 # Copyright © 2009 Pearson Education, Inc. Admin: Discussion sections and Labs start this week. Do the pre-lab (2 points)!

## Presentation on theme: "Copyright © 2009 Pearson Education, Inc. Admin: Discussion sections and Labs start this week. Do the pre-lab (2 points)!"— Presentation transcript:

Copyright © 2009 Pearson Education, Inc. 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. 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: EMF and Terminal Voltage (Ch 26)

Copyright © 2009 Pearson Education, Inc. This resistance behaves as though it were in series with the emf. EMF and Terminal Voltage

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, V ab, and (c) the power dissipated in the resistor R and in the battery’s internal resistance r.

Copyright © 2009 Pearson Education, Inc. A series connection has a single path from the battery, through each circuit element in turn, then back to the battery. Resistors in Series and in Parallel

Copyright © 2009 Pearson Education, Inc. 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: Resistors in Series and in Parallel

Copyright © 2009 Pearson Education, Inc. From this we get the equivalent resistance (that single resistance that gives the same current in the circuit): Resistors in Series and in Parallel

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

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: R3R3

Copyright © 2009 Pearson Education, Inc. A parallel connection splits the current; the voltage across each resistor is the same: Resistors in Series and in Parallel

Copyright © 2009 Pearson Education, Inc. The total current is the sum of the currents across each resistor: Resistors in Series and in Parallel,

Copyright © 2009 Pearson Education, Inc. This gives the reciprocal of the equivalent resistance: Resistors in Series and in Parallel

Copyright © 2009 Pearson Education, Inc. 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. Resistors in Series and in Parallel

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) Current Divider:

Copyright © 2009 Pearson Education, Inc. 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?