Physics 2 Lecture Notes

1
A
B
C
Which of the circuits shown above are wired in
parallel? 1) A only 2) B only 3) C only 4)
B and C 5) all of them
2
5 A
P
8 A
2 A
What is the current at point P? A) 2 A B) 3
A C) 6 A D) 10 A E) need more info to answer
3
Kirchhoffs Rules (rules of thumb)

• The SUM OF CURRENTS entering any junction
• equals the sum of all currents leaving.

I1
I2
IB
I3
I1 I2 I3
IC
IA
IC IA IB
This follows from Conservation of Charge!
4
12pF
When fully charged at QCV(12pF)(24v) the
circuit has two competing voltage steps 24v
24v 0
-
24 volts
5
R3
R2
6 ?
12 ?
3 ?
R1
12 V
The voltage drop is

• greatest across R1.
• greatest across R2.
• greatest across R3.
• 4. the same across each of the three resistors.

6
2 ?
4 ?
I
12 V
The current through the 2? resistor is

• half the current through the 4?.
• the same as thru the 4?.
• twice the current through the 4?.
• ½ Ampere 2. 1 Ampere
• 2 Ampere 4. 3 Ampere
• 5. 4 Ampere 6. 6 Ampere

7
Current in each resistor? Total resistance 6
? Ohms Law I V / R 12 V / 6 ? 2 A
2 ?
4 ?
I
12 V
Voltage drop across a resistor? V I R
12 V
8 V
4 V 0 ?
?V
8
Kirchhoffs Rules (rules of thumb)
2. The SUM OF POTENTIAL DIFFERENCES across
all components of ANY closed circuit loop is ZERO.
R2
R1

• IR1 IR2 IR3 IR4
• or
• - IR1 - IR2 - IR3 - IR4 0

e
R3
R4
This follows from Conservation of Energy!
9
Voltage drops enter with a sign and voltage
gains enter with a sign in this equation.
10
Loop Rule Example(without numbers)
The first thing we have to do is? Define
currents!
I2R1
I2R2
Now apply Loop rule to each loop
R1
R2
?
I2
?
I2R1
I2R2
?V
0
I1
I3
?
R3
?
I3 R3
?V
0
I3R3
11
Circuits
ConcepTest
The light bulbs in the circuit are identical.
When the switch is closed
A) both bulbs go out B) the intensity of both
bulbs increases C) the intensity of both bulb
decreases D) nothing changes
12
Circuits
ConcepTest
The light bulbs in the circuit are identical.
When the switch is closed,
a) the intensity of bulb A increases b) the
intensity of bulb A decreases c) the intensity of
bulb B increases d) the intensity of bulb B
decreases e) nothing changes
13
When resistors are in parallel, the voltage drop
across each is the same.
1) A only
C) 6 A
In series circuits the same current
surges through each resistor! The voltage
drop across is IR. Same I, but different Rs!
3. greatest across R3.
2. the same as thru the 4?.
3. 2 Ampere
D) nothing changes
See discussion on following slide
e) nothing changes
14
Answer

• Write a loop law for original loop

12V - I1R 0 I1 12V/R

• Write a loop law for the new loop

12V 12V - I0R - I0R 0 I0 12V/R