Download ap physics b lesson 72 kirchoff`s laws

• You will need your notes and calculator
today
• Turn in the HW (75pts)
• Blizzard Bag 2 is due this Friday.
Kirchoff’s Laws
Voltage and Current
Kirchoff’s Loop (current) Law
• Kirchoff’s Current Law – The same amount
of charge must enter a junction in a given
time interval as the amount of charge that
leave the junction in the same time interval.
– Itotal = I1 + I2 + I3 ……
Example I
• Find I1
I1
3A
5A
R
R
Answer
• 8A
Example II
• Find IT
IT
30V
5Ω
10Ω
Answer
• 9A
Kirchoff’s Voltage Law
• At any instant, the algebraic sum of the
voltage changes (increases or decreases) in
a loop in an electric circuit must be equal to
zero. (the voltage drops across all resistors
must add up to the total )
– VT = V1 + V2 ….
Example III
• Find the voltage change from points A to C
(VCA)
6V
C
3V
15V
2V
A
8V
Answer
• 9V
Example IV
• Determine a) the current in R2 and b) the
current through R3
10V
R3 = 7Ω
R2 = 4Ω
IT = 2.2A
R1 = 2Ω
Answer
• 1.4A, 0.8A