1 / 12

PHYS 221 Recitation

PHYS 221 Recitation. Kevin Ralphs Week 4. Overview. HW Questions Current Resistance Resistivity Kirchhoff's Rules Capacitors, Current, Time. HW Questions. Ask Away…. Current. What does it tell me? The amount of charge flowing through a boundary The unit of measure is the ampere:

nen
Download Presentation

PHYS 221 Recitation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PHYS 221 Recitation Kevin Ralphs Week 4

  2. Overview • HW Questions • Current • Resistance • Resistivity • Kirchhoff's Rules • Capacitors, Current, Time

  3. HW Questions Ask Away….

  4. Current • What does it tell me? • The amount of charge flowing through a boundary • The unit of measure is the ampere: • The word “flow” implies there should be an equation similar to flux that describes this is the drift velocity. It is the average velocity of the charge carriers, is the charge density, is the number density (# of charge carriers/unit volume)

  5. Resistance • What does it tell me? • The ratio of the potential drop in the “direction” of the current and the current in a segment and is measured in ohms () • Essentially it is telling you how tough it is to push charge through an object • Why do I care? • All things have resistance so it is critical to understand how it affects electric current • The resistor is another one of our linear electronic components • The potential difference across a resistor is given by Ohm’s Law ΔV: potential difference across a resistor, I: current passing through the resistor, R: resistor’s resistance, the sign depends on the direction of the current

  6. Resistivity • The resistivity () tells us how easy it is to push charge through a material, regardless of its dimensions R: resistance, ρ: resistivity, L: length of resistor, A: cross-sectional area of resistor (assumes A is constant along the resistor’s length) • It has temperature dependence ρ: resistivity at temperature T, ρo: Resistivity at To =20°C

  7. Kirchoff’s Rules • Loop Rule • Based on conservation of energy • Node Rule • Based on conservation of charge

  8. Kirchhoff’s Rules General Procedure: • Choose loops so that every branch is covered by at least one loop • Choose current directions in each branch – this does not have to correspond to the direction of your loop • Write down each loop/node equation and solve using method of your choice. You need as many independent equations as you have currents to solve.

  9. Kirchhoff’s Rules • The most common errors in applying Kirchhoff’s rules are sign errors Voltage Source Resistor (Black arrows denote a positive change in voltage; red negative) Current

  10. Capacitors, Current, Time • You will be responsible for understanding how a capacitor behaves at various times during charging/discharging • The end points are simple • If uncharged, a capacitor acts as a short • If charged, a capacitor acts as an open • The words “after a long time” or similar mean it has gone from charged to uncharged or vice versa

  11. Capacitors, Current, Time • In between it is governed by equations • Note: vary depending on whether you are charging/discharging and what resistors you are charging/discharging through

  12. Practice Problem

More Related