Transcript Lecture 4
Admin: • Discussion sections and Labs start this week. • Do the pre-lab (2 points)! • http://www.physics.udel.edu/~jholder/Phys245/PHYS245.html • Lab reports: typed neatly. Clear and well-structured. • One week to complete them, but most can be done during the lab itself. • Combined reports are OK (and recommended) • Register for MasteringPhysics • www.masteringphysics.com • Course ID: MPHOLDER67874 • First graded assignment is posted. • Due Monday 23rd Copyright © 2009 Pearson Education, Inc. What was the plan? What did I do? What were my results? Copyright © 2009 Pearson Education, Inc. Clear figures Clear tables Copyright © 2009 Pearson Education, Inc. What does it mean? Copyright © 2009 Pearson Education, Inc. Wrap it up. Copyright © 2009 Pearson Education, Inc. EMF and Terminal Voltage (Ch 26) An Electric circuit needs a battery or generator to produce current – these are called sources of emf. (electromotive force, ε) A battery is a nearly constant voltage source, but does have a small internal resistance, which reduces the actual voltage from the ideal emf: Copyright © 2009 Pearson Education, Inc. EMF and Terminal Voltage This resistance behaves as though it were in series with the emf. Copyright © 2009 Pearson Education, Inc. EMF and Terminal Voltage Example : Battery with internal resistance. A 65.0-Ω resistor is connected to the terminals of a battery whose emf is 12.0 V and whose internal resistance is 0.5 Ω. Calculate (a)the current in the circuit, (b) the terminal voltage of the battery, Vab, and (c) the power dissipated in the resistor R and in the battery’s internal resistance r. Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel A series connection has a single path from the battery, through each circuit element in turn, then back to the battery. Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel The current through each resistor is the same; the voltage depends on the resistance. The sum of the voltage drops across the resistors equals the battery voltage: Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel From this we get the equivalent resistance (that single resistance that gives the same current in the circuit): Copyright © 2009 Pearson Education, Inc. Resistance Limits: Open and Short Circuits • Short Circuit: A wire! R=0, V=0 for any i. – Particularly bad for any voltage source • Open circuit: A break! R→, i=0 for any V. – Particularly bad for current source Copyright © 2009 Pearson Education, Inc. Series Resistors and the Voltage Divider Rule For N resistors in series: Same current through each resistor – so voltage is split according to their resistance Voltage Divider: R1 v1 (1.5V) REQ R v2 2 (1.5V) REQ v3 R3 (1.5V) REQ R3 Resistors in Series and in Parallel A parallel connection splits the current; the voltage across each resistor is the same: Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel The total current is the sum of the currents across each resistor: , Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel This gives the reciprocal of the equivalent resistance: Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel An analogy using water may be helpful in visualizing parallel circuits. The water (current) splits into two streams; each falls the same height, and the total current is the sum of the two currents. With two pipes open, the resistance to water flow is half what it is with one pipe open. Copyright © 2009 Pearson Education, Inc. Parallel Resistors and the Current Divider Rule For N resistors in parallel: Current is split according to the inverse of their resistance (more current through a wider (lower resistance) pipe) v REQ iS Current Divider: i1 R1 R1 REQ v i2 iS R2 R2 v REQ i3 iS R3 R3 Resistors in Series and in Parallel Example: Series or parallel? (a) The lightbulbs in the figure are identical. Which configuration produces more light? (b) Which way do you think the headlights of a car are wired? Copyright © 2009 Pearson Education, Inc. Resistors in Series and in Parallel Example: Circuit with series and parallel resistors. How much current is drawn from the battery shown? Copyright © 2009 Pearson Education, Inc.