Direct Current Circuits
Vocab
Ammeter: An ammeter is a meter used to measure electric current. The meter reads the
current passing through it.
Circuit breaker: the circuit breaker or fuse is used as a safety device that breaks the
circuit (in other words, opens it) whenever current exceeds a certain designated amount,
usually about 100 A
Ground-fault interrupter: circuit breaker at the point of use
Open Circuit: incomplete loop – a path that that is not closed.
Parallel circuit: Resistors are said to be in parallel when both ends of each resistor are
connected to the ends of the other resistors by ideal conducting paths. Since the
connected ends must be at the same potential, the potential difference across each of the
parallel resistors is the same.
Series circuit: a circuit having its parts connected serially
Short circuit: A short circuit is an ideal conducting path connected across a circuit
element or across a combination of elements
Voltage divider: A number of resistors in series provided with taps at certain points to
make available a fixed or variable fraction of the applied voltage.
Voltmeter: A voltmeter is a device used to measure the difference in electric potential
between two points in an electric circuit
Direct Current Circuits
Chapter notes – IF ANYTHING IN THIS WAS NOT AT ALL CLEAR, email
jannex96@gmail.com, tweet @jannex96, or message me on Facebook (I would prefer the
Facebook message or email as I don’t check twitter)
•
Kirchhoff’s Rules
o First Rule
 The sum of the currents entering any branch point is 0
o Second Rule
 The Sum of the voltage drop around any closed path is 0
o Implications of the rules
 First Rule
• Equal quantities of charge flow into and away from a
branch point. This follows from the fact that charge is
conserved and does not accumulate at the branch point.

Second Rule
• All the positive voltage drops (decreases in potential)
must be canceled by negative voltage drops (increases
in potential); the sum of the voltage drops must equal
zero.
•
o By understanding these, you can pretty much figure out how circuits work
Equivalent Resistance
o Equivalent Resistance: the resistance of a single resistor that would
produce, in the remainder of the circuit, the same effect as that produced
by the network.
 Req=Vab/I
 This gives you the voltage drop
 Notice that it is just an application of Ohm’s law
o Series Resistors
 When there is just 1 path around the battery, ex
 To find the resistance do the following
 R1+R2+R3…=RT
o Parallel Resistors
 A branch comes off of the wire, ex
Direct Current Circuits

•
•
•
To find resistance use this:
• 1/R1+1/R2+1/R3…=RT
 NOTE
• When using this, not the total resistance of the entire
circuit, just of that one section
• This can be used to help you find the current by making it 1
resistance because you can make the entire circuit into a
series circuit
Multiple Circuit problems
o The key to solving these is to simplify using your knowledge of parallel
and series circuits to simplify the resistance down to the point where you
can just use Ohm’s law
Measurement of Current, Potential Difference, and Resistance
o Ammeter
 Measures Current
o Voltmeter
 Measures Voltage
o Ohmmeter
 Measures Resistance
RC Circuits- I don’t believe that we went over these in class, or that they were on
the final, but I would check with Jim on that
o A circuit in which time is a variable
o Suppose that the capacitor is initially uncharged and the switch is open.
Then at t 0 let the switch be closed. Obviously no steady-state current
can exist in this circuit because the capacitor makes it an open circuit. As
soon as the switch is closed, however, there is a temporary, or “transient,”
current. At any instant this current is the same at all points in the circuit
except for the region between the capacitor plates, where there is no
current. We find the instantaneous current by applying Kirchhoff’s second
rule:
o
2 things to know if you are walking into the final
1. Resistance is in Ohms and Current is it amps
Direct Current Circuits
2. Formulas to know are:
a. V=IR
b. R1+R2+R3…=RT
c. Req=Vab/I
d. 1/R1+1/R2+1/R3…=RT
Direct Current Circuits
Problems
Multiple Choice
1. Current is in:
a. Amps
b. Watts
c. Ohms
d. Joules
e. Kumquat
2. Which of the following is a circuit in series:
a. Left image
b. Right image
3. Bill made a circuit. He thinks he did a pretty dam good job of it to. He uses ohms
law and finds out that he has 10V left over at the end of the circuit. He thinks,
“Wow, all those volts have been saved.” What happens when Bill tries to turn the
circuit on?
a. It works incredibly well and he is given a Nobel prize in physics for
creating the best circuit ever.
b. It fails, miserably
c. He made a math error
d. Both A and C
e. Both B and C
4. Which of the following is the current of the diagram below?
a. .50A
Direct Current Circuits
b. .50W
c. 1 W
d. 1 A
e. .50 Kumquats
f. 2 A
g. 3 A
5. What measures current?
a. Voltmeter
b. Geiger Counter
c. A ruler
d. A meter stick
e. Measuring tape
f. Ammeter
Direct Current Circuits
I understand that I didn’t write the problems out myself. I wanted to make sure that you
guys have GOOD problems to practice with rather than just have me show effort because
the entire purpose of this is to prepare us for the final. Please do not consider this to be
laziness; it really took a lot of time to make sure I found the best possible problems for
you guys to practice with.
I would also recommend checking out this website. It has a bunch of plug and chug
ohms law questions, which could be helpful to you.
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=dce11904
Also, this is an ohm’s law calculator, not sure if it helps
http://www.rapidtables.com/electric/ohms-law.htm
If you want to do more problems, I would check out page 926 of the textbook because
that has the problems that have answers for them, so you can check right away if you
were correct. If you don’t want to look, they are:
1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55,57,59
The problems start on page: 538
Short answer:
1.
Solve for R
2. Solve For Current
Direct Current Circuits
3. A CD player with a resistance of 40 ohms has a current of 0.1 amps flowing through it.
Sketch the circuit diagram and calculate how many volts supply the CD player.
4. What is the resistance of the circuit conductors when the conductor voltage drop is 3
volts and the current flowing through the conductors is 100 amperes?
5. V=20
R1= 10
R2 = 10
I=?
Direct Current Circuits
Answer Key
MC:
1. A
2. B
3. B, C
4. A
5. F
Short Answer:
1. 3 ohms
2. 8 Amps
a. Basically you just simplify the resistance
i. First you do the 4 on top, then you add it to the other one that is in
series
ii. Then you solve with the 3 ohm resister that is in parallel
iii. Add the last resistance
iv. Use ohms law
v. http://www.khanacademy.org/science/physics/electricity-andmagnetism/v/circuits--part-4
1. He explains it very well, but I think my explanation is
sufficient
3. 4.0 v
Diagram:
4. 3 ohms
5. 5 amps
Websites used:
http://www.rtc.edu/cce/resources/products/mathtoolbox/files/5_lpelect&ohmslaw5.pdf
http://www.khanacademy.org/science/physics/electricity-and-magnetism/v/circuits--part4
http://www.wisc-online.com/Objects/ViewObject.aspx?ID=dce11904
http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.example.ohm.html