The Fundamentals of Circuits

Transcription

1 The Fundamentals of Circuits Now that we have an understanding of current and resistance, we re ready to start studying basic direct current (DC)circuits. We ll start with resistor circuits, and then move on to resistor-capacitor (RC) circuits. Note: engineering majors will return to this topic in ECE205; physics majors in PHY292/294. Common Circuit Elements: picture Diagram symbol open = no current closed = current no current, but can charge Changes current, i.e. branches A resistor that heats and glows when a current goes through it Ideal wire, R = 0, all points along wire at same potential, current is constant Ideal source of EMF, + and terminals labelled (usually assume no internal resistance)

2 Circuit Diagrams A pictorial representation of a circuit with batteries, resistors, etc. can be very confusing to follow. Where are the junctions? What elements are in series or parallel (more on this later)? It is important that you learn to follow and draw accurate circuit diagrams using the circuit element symbols on the previous slide. For example, the above circuit with a battery, resistor, and capacitor looks like this when properly diagrammed.

3 Measuring Current and Voltage Lab In this exercise, you will measure the current and potential along a piece of Nichrome wire. Important point: in most of what we re doing in chapter 28, we assume that wires are perfect conductors with R = 0. In this exercise, we will not make that assumption. The wire that you re working with has some resistivity so that the longer the wire, the larger the resistance which affects the current. The materials that you ll need are: 1 source of EMF (battery) 1 multi-meter 1 wire sample on a length scale 2-3 connector wires and 1-2 probe wires When you want to take a measurement, press and hold down the red button on the battery; this completes the circuit. This will ensure that the battery doesn t run-out. Follow the directions in the handout, and make sure to put your group name and all of your names on it.

4 Part 1: Measuring the Current Part 2: Measuring the Voltage

5 The Basic Laws: Kirchoff s Laws To solve any circuit, we need only two basic laws which we already know: the Kirchoff Loop and Junction Laws. (Note that in all that we do in Chapter 28, we will use the conventional current, I) Kirchoff Junction Law*: At any junction: *This is just really just the Conservation of Current

6 Whiteboard Problem: 28-1 In the figure of the piece of a circuit below, what is the current, I, in the wire to the right of the junction? Does this current flow to the right or to the left? Hint: current flows from high to low potential across a resistor Hint: assume whatever direction you want for the current I, and calculate it. If your answer is positive, you chose the right direction; if your answer is negative, the magnitude is right, but the direction is opposite of your choice.

7 The Basic Laws: Kirchoff s Laws Kirchoff Loop Law*: For any closed circuit loop: circuit elements, e.g. resistors, batteries, capacitors, etc. *this is a consequence of energy conservation

8 Sign Conventions with the Loop Law When writing loop equations for a circuit, it is very important to follow the the proper sign conventions for potential changes: Across a battery or source of EMF: + - Loop equation + direction - Loop equation direction What is on your equation sheet:

9 Across a resistor: Sign Conventions with the Loop Law loop equation direction with the current loop equation direction against the current What is on your equation sheet:

10 Whiteboard Problem 28-2 What is the magnitude and direction (CW or CCW) of the current in the 10 Ohm resistor? (LC, just current magnitude) Hint: assume whatever direction you want for the current, and calculate it. If your answer is positive, you chose the right direction; if your answer is negative, the magnitude is right, but the direction is opposite of your choice.

11 Power in Circuits For a simple circuit: + - Your author shows: Power supplied by the battery: Units = Watts This energy is dissipated in the resistor into thermal energy: Note: for the resistor, the potential difference must be the potential across the resistor and the current is the current through the resistor.

12 Whiteboard Problem: 28-3 For the circuit below, how much power is dissipated by each resistor? (LC, enter the power dissipated in R 2 )

13 PhET Lab: Ohm s Law and Power Using the group s computer, one of you log onto Masteringphysics and begin the exercise: PhET: Ohm's Law and Power. Use the computer to complete the exercise, but record your answers on the sheet to hand in that s what will be graded. In the circuit construction kit, you can make circuits with ideal wires and batteries. You can add in resistors and lightbulbs and change their resistances. You can select lifelike visuals of the components or a schematic representation that looks like the diagrams that we use. When you re done with the exercise, make sure your Group s name and all of your names are on it, and turn it in. Feel free to play with the simulation if you have time.