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• How many electrons go past a point every second in a wire if the current is 12 Amps?

Objectives

• Understand that current tells us how much charge (think moving electrons! – but more to come on this) moves past a certain point every second.

• We all know “Resistance is futile”…but what is resistance when we are talking about Current?

Homework

• Castle Learning – Current and Resistance

Test• Corrections today post and tomorrow.

• I do have a meeting today but I will be in here so feel free to come today for corrections.

• Please look over, ask questions, check the scoring and then return to your mailbox.

Worksheet

• Current

4.2.2 Electrical Resistance

Electrical Stopping Power

Definitions• resistance: OPPOSITION TO CURRENT

• unit: Ω• factors that change resistance:

• resistivity: MATERIAL • length• cross-sectional area• temperature

A

LR

Resistance Factors

R

ρ

R

L

R

Temp.

NEXT

R

A

Resistance Factors

R

ρ

R

L

R

Temp.

To build an “ideal” conductor with the smallest possible resistance you would

select one that is:

LOW RESISTIVITY, SHORT, WIDE, COLD

• Determine the resistance of a 1.0 meter long copper wire with a cross-sectional area of 0.01 meter2.

Example #1

R = ρL / A

R = (1.72 x 10-8 Ω·m)(1.0 m) / (0.01 m2)

R = 1.72 x 10-6 Ω

• A piece of wire that has a length of 5.0 x 107 meters and a cross-sectional area of 0.025 meter2 has a resistance of 31.8 ohms.– What is the composition of this wire?

Example #2

R = ρL / A

ρ = RA / L

ρ = (31.8 Ω)(0.025 m2) / (5.0 x 107 m)

ρ = 1.59 x 10-8 Ω·m

End of 4.2.2 - PRACTICE