Electric Current Electric current I is the rate of the flow of charge Q through a cross-section A in a unit of time t. One ampere A is charge flowing.

Electric CurrentElectric Current

Electric currentElectric current I I is the is the rate of the flow of charge rate of the flow of charge QQ through a cross-section through a cross-section A A in a unit of time in a unit of time tt..

QI

tQ

It

1C1 A

1 s

1C1 A

1 s

One One ampereampere A A is charge flowing is charge flowing at the rate of one at the rate of one coulomb per coulomb per secondsecond..

One One ampereampere A A is charge flowing is charge flowing at the rate of one at the rate of one coulomb per coulomb per secondsecond..

AA++

--WireWire

+Q+Q

tt

Example 1.Example 1. The electric current in The electric current in a wire is 6 A. How many electrons a wire is 6 A. How many electrons flow past a given point in a time of flow past a given point in a time of 3 s?3 s?

I = 6 AI = 6 A; q

I q Itt

; q

I q Itt

qq = (6 A)(3 s) = 18 = (6 A)(3 s) = 18 CCRecall that: 1 eRecall that: 1 e-- = 1.6 x 10 = 1.6 x 10-19-19 C, then C, then

convert:convert:

-20

-19

1e18 C 18 C 1,125 x 10 electrons

1.6 x 10 C

In 3 s: 1.12 x 1020 electrons

Factors Affecting Factors Affecting ResistanceResistance

1. The 1. The length Llength L of the material. Longer of the material. Longer materials have greater resistance.materials have greater resistance.

1 1

LL

2 2

2L2L

2. The cross-sectional 2. The cross-sectional area Aarea A of the of the material. Larger areas offer material. Larger areas offer LESSLESS resistance.resistance.

2 2 AA

1 1

22AA

Factors Affecting R (Cont.)Factors Affecting R (Cont.)3. The 3. The temperature Ttemperature T of the material. of the material.

The higher temperatures usually The higher temperatures usually result in result in higherhigher resistances. resistances.

4. The kind of 4. The kind of materialmaterial. Iron has more . Iron has more electrical resistance than a electrical resistance than a geometrically similar copper geometrically similar copper conductor.conductor.

RRoo

R > RR > Roo

RRii > R > RccCopperCopper IronIron

Resistivity of a MaterialResistivity of a Material

The The resistivity resistivity is a property of a is a property of a material that determines its electrical material that determines its electrical

resistance resistance RR..Recalling that Recalling that RR is directly is directly proportional to length proportional to length LL and and inversely proportional to area inversely proportional to area AA, , we may write:we may write:

or L RA

RA L

or L RA

RA L

The unit of resistivity is the The unit of resistivity is the ohm-meter ohm-meter ((m)

Temperature CoefficientTemperature CoefficientFor most materials, the resistance For most materials, the resistance RR changes in proportion to the initial changes in proportion to the initial resistance resistance Ro and to the change in and to the change in temperature temperature tt..

0R R t 0R R t Change in Change in resistance:resistance:

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

00

1; Units:

C

R

R t

00

1; Units:

C

R

R t