DC CircuitDC Circuit Overview OverviewExamination #2 Review

March 1, 2010

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This WeekThis Week

Review TodayNo problem session after class todayExam #2 on Wednesday

◦There will be a class hour after the exam and we will start some new material.

No quiz on WednesdayDon’t forget to bring in Lab Notebooks on

Friday

Next topic:

Things To Remember:This chapter (20) does not exist by itself.

The understanding of POTENTIAL is critical to the chapter so make sure that you at least look at chapter 19 of you are unsure of the concept.

The exam structure:8 reasonably short conceptual problems, some

from your lab sheets. (Total of 60 points)1 Problem on Kirchoff’s Laws (you knew that,

right?) (20 points)1 Other problem. (20 points)

What is a battery and what does it do??

The battery has two TERMINALS that maintain a potential difference between them

That potential difference is usually referred to as the “voltage of the battery”.

Any two points in a circuit that are DIRECTLY connected to a battery will have the battery voltage across it.

The (+) terminal is defined as having the higher potential and the (–) terminal is often referred to as the lower potential and sometimes it is referred to as the ground or reference potential.

The symbol for the battery is shown above. There are MANY types of batteries.

6 Volts 1

.5 V

olt

s

/ secQ

I Coult

Number of electrons transported

through an area of a wire is

Ne

Q I t

Q I t

e e

Current & Wires

The electrons are moving in the wire.There MUST be an electric field in the wire.The field is V/d. d is the length of the wire. V is very

small in most circuits.Current goes the opposite way that electrons go. This

is rarely a problem.Some wires make it hard to move charge. These are

called Resistors. A simple example is the W wire that you played with.

For such materials, the relationship between the Voltage across the element and the current through it is given by Ohm’s Law: V=IR

~

resistivity ohm-meter

LR

AL

RA

The filament in an incandescent light bulb is made from tungsten. The light bulb is plugged into a 60 V outlet and draws a current of 1.04 A. If the radius of the tungsten wire is 0.0024 mm, how long must the wire be?

A wire has a resistance of 21.1 Ω. It is melted down, and from the same volume of metal a new wire is made that is four times longer than the original wire. What is the resistance of the new wire?

Two wires are identical, except that one is aluminum and one is iron. The aluminum wire has a resistance of 0.19 Ω. What is the resistance of the iron wire?

22

' Law : V=IR

P=I IR=I

Work q VP I V IV

time tOhm s

ER

R

A cigarette lighter in a car is a resistor that, when activated, is connected across the 12-V battery. Suppose that a lighter uses 27.1 W of power.

(a) Find the resistance of the lighter.

(b) Find the current that the battery delivers to the lighter.

2

2

27.1

1446

27.1 27.1

12 / 6 2

EP

R

ER

EI Amp

R

1 2

1 2

......

1 1 1.......

SERIES

R R R

PARALLEL

R R R

The circuit in the Figure has been connected for a long time.What is the voltage across the capacitor?

Determine the power supplied to the 7.0 Ω resistor in the circuit shown in the drawing. (R1 = 4.0 Ω, R2 = 7.0 Ω and V1 = 15 V.)

Kirchoff’s Law #1The sum of all the currents entering a node of

a circuit is equal to the sum of all the currents leaving the node.Current Entering= - Current Leaving.Sometimes stated: The sum of all the currents

entering a node is equal to zero.

The sum of the voltage RISES in going around a current loop is equal to zero.

H L

RISE

DROP

I

L H

DROP

RISE

Emf of Battery

Loop: E –Ir – IR =0Voltage Supplied = V-Ir