Part 1

Current & Energy Transfer

Electric Current

Objectives

Define an electric current, and the ampere Describe conditions that create current in an

electric circuit Define power in electric circuits Draw circuits and recognize that they are

closed loops Define resistance and state Ohm’s Law Use equations to solve problems

Electric Circuit

o Closed path along which charged particles move

Current

Flow of charged particles Unit is ampere (A) or “amp”

Rate at which charge passes a point in a circuit 1 ampere = 1 coulomb per second

Symbol is “I”

Iq

t

Electric Current

Conducting path Closed circuit

Supply of energy Potential difference (voltage)

Conventional current Flows from positive to negative Electrons actually move from

negative to positive

Conductivity

Ability of a material to conduct electricity Depends on number of free charges per unit

volume Charges move under the influence of an electric

field Potential difference created between two points

Pure metals are good conductors Many electrons are not bound or are loosely bound to

a particular atom

Resistivity

Reciprocal of conductivity Characteristic of a material that depends on its

electronic structure & temperature Increases with temperature Conductors

Low resistivity Insulators

High resistivity Electrons are tightly bound; few are free to move

Energy Transfer

Power Rate at which

energy (work) is transferred

Units are watts (W) Joules per second

Multiply the potential difference by the current

P I V

PW

t

Power

PW

t

EF

q

VW

q

VW

qW q V

Iq

ttq

I

PW

tP

q VqI

I V

Example

Textbook, Page 509 Current through the

motor is 3.0 coulombs per second

Potential difference is 120 volts 1 volt = 1 joule per

coulomb

What is the power? 1 watt = 1 joule per sec

P I V

P Cs

JC

P Js W

30 120

360 360

.e je j

Practice Problem

A 6.0 V battery delivers a 0.50 A current to an electric motor that is connected across its terminals What power is consumed by the motor? If the motor runs for 5.0 minutes, how much

electric energy is used? If the motor does 600 J of work, what is the

efficiency of the motor?

Practice Problem

Draw a diagram Show the conventional current

List the knowns and unknowns

Known:

min

V V

I A

t

6 0

050

50

.

.

.

Unknown:

P W

W J

eff

?( )

?( )

?(%)

Practice Problem

Known:

min

Unknown:

V V

I A

t

P W

W J

eff

6 0

050

50

.

.

.

?( )

?( )

?(%)

Do Textbook, Pg. 511 #1 – 4

P I V

A V

W

PW

tW P t

W J s

x J

effJ

J

050 6 0

30

30 50 60 1

9 0 10

600

900100% 67%

2

. .

.

. / . min sec/ min

.

b gb g

b gb gb g

Part 2

Resistance & Ohm’s Law

Electric Current

Resistance

Property that determines how much current will flow between two points at a given potential difference Measured by placing a potential difference across

two points on a conductor, and measuring the current

Resistance (R) is the ratio between the voltage (V) and the current (I)

Resistance

Opposition to the flow of electric current Ratio of potential difference to current flow Unit is “ohm” (Ω)

Ohm’s law Device which obeys this relationship is “ohmic” Not a general physical “law”

RV

I

Resistance Unit of resistance is the ohm ()

One ohm is the resistance that will allow one ampere of current to flow when the potential difference is one volt

Ohm’s law Describes the behavior of a “pure” resistor Devices that behave this way are called, “ohmic”

Ohm’s Law

CurrentR = slope of the VI curve

RV

I

Ohm’s Law

RV

IV R I

IV

R

Application

Circuit diagrams Pg. 512

Potentiometer (variable resistor) Pg. 513

See Pg. 514, Table 22-1

Example

A 30.0 V battery is connected to a 10.0 ohm resistor What is the current in the circuit?

Draw a diagram List the knowns and unknowns Calculate your answer Check your answer

Example

Known:

Unknown:

V V

R

I A

30 0

10 0

.

.

?( )

IV

R

IV

I A

30 0

10 0300

.

..

Resistance of a Wire

Depends on its physical properties Length Cross-sectional area (thickness) Resistivity of material (ρ)

RL

A FHG

IKJ

Units of Resistivity

What are the units of resistivity?

Ohm·meter (Ω·m)

RL

A

RA

L

m

m

2

Example

What is the resistance of 500 yards of 12AWG copper wire?

If one amp of current is flowing in the wire, what is the potential difference (voltage)?

SolutionL m

A r

rD x m

x m

457 2

2

2 0525 10

2172 10

2

3

8

.

.

.

RL

A

Rx m m

x m

x m m

x m

FHG

IKJ

172 10 457 2

2 0525 102

4 172 10 457 2

2 0525 10

8

3 2

8

3 2

. .

.

. .

.

c hb g c hb gc h

Check

Rft

Rft

ft

yd

yd

15883

1000

15883

1000

3

1

500

1

.

.

Temperature Coefficient of ResistanceTemperature Coefficient of Resistance

The The temperature coefficient of resistancetemperature coefficient of resistance is the is the change in change in resistanceresistance per unit resistance per degree change in per unit resistance per degree change in temperature. temperature.

R

R t0

R

R t0

Where:

= temperature coefficient of resistance

R = change in resistance

R0 = initial resistance

t = change in time

Where:

= temperature coefficient of resistance

R = change in resistance

R0 = initial resistance

t = change in time

SuperconductivitySuperconductivity

Superconductivity Superconductivity is a condition of is a condition of zero resistancezero resistance encountered at relatively low temperatures for certain encountered at relatively low temperatures for certain materials.materials.

The The transition temperaturetransition temperature is is the temperature at which the the temperature at which the resistivity of a resistivity of a superconductor material superconductor material rapidly approaches zerorapidly approaches zero..

Extra Credit

What animal is capable of producing nearly 1 amp of current? Genus & species Where found

Practice Problems

Drawing schematic diagrams Pg. 516

Textbook, Pg. 515 #5 – 10 Textbook, Pg. 517 #11 – 13