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
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
When resistors are in parallel, the voltage drop
across each is the same.
3
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
4
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!
5
12pF
When fully charged at QCV(12pF)(24v) the
circuit has two competing voltage steps 24v
24v 0
-
24 volts
6
6 ?
12 ?
3 ?
12 V
The voltage drop is

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

7
2 ?
4 ?
12 V
The current through the 2? resistor is

1. half the current through the 4?.
2. the same as thru the 4?.
3. twice the current through the 4?.

8
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

9
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

What is the VOLTAGE DROP across each resistor?
10
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
11
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!