CHAPTER 19

How does the energy generated by wind farms get to people’s houses to power their appliances?

Current is the rate of change of electric charge

A current exits whenever there is a net movement of electric charge through a medium

The unit for current is the ampere 1 ampere= 1 Coulomb

second

t

QI

interval time

areagiven a through passing Charge Charge Electric

In a particular television tube, the beam current is 60 µA. How long does it take for 3.75 x 1014 electrons to strike the screen?First calculate the electric charge of 3.75 x 1014

electrons.1 electron has a charge of 1.60 x 10-19 C(3.75 x 1014 ) (1.60 x 10-19 C)= 6 x 10-5 C

sec0.11060

1066

5

Ax

Cx

I

Qt

Batteries and generators work by converting other forms of energy into electrical potential energyBatteries convert chemical energy into

electrical potential energyGenerators convert mechanical energy (KE and

PE) into electrical potential energy

Potential Difference, ΔV, is the driving force behind currentIncreasing potential difference results in a

greater currenti.e. using a 9.0 V battery generates a greater

current than a 6.0 V battery

V is measured in volts1 volt= 1 Joule/Coulomb

Some conductors allow charges to move through them more easily than others

The opposition to the motion of charge through a conductor is the conductor’s resistanceThe unit for resistance is the ohm (Ω)

Ohm’s Law: I

VR

Current

Difference PotentialResistance

Resistance is inversely proportional to currentAs the resistance increases, the current

decreases

For most materials, resistance is independent of V.Therefore, changing V affects the current, not

the resistance

The current in a certain resistor is 0.50 A when it is connected to a potential difference of 110 v. What is the current in this same resistor ifa. The operating potential

difference is 90.0 V?b. The operating potential

difference is 130 V?

I= 0.50 A, V = 110 VWe’re looking for the new current if the

potential difference is changedAccording to Ohm’s Law:

We’re missing R. Let’s find it

R

VI

22050.0

110

A

V

I

VR

Let’s find the new current for each potential differenceA.

B.

A 41.0220

90

V

R

VI

A 59.0220

130

V

R

VI

Superconductors have zero resistance below a certain temperature called the critical temperature.Once a current is established in a

superconductor it will continue even if the potential difference is removed

Electric power is the rate at which electrical energy is converted to other types of energyPower is measured in Watts

R

VRIVIP

22

Circuits and Circuit Elements

Chapter 20

A diagram that depicts the construction of an electrical apparatus is a schematic diagram

Schematic Diagrams (p. 731)

An electric circuit is a path through which charges can be conducted

Electric Circuits

Necessary Parts of an electrical circuitThe wire: Current flows through the wireThe resistor: Can be a light bulbThe emf source: Provides the potential

difference…it’s usually a battery

Series CircuitsWhen resistors are connected in series, all

the charges have to follow a single path

When one light bulb goes out, they all go out

Series CircuitsWhen resistors are connected in series, the

current in each resistor is the same!!!

The total current in the circuit depends on how many resistors are present

The equivalent resistance is the sum of the circuit’s resistances

THE EQUIVALENT RESISTANCE SHOULD ALWAYS BE GREATER THAN THE LARGEST RESISTANCE IN THE CIRCUIT

1 2 3...eqR R R R

Series CurrentTo find the total current in the circuit, first find

the equivalent resistance and then use Ohm’s Law

Although the current in each resistor has to be the same, the potential difference doesn’t have to be the same.

eq

VI

R

Sample Problem p. 739 #2A 4.0 Ω resistor, an 8.0 Ω resistor and a 12.0

Ω resistor are connected in series with a 24.0 V battery A. Calculate the equivalent resistance

B. Calculate the current in the circuit

What is the current in each resistor?For resistors in series, the current in each

resistor is the same…so the answer is 1.0 A

1 2 3... 4 8 12 24eqR R R R

241 A

24eq

V VI

R

Parallel CircuitsA parallel circuit is a wiring arrangement that

provides alternative pathways for the movement of charges

Parallel CircuitsThe total current in a parallel circuit is equal to the

sum of the current in each resistor

The equivalent resistance in a parallel circuit is calculated using the following equation

The potential difference across each resistor is the same

1 2 3...totalI I I I

1 2 3

1 1 1 1...

eqR R R R

Sample Problem p. 744 # 2

An 18.0 Ω, 9.00 Ω, and 6.00 Ω resistor are connected in parallel to an emf source. A current of 4.0 A is in the 9.00 Ω resistor. a. Calculate the equivalent resistance of the circuit.

B. What is the potential difference across the source?

C. Calculate the current in the other resistors

1 2 3

1 1 1 1 1 1 13.0

18 9 6eqR R R R

(4.0 )(9 ) 36 VeqV IR A

362A

18eq

V VI

R

36

6A6eq

V VI

R

Complex CircuitsMost circuits have both series and parallel

components

Complex Circuits (p. 747)To determine the equivalent resistance for a

complex circuit, you have to simplify the circuit into groups of series and parallel resistors

Sample Problem 20C (p. 747)Since the 6.0 Ω and 2.0 Ω resistor are connected in series, their equivalent resistance is 8.0 Ω

1 2 3...eqR R R R

Sample Problem 20C (p. 747)The new 8.0 Ω resistor and

4.0 Ω resistor are connected in parallel. Their equivalent resistance can be found using the following equation:

Req= 2.7 Ω

1 2 3

1 1 1 1...

eqR R R R

Finally, the last three resistors are connected in series so their equivalent resistance= 9.0 Ω + 2.7 Ω + 1.0 Ω= 12.7 Ω

The circuit can now be redrawn with the equivalent resistance connected to the original emf source

To find the current and/or potential difference across a particular resistor in a complex circuit you must first find the equivalent resistance for the circuit

Then you must rebuild the circuit in steps and calculate the current and potential difference for each group

Sample problem 20D is a continuation of sample problem 20C.

We already determined the equivalent resistance for the circuit…12.7 Ω

Next we need to rebuild the circuit and find the potential difference and current for each group.

A 71.07.12

0.9

V

R

VI

Work backward to find the current and potential difference for the next group.

These three resistors are connected in series. That means the current across all three resistors is the same (I=0.71 A).

We only care about the middle resistor because it’s the only one that leads to the 2.0 Ω resistor

VAIRV 9.1)7.2)(71.0(

Work backward to find the current and potential difference for the next group.

The 2.7 Ω resistor is composed of the 8.0 Ω and 4.0 Ω resistors in parallel

This means they have the same potential difference. (V=1.9 V)

We only care about the 8.0 Ω resistor because it’s the only one that leads to the 2.0 Ω resistor

A 24.00.8

9.1

V

R

VI

Work backward to find the current and potential difference for the next group.

The 8.0 Ω resistor is composed of the 6.0 Ω and 2.0 Ω resistors connected in series.

This means they share the same current (I=0.24 A)

Solve for the potential difference and you’re done

V 48.0)0.2)(24.0( AIRV

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