CURRENT, RESISTANCE AND ELECTROMOTIVE FORCE

Current • A current is any motion of charge from

one region to another.

• The current is the rate at which charge flows through this surface.

Drift Velocity

• The speed of the charge carriers vd is an average speed

Current Density

• Current per cross-sectional area.

• A wire carries a current of 1A. How many electrons pass any point in the wire each second?

Electrolysis

• The liberation of free elements from liquids through which electric currents are passed is called electrolysis.

• Faraday’s laws of electrolysis state that the mass m of an element set free at either electrode is proportional to two quantities:

1. The total charge Q passed through the liquid, which is equal to the product It of current I and time t.

2. The molar mass A of the element divided by the number v of electron charges (plus or minus) on its ions.

Faraday’s Laws can be combined in the formula:

Fv

ItA

Fv

QAm

• F, is called the faraday, is equal to 96, 500 C and represents the total charge of 1 mol of electrons. Thus the passage of 1 F of charge will liberate 1 mole of element X from a solution that contains X+ or x- ions, ½ mol of element Y from a solution that contains Y2+ or Y2- ions, 1/3 mol of element Z from a solution that contains Z3+ or Z3- ions, and so forth.

Electrolysis Example

• A 12-A current is passed through an electrolytic cell in which the anode (positive electrode) is a zinc bar, and the electrolyte contains Zn2+ ions. How much zinc is plated on the cathode (negative electrode) after 2h?

RESISTIVITY

Resistance

1. A 120-V toaster has a resistance of 12Ω. What must be the minimum rating of the fuse in the electric circuit to which the toaster is connected?

2. What is the resistance of a copper wire 0.50mm in diameter and 20m long? The resistivity of copper is 1.7 x 10-8 Ω.m.

3. A platinum wire 80 cm long is to have a resistance of 0.1 Ω. What should be its diameter? The resistivity of platinum is 1.1x10-7 Ω.m

4. The specific resistance of Nichrome is 600 Ω.cmil/ft. How long should a Nichrome wire 20 mils in diameter be for it to have a resistance of 5 Ω?

• A copper wire has a resistance of 10.0 Ω at 200C (a) What will its resistance be at 800C? (b)at 00C? The temperature coefficient of resistance of copper is 0.004/C0.