Electric Current Notes

7/18/2019 Electric Current Notes

http://slidepdf.com/reader/full/electric-current-notes 1/25

Electric Circuits Video

What Do You Think?

If static electricity relates to the presence of an

electric charge, what is current electricity?

What is needed to have an electric current?

https://www.youtube.com/watch?v=BhYbmcbYJn8

https://www.youtube.com/watch?v=BhYbmcbYJn8



Current Electricity

• An Electric Current is the flow of charge. Charge (or

electrons) flow due to a difference in electric potential.• Electrical Potential:

– i!ilar to potential energy (lifting so!ething higher

against the force of gravity gives it greater potential to

do wor", increasing its potential energy.) – When given the opportunity, o#$ects will !ove fro!

higher potential energy to an area of lower potential

energy



Batteries - Electrochemistry%. Converts che!ical energy into electrical energy

&. Consists of two different !etals – the electrodes

'. lectrodes i!!ersed in a che!ical #ath* that conducts electricity

called the electrolyte

+. he part of the electrodes a#ove the electrolyte is the ter!inal and

used to connect the #attery to the circuit.

• here is a che!ical reaction #etween the electrodes and the electrolyte resulting in a#uildup of electrons on one of the ter!inals (it #eco!es the - ter!inal)

• he other ter!inal gives up its electrons and #eco!es the * ter!inal.• his difference sets up the electrical potential difference of the syste! or /oltage



Dry Cell and Wet Cell

• Wet Cell – the electrolyte is a liquid (car attery!

– In a car #attery, electrolyte is sulfuric acid the * ter!inal is lead o0ideand the - ter!inal is lead !etal

• Dry Cell – the electrolyte is not really dry" ut is a #aste

– tandard AA, C, 1 type #atteries



$olta%e• $olta%e2 Causes current to flow through an electrical circuit. he che!ical energy

of the #attery is transferred to the electron as it !oves through the #attery.

• $olt: 3nit of !easure to !easure this potential• A $olta%e &ource (#attery or generator) is re4uired to !aintain the electrical

potential in a circuit.



current

2 J of energy changed

into heat and heat and

light energy by each

coulomb of charge

4 J of energy givento each coulomb of

charge

2 J of energy

changed into heat

and heat and light

energy by each

coulomb of charge

' $ ' $

$

) attery is

connected to t*o

identical li%htuls+



Current is the rate at which charge is flowing in a circuit. It is the a!ount of charge that passes through any point of the circuit per unit ti!e.

Current is !easured in a!pere, A, where % A 5 % C6s.

Electric Current

Conductor

I =Q

t



Electric Current

• cientist first thought that positive charges flow fro! thepositive ter!inal of a cell to the negative ter!inal. his is

called the con,entional current direction.

• 7owever, it was found that a current in a !etal wire is in

fact a flow of negatively-charged electrons in theopposite direction. 8evertheless, the conventional

current is still used.+ –

electron flow

convention

current



Electric Current

• Water flowing thru a pipe depends on !ore than the angle

of the pipe. It also depends on the length of the pipe,dia!eter of the pipe and if the pipe is clogged or open.

• A!ount of electrical current (a!ps) depends on !ore than

$ust ,olta%e, it depends on the resistance found in the

circuit.



Eam#le .

A hair dryer draws a current of ' A. If it is switched on for 9 !inutes,(a) how !uch charge, and(#) how !any electrons have passed through it?

&olution:

(a)

(#) charge of % electron 5 %.: 0 %;-%< C

no. of electron 5 <;;C 6 (%.: 0 %;-%<) 5 9.:&9 0 %;&%

electrons

I = Qt

3 A= Q

5min( ) 60sec

min

÷

⇒Q= 900C



Electric Current

A sim#le circuit contains the

!ini!u! things needed to have

a functioning electric circuit. Asi!ple circuit re4uires three

things2• A source of electrical potential

difference or voltage. (typically a

#attery)

• A conductive path which wouldallow for the !ove!ent of

charges. (typically !ade of wire)• An electrical resistance

(resistor) which is loosely

defined as any o#$ect that uses

electricity to do wor". (a light

#ul#, electric !otor, heating

ele!ent, spea"er, etc.)

he attery here is represented #y an escalator

which raises charges to a higher level of energy. As the charges !ove through the resistor

(represented #y the paddle wheel) they do wor" on

the resistor and as a result, they lose energy.

=y the ti!e each charge !a"es it #ac" to the #attery,

it has lost all the energy given to it #y the #attery.

As the charges !ove through a wire, they lose noenergy (assuming the wires are short and are a good

conductor).

he potential drop ( - #otential di//erence) across

the resistor is the sa!e as the potential rise (

#otential di//erence) across the #attery. his

de!onstrates that a charge can only do as !uchwor" as was done on it #y the #attery.



Connected to a source of voltage, an electric la!p circuit loo"s

so!ething li"e this2

=ecause it ta"es energy to force electrons to flow against the opposition

of a resistance, there will #e a drop in voltage #etween any points in acircuit with resistance #etween the!. It is i!portant to note that although

the a!ount of current (the 4uantity of electrons !oving past a given

point every second) is unifor! in a si!ple circuit, the a!ount of voltage

(potential energy per unit charge) #etween different sets of points in a

single circuit !ay vary considera#ly.





• 0esistance: he opposition to the flow of electricity

• >easured in 1hms – sy!#ol is the ree" letter @!ega -• lectricity will always ta"e the path of least resistance

• he greater the resistance, the less current there is for agiven voltage.

Electrical 0esistance

hm!s "aw

V = IR



&eries Circuits• &eries Circuits2 rovides only one path for the electrons to follow

– A #rea" in the circuit stops the flow of electricity to all otherparts of the circuit

– With !ultiple light #ul#s (!ore resistance) the current reduces

and the di!!er the lights #eco!e

– A!!eters (to !easure the current) !ust #e wired in series.



&eries Circuits

Besistors are in series have e4ual currents through the! #ecause there

is only one path for the current to follow.

he voltage across each resistor in series will add to the total voltage of

the #attery.

everal resistors in series can #e replaced #y a single resistor that will

have the sa!e current through it as the resistors it replaces. his is the

total resistance* of the co!#ination.

32#

32#

32#

32#

and

R R R R

IR IR IR IR

I I I I IRV V V V V

T

T

T

T

++=

++=

====

++=

$% &



Parallel Circuits• Parallel circuits: different parts of the circuit are on separate

#ranches – electrons must choose a path

• A #rea" (#urned out light #ul#) in the circuit does not stop the flow tothe re!aining devices

• >ultiple identical light #ul#s will re!ain the sa!e #rightness since

the resistance is not decreasing as it does in a series circuit.

• ach pathway can #e separately switched off w6out affecting the

others• 7ousehold circuits are wired in parallel, with a standard of %&; volts

• /olt!eters are wired in parallel to !easure the chan%e in volts

#etween two points in the circuit.



Parallel Circuits

Besistors in parallel have the sa!e

voltage across the! #ecause the

current !uch choose a path to

follow.

everal resistors in parallel can #ereplaced #y a single resistor that will

have the sa!e voltage across it as

any of the resistors it replaced. his

is the total resistance of the

co!#ination.

he total current will #e the su! of

the currents through the resistors it

replaced.32#

32#

32#

32#

####

and

R R R R

RV

RV

RV

RV

V V V V IRV

I I I I

T

T

T

T

++=

++=

====

++=



Parallel Circuits

• he more #aths to choose /rom the 2E&& the

resistance – Water e0a!ple again2 Added pipes co!ing fro! a

large tan" will allow !ore water to flow out than a

single pipe.

– herefore as resistance decreases3 currentincreases" they are inversely proportional

& h ti Di



&chematic Dia%rams• All circuits need at least the All circuits need at least the

following2following2

– ower supply, wire, andower supply, wire, and

resistance.resistance.

• here is a set of standard sy!#olshere is a set of standard sy!#ols

used to represent these ite!s in aused to represent these ite!s in a

diagra! of the circuitdiagra! of the circuit





Electric Po*er

lectric power – Be!e!#er that power is the rate at which

wor" is done and the unit of power is the Watt. owerapplies to electrical power as well as !echanical power.

P = IV ' ( )ower in *atts

& ( current in +m)eres

$( voltage in $olts

IV V t q

t qV

t W P ====



5 / 0 I

5 : / 0 ;.9 A

5 ' Watts

A potential difference of %&;/ is operating on a 9;;

Watt !icrowave oven. What is the current through the

oven?

5 / 0 I

9;; 5 %&; 0 I

I 5 +.& A

Eam#le

A : /olt #attery produces a current of ;.9 A. What is the power in the circuit?



Calculatin% Ener%y Cost

• aying for energy2 We are charged #y the electric

co!pany for the power we use. It is calculated and #illedto us #y the "ilowatt hour.

– nergy 5 ower 0 i!e

– ilowatt hours 5 ilowatts 0 7ours



Eam#le

A coffee pot operates on & a!peres of current on a %%; / circuit for ' hours. Calculate

the total "ilowatt hours used.

' , $ - &

' , ##0 - 2 , 220 *atts , 0.22 /*

nergy used in /ilowatt(hours0.22 /*( ) x 3 hours( ) = 0.66 /*hr

Electric Current Notes

Documents