Download Lecture 7

Admin:
Lab today:
Light bulb and resistors II: current - voltage (I - V) curves
Don’t forget to complete the pre-lab.
Proposed mid-term date: Wednesday March 18th.
• Assignments: Still a few people not registered for Mastering Physics
• Second graded assignment is posted.
• Due Monday March 2nd
• Grading setting changed from 3 to 4 attempts.
• If you’re not completely sure of your answer, use the hints.
• Don’t forget the other resources available to you – my office hours, my
email, the discussion TAs and their office hours and emails.
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Today
• Sources
• Finishing up combination rules
• Kirchoff’’s laws
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More on Sources
• Sources and Loads
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Source + load = simplest description of any electrical system
Voltage sources and current sources
provide prescribed voltages / currents
Ideal sources: can provide any amount of energy
In the ideal case, Voltage / Current values are not affected by load
– Ideal voltage source:
• Output voltage does not change with current
• Current supplied is determined by load
• Zero internal resistance
– Ideal current source:
• Output current does not change with voltage
• Voltage generated is determined by load
• Infinite internal resistance
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battery
Practical Voltage Sources
Modelled with an ideal source and a
series resistor
Ideal voltage source: rS=0
vL =
RL
vS
rS + RL
voltage divider
rule
So for a practical voltage source, the
output voltage depends upon RL
If rS<< RL , vL = vS , independent of RL
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Some voltage sources
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Series and Parallel EMFs
EMFs in series in the same direction: total
voltage is the sum of the separate voltages.
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Series and Parallel EMFs; Battery
Charging
EMFs in series, opposite direction: total
voltage is the difference (but the lowervoltage battery is charged).
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Series and Parallel EMFs
EMFs directly in parallel only make sense if the
voltages are the same; this arrangement can
produce more current than a single emf.
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Some current sources
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Practical Current Sources
Modelled with an ideal source and a
parallel resistor
Ideal current source: rS= 
iL 
REQ =
1
REQ
RL
iS
current divider rule
rS RL
=
æ1 1 ö r +R
L
ç + ÷ S
è rS RL ø
iL 
rS
iS
rS  RL
The output current now depends upon RL
If rS>> RL , iL = iS , independent of RL
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Series and Parallel EMFs
Current sources in parallel provide a current
equal to their sum. Current sources in series
does not happen.
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Circuit analysis method 1:
Apply element combination rules
Series resistors
Parallel resistors
Series voltage sources
Parallel current sources
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Another Example
R=22Ω
R1=10Ω
R2=20Ω
R3=30Ω
V=10V
Find the equivalent resistance and the current at I
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I= 10V/22Ω = 0.45V