Download A) 2

Circuits
Which of the equations is valid for the circuit shown below?
1
A) 2 – I1 – 2I2 = 0
B) 2 – 2I1 – 2I2 – 4I3 = 0
I2
C) 2 – I1 – 4 – 2I2 = 0
D) I3 – 2I2 – 4I2 = 0
E) 2I1 – I1 – 4I2 = 0
2
6V
22VV
4V
I1
1
3
I3
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
Solve first equation for y:
x+y =7
y =7–x
Plug into second equation
3x – 2(7 – x) + 1 = 0
E) 17 / 4
3x – 14 + 2x + 1 = 0
5x – 13 = 0
5x = 13
x = 13 / 5
In the circuit below, if all the resistors are identical,
what is the voltage drop across resistor R2?
Same current as R1
and same resistance
V =V
V
V
right loop
V - V = 0
V
R1
R2
V = ?
=8V
Loop rule
(bottom left loop)
V = –8 V
How do the currents
compare here and here?
Give me an expression that describes the current I1 through R1
In the circuit below all the resistors are identical.
Same current as R1
and same resistance
V =V
I1 = 2I2
V
V
right loop
V - V = 0
V
R1
R2
V = ?
=8V
Loop rule
(bottom left loop)
V = –8 V
Both same
current, I2
What equation best describes the CW full outermost loop?
1) 8v + I2R1 + I2R - I2R2 = 0
2) -8v + I2R1 + I2R + I2R2 = 0
3) -8v + 2I2R1 + 2I2R + I2R2 = 0
4) 8v - 2I2R1 - 2I2R - I2R2 = 0
In the circuit below all the resistors are identical.
Same current as R1
and same resistance
V =V
I1 = 2I2
V
V
right loop
V - V = 0
V
R1
R2
V = ?
=8V
Loop rule
(bottom left loop)
Both same
current, I2
V = –8 V
What is the voltage drop across resistor R2?
1) 0.2 volts
3) 1.2 volts
5) 2.0 volts
2) 1.0 volts
4) 1.6 volts
6) 4.0 volts
Sample Problem
I2
V1 = 9 V
Determine the magnitudes and
directions of the currents
through R1 in the figure at right.
2
R2 = 15 
I1
Label Currents
1
Junction Rule
I1 = I2 + I3
Loop Rule (1)
+V2 – I1R2 = 0
Loop Rule (2)
+I1R2 + I2R1 + V1 = 0
Algebra
Solve Loop Rule (1) for I1
Plug into Loop Rule (2)
Comments
R1 = 25 
I3
V2 = 6 V
I2 =
–V1 – V2
R1
I2 = –3/5 Amp What does the minus sign mean?
Current goes in opposite direction of arrow