Physics 2 Lecture Notes

1
Algebra Test
Suppose x y 5 2 and 3x 2y 1 0. What
is x? A) zero B) 13 / 5 C) 2 / 3 D)
7 E) 17 / 4
2
Algebra Test
Suppose x y 5 2 and 3x 2y 1 0. What
is x? A) zero B) 13 / 5 C) 2 / 3 D)
7 E) 17 / 4
Solve first equation for y x
y 7 y 7 x Plug
into second equation 3x 2(7 x) 1 0 3x
14 2x 1 0 5x 13 0
5x 13
x 13 / 5
3
ConcepTest
Circuits
Which of the equations is valid for the circuit
shown below?
1 ?
A) 2 I1 2I2 0 B) 2 2I1 2I2 4I3
0 C) 2 I1 4 2I2 0 D) I3 2I2 4I2
0 E) 2I1 I1 4I2 0
I2
2 ?
6 V
2 V
2 V
4 V
I3
I1
1 ?
3 ?
4
ConcepTest
Circuits
Which of the equations is valid for the circuit
shown below?
1 ?
A) 2 I1 2I2 0 B) 2 2I1 2I2 4I3
0 C) 2 I1 4 2I2 0 D) I3 2I2 4I2
0 E) 2I1 I1 4I2 0
I2
2 ?
6 V
2 V
2 V
4 V
I3
I1
1 ?
3 ?
5
ConcepTest
R1
R2
? 8 V
In the circuit above, if all the resistors are
identical, what is the voltage drop across
resistor R2? A) 0 V B) 2 V C) 4 V D) 6 V E)
20 V
6
ConcepTest
Loop rule ? (right loop and top left loop)
Same current as R1 and same resistance
R1
?V 6 V
R2
?V ?
4 V
? 8 V
Loop rule (outer loop)
Loop rule (bottom left loop)
In the circuit above, if all the resistors are
identical, what is the voltage drop across
resistor R2? A) 0 V B) 2 V C) 4 V D) 6 V E)
20 V
7
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
8
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
9
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
10
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
11
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
12
Sample Problem
V1 9 V
I2
R1 25 ?

• Determine the magnitudes and directions of the
  currents through R1 in the figure at right.

2
R2 15 ?
I1
Label Currents
1
I3
Junction Rule
I1 I2 I3
Loop Rule (1)
V2 I1R2 0
V2 6 V
Loop Rule (2)
I1R2 I2R1 V1 0
Solve Loop Rule (1) for I1 Plug into Loop Rule (2)
Algebra
Comments
I2 3/5 Amp What does the minus sign
mean? Current goes in opposite direction of arrow