Direct Current Circuits

Transcription

1 Vocab Direct Current Circuits 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

2 Chapter notes IF ANYTHING IN THIS WAS NOT AT ALL CLEAR, or message me on Facebook (I would prefer the Facebook message or 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. R eq =V ab /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 R 1 +R 2 +R 3 =R T o Parallel Resistors A branch comes off of the wire, ex

3 To find resistance use this: 1/R 1 +1/R 2 +1/R 3 =R T 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

4 2. Formulas to know are: a. V=IR b. R 1 +R 2 +R 3 =R T c. R eq =V ab /I d. 1/R 1 +1/R 2 +1/R 3 =R T

5 Problems Direct Current Circuits 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

6 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

7 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. Also, this is an ohm s law calculator, not sure if it helps 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

8 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=?

9 Answer Key MC: 1. A 2. B 3. B, C 4. A 5. F Direct Current Circuits 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 He explains it very well, but I think my explanation is sufficient v Diagram: 4. 3 ohms 5. 5 amps Websites used: