Electric current and direct-current circuits A flow of electric charge is called an electric...

Electric current and direct-current circuits

A flow of electric charge is called an electric current.


QI

t


1 ampere =

CSI unit A

s


When electric charge flows through a closed path and returns to its starting point the path is called an electric circuit.


When electric charge flows through a closed path in one direction the path is called a direct-current circuit.


When electric charge flows through a closed path and periodically reverses direction the path is called a alternating-current circuit.


A battery produces a difference in electric potential

between its terminals through chemical reactions.


The symbol for a battery is


The terminal designated + corresponds to the higher potential, while the terminal designated by a – corresponds to the lower potential.


By convention we say that the direction of the current is the

direction in which a positive charge would move.

Figure 21-4Direction of Current and Electron Flow


The electromotive force (emf) (ξ) is the potential across the

terminals (voltage) of a battery under ideal conditions.


The charges that actually move through a conductor, are electrons.


In a real conductor there is always some resistance to electron flow,

and a potential difference is necessary to keep them flowing.


Ohm’s Law relates the potential(V), resistance (R)and current (I)in a

circuit


Ohm’s Law

V IR


Ohm’s Law

11 = 1ohm

V VR

I A


Ohm’s Law

LR

A


Ohm’s LawUnit for resistivity is

m


When an electric charge moves across a potential difference the potential energy changes by the

amount

( )U Q V


SI unit; watt, W

( )U Q Vpower P IV

t t


Other expressions for electric power

22 V

P I RR


Resistors in a series are connected end to end.

Example 21-5Three Resistors in Series


The equivalent resistance for resistors in series is just the

sum of the individual resistances


For the example given

1 2 3eqR R R R


Each of the resistors connected in series has the

same current going through it.


Resistors connected in parallel are connected across the same potential difference.

Example 21-6Three Resistors in Parallel


The equivalent resistance for resistors in parallel is calculated by adding the reciprocal values of the individual resistors.


This gives the reciprocal of the equivalent resistance



1 2 3

1 1 1 1

eqR R R R


The current going through individual resistors connected in parallel is not necessarily the same.


The sum of the currents will be equal to the current calculated for the individual resistors.


For circuits that contain resistors connected both in series and in parallel, we first calculate the equivalent resistances.


We then treat the result as if it were just another resistor in series. Ex.21-7 on page 693.

Example 21-7Combination Special


The sum of the voltage drops in a circuit must be equal to the voltage applied to the circuit.

Figure 21-16Capacitors in Parallel


The equivalent capacitance for capacitors in parallel is

just the sum of the individual capacitances



1 2 3eqC C C C


The sum of the individual charges on the capacitors is equal to the charge on the

equivalent capacitor.

Figure 21-17Capacitors in Series


The equivalent resistance for capacitors in series is calculated by adding the reciprocal values of the individual capacitors.


This gives the reciprocal of the equivalent capacitance.



1 2 3

1 1 1 1

eqC C C C


Active example 21-3 p 700.


Kirchoff’s rules1. The sum of the currents entering a junction, must equal the sum of the currents leaving that junction (result of charge conservation).


Kirchoff’s rules2. The algebraic sum of the potential differences around a closed loop is zero. The potential increases in going from the negative to the positive terminal of a battery, and decreases when crossing a resistor in the direction of the current. (energy conservation).


Batteries – all non-ideal batteries have an internal resistance. The voltage measured across the terminals of a battery will be less with current flowing than without current flowing.


Ammeters are connected in series with the part of the circuit being tested. The ideal resistance of an ammeter is 0 .


Voltmeters are connected in series with the part of the circuit being tested. The ideal resistance of a voltmeter is ∞.

Electric current and direct-current circuits A flow of electric charge is called an electric...

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