Electric Current - General Physics - Lecture Slides, Slides of Physics

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Chapter 21

Electric Current and Direct-

Current Circuits

Units of Chapter 21

Electric Current

Resistance and Ohm’s Law

Energy and Power in Electric Circuits

Resistors in Series and Parallel

Kirchhoff’s Rules

Circuits Containing Capacitors

RC

Circuits

Ammeters and Voltmeters

21-1 Electric Current

A battery uses chemical reactions to produce a

potential difference between its terminals. It causes current to flow through the flashlight

bulb similar to the way the person lifting the

water causes the water to flow through the

paddle wheel.

21-1 Electric Current

A battery that is disconnected from any circuithas an electric potential difference between itsterminals that is called the electromotive force or emf

Remember – despite its name, the

emf

is an

electric potential, not a force.The amount of work it takes to move a charge ∆

Q

from one terminal to the other is:

21-1 Electric Current

Finally, the actual motion of electrons along awire is quite slow; the electrons spend most oftheir time bouncing around randomly, and haveonly a small velocity component opposite tothe direction of the current. (The electric

signal

propagates much more quickly!)

21-2 Resistance and Ohm’s Law

Under normal circumstances, wires presentsome resistance to the motion of electrons.Ohm’s law relates the voltage to the current:

Be careful – Ohm’s law is not a universal lawand is only useful for certain materials(which include most metallic conductors).

21-2 Resistance and Ohm’s Law

Two wires of the same length and diameter willhave different resistances if they are made ofdifferent materials. This property of a material iscalled the resistivity.

21-2 Resistance and Ohm’s Law

The difference betweeninsulators,semiconductors, andconductors can be clearlyseen in their resistivities:

21-3 Energy and Power in Electric CircuitsWhen a charge moves across a potentialdifference, its potential energy changes:

Therefore, the power it takes to do this is

21-3 Energy and Power in Electric Circuits

In materials for which Ohm’s law holds, thepower can also be written:

This power mostly becomes heat inside theresistive material.

21-4 Resistors in Series and Parallel

Resistors connected end to end are said to be inseries. They can be replaced by a singleequivalent resistance without changing thecurrent in the circuit.

21-4 Resistors in Series and Parallel

Since the current through the series resistorsmust be the same in each, and the total potentialdifference is the sum of the potential differencesacross each resistor, we find that the equivalentresistance is:

21-4 Resistors in Series and Parallel

Using the fact that the potential differenceacross each resistor is the same, and the totalcurrent is the sum of the currents in eachresistor, we find: Note that this equation gives you the inverse ofthe resistance, not the resistance itself!

21-4 Resistors in Series and Parallel

If a circuit is more complex, start withcombinations of resistors that are either purelyin series or in parallel. Replace these with theirequivalent resistances; as you go on you will beable to replace more and more of them.