work is done per unit of time
W
P
t
Units: J/s  Watt (W)
James Watt
invented the
steam engine
In an electric circuit
power is the product
of current and voltage:
Power  I V
Concept Test #1
The circuit in front of you initially contains a single light bulb,
bulb A, that burns with a certain brightness. We will now add an
identical bulb, bulb B, in series with bulb A. What will happen?
(Hint: brightness is determined by power and hence current.)
1) Both will shine with the same brightness as bulb A did
before.
2) Both will shine with equal brightness, but they will be
dimmer than before.
3) Both will shine with equal brightness, but they will be
brighter than before.
4) Whether A or B is brighter depends on the direction of
the current.
A
The total resistance in the circuit increases after adding light
bulb B, so the current decreases. Because the light bulbs are
in series, the current though each light bulb is the same.
B
Concept Test #2
A
The circuit in front of you contains a single light
bulb, bulb A, that burns with a certain
brightness. We will now add an identical bulb,
bulb B, in parallel with bulb A. What will
happen?
1) Both will shine with the same brightness
as bulb A did before.
2) Both will shine with equal brightness, but
they will be dimmer than before.
3) Both will shine with equal brightness, but
they will be brighter than before.
4) Whether A or B is brighter depends on the
direction of the current.
B
In parallel, bulb B is
connected across the
battery in exactly the
same way as bulb A
and will draw the same
current as A, resulting
in the same brightness
as bulb A.
Kirchhoff's Rules
I1 = I2 + I3 + I4
Two simple rules which
apply to any circuit:
1.
2.
Junction rule: the total
current into any point =
total current out (what
flows in must flow out…)
Continuity for current!
Loop rule: around any
closed loop, increases in
potential equal decreases
in potential  the electric
force is conservative no
work done in a closed
loop  you end up at the
same potential (energy) as
where you started (just
like gravity)!
I2
I3
I1
I4
A
B
Start and
End Here
Gustav Robert Kirchhoff
V12  VA V23  VB
6 V  VA  VB  0
 6 V  VA  VB