Review Lecture 1. Copyright © 2009 Pearson Education, Inc. Admin for next week: Mid-term date...

Review Lecture 1

Copyright © 2009 Pearson Education, Inc.

Admin for next week:Mid-term date Friday March 20th.•Labs as usual•NO DISCUSSION SECTIONS THIS WEEK•Extra Office hours

• 2-4pm Monday: Dr Holder Sharp Lab 222• 2-4pm Tuesday: Dr Holder Sharp Lab 222• 2-4pm Wednesday: Seth Sharp Lab 308• 2-4pm Thursday: Zach Sharp Lab 320

•Email is also OK – or stop by my office any time Monday or Tuesday

•I will be away Wednesday – Friday. •Dr Ji will give Wednesday’s class and the exam.


Admin:

•Assignments:

• Fifth Assignment is posted.• Not due until Monday (23rd), but good exam practice• Three questions on Thevenin circuits – I’ll go through

one of them today


What does the test cover?• What are current, voltage and charge?• Ohm’s law: V=IR• Resistivity and resistance• Temperature dependence• What is power?• P=IV• Resistors in series and parallel• Calculating equivalent resistance• The voltage divider rule• The current divider rule• Ammeters, Voltmeters and Ohmmeters• Voltage sources and current sources• Ideal and Practical sources• Combining sources


What does the test cover?

•Kirchhoff’s rules• Junction rule (current law)• Loop rule (voltage law)

•Circuit analysis• Applying Kirchhoff’s rules• Mesh analysis• Thevenin/Norton Equivalents


What formulae will I have?Note that the equations for current, voltage, power, etc, are not given, because they are implicit in the definition: if you don’t know the equation, you don’t understand what they are.Any required constants will be given (e.g. resistivity values)


What will be the format of the exam?

Roughly…5 short answer questions: 2 points each. 4 longer questions: 5 points each.

Rules•Closed book exam – no notes or textbooks.•Please seat yourselves with at least one space between you.•A scientific calculator is required•Graphing calculators are OK – but I can’t give partial credit for the algebra if you don’t show it.•No phones/ phone calculators

•Students with accommodations can use the test center – but fill out the DSS form.


How should I study?


How should I study?

Hard!


How should I study?

Take it seriously: 15% of your gradeI aim for “fair but challenging”

The best resource is the lecture notes – in particular, make sure you can solve all of the worked examples, without the solutions in front of you.

The textbook (Ch 25 & 26) is also useful.

Review the discussion problems, and homework problems.

Come to office hours (soon!) if anything is not clear – or drop me an email, but office hours are usually much more effective


Clearing up a misconception.

Current flows through the elements of a circuit.

Voltage is measured across the elements of a circuit.

Current only changes if there is a junction.

The voltage between two points is the same, regardless of the path taken to calculate it.


Light bulbs

Power defines how bright a light bulb gets.P=IV, P=I2R, P=V2/R

Rank these light bulbs in brightness, each with the same resistance:

What happens to A if we unscrew E?

Total resistance increasesCurrent decreases (V=IR)Bulb A gets dimmer!

1. C2. A & B3. D & E


Light bulbs

Power defines how bright a light bulb gets.P=IV, P=I2R, P=V2/R

Two lightbulbs of resistance R1 and R2 (R2 > R1) and a battery are all connected in series. (a) Which bulb is brighter? (b) What if they are connected in parallel?

Draw it!

A: the bulb with resistance R2B: the bulb with resistance R1


Voltage dividers

What is the voltage across the 2 Ohm resistor? Use intuition… remember Kirchhoff..

The voltage across each R is split according to the fraction of the total resistance

VR= VIN (R/REQ)


Current dividers

What is the current through resistor R2? Use intuition…

The current through each R is split according to the inverse of the fraction of the total resistance.

IR= IIN (REQ/R) (note that REQ is always less than R – so IR cannot be greater than IIN)

3 Amps

10Ω 20Ω


Thevenin Equivalent (a freebie)


Mesh analysis

VS1=12V VS2 =5V R1 =50Ω R2 =R3 =20Ω R4 =10Ω R5 =15Ω

Find the mesh currents, and the voltage across R4


http://www.math.odu.edu/~bogacki/cgi-bin/lat.cgi?c=sys

Hint - you can use this to check your algebra:


Kirchhoff’s law circuit analysis

Example 26-10: Jump starting a car.A good car battery is being used to jump start a car with a weak battery. The good battery has an emf of 12.5 V and internal resistance 0.020 Ω. Suppose the weak battery has an emf of 10.1 V and internal resistance 0.10 Ω. Each copper jumper cable is 3.0 m long and 0.50 cm in diameter, and can be attached as shown. Assume the starter motor can be represented as a resistor Rs = 0.15 Ω. Determine the current through the starter motor (a) if only the weak battery is connected to it, and (b) if the good battery is also connected.


See page 687 for full solutionI1=76AI2=-5A (charging)I3=71A

Kirchhoff’s law circuit analysis

Review Lecture 1. Copyright © 2009 Pearson Education, Inc. Admin for next week: Mid-term date...

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