Physics - Summaries - Current Electricity

8/14/2019 Physics - Summaries - Current Electricity

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Topic 8: Current of ElectricityThere ore o number oJ definitions to recallfor this chapter. Checkthot you know the Jollowjng.

Electric CurrentElectric current is the rate offlow of electric charges.

where Q = total charge, In terms of units:n = number of charge carriers,q = charge on each charge carrier

1A=1Cs1

For an electron, the elementary charge is e = 1.6 x lole C.Charge carriers can be:(a) Free electrons (in conductors and semiconductors)(b) lons (in fluids)(c) Holes (in semiconductors)Direction of convention current:. opposite to the direction of the electron/negative ion flow;. in the same direction as the flow of positive ions_

an electrical property oI particles, such as electrons and protons, whichthem to attract or repel each otherthe quantity of electric charge that passes through a given section when asteady current of one ampere flows for one secondPotentialdifference potential difference between two points in a circuit is the amount of electricalenergy converted to other forms of energy when unit charge pasies from onepoint tothe otherthe potential difference between two points in a circuit if one joule of electricalenergy is converted to other forms of energy when one coulomb of chargefrom one point to the other

Resistance the ratio of the potential difference across the conductor to the current flowthroush itthe resjstance ol a conductor if a current of one ampere flows in it when apotential difference of one volt is applied across itthe energy converted into electrical energy when a unit charge js delivered byan e.m,f source

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a

Related formulaewhere W = work done in drivingthecharge between the 2 points

Ohm's Law

ln terms of units: 1V=1lC-lV=lR

where P = power used to drive the current between the 2 points

E : 12 RtNote 1: When calculating power or energy lq$ through joule heating (heating of circuitcomponents when current flows through. This is due to non-zero reslstance in them), weusually use P = I2 R and E : 12 Rt, where 1is the current flowing in the component and R is theresistance of the component.Note 2: At times you are required to compare brightness of bulbs when switches are togg,ed onand off. Remember that brightness of a bulb o< power delivered to it (so use the equationsinvolving P to Iind lhe answer).Example: Three identical light bulbs are connected as shown in the circuit below,

Readings on... @ @ @(A) lf bulb A blows, 24r) o 0(B) lf bulb B blows, 120 720 720(C) lf bulb C blows, a20 t20 120(D) lf wire X breaks, o 0 o

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n=4Resistance and Resistiviw

For example,

When connected this way

whereP = resistivity'/ = length (distance that current flows across);I = cross-sectional area {note: this must be the area through which currentflows)

When connected thisway A:xxyI=z

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I - V Characteristic Graphs that you must be famiiiar with {including the explanations)Material Explanation

Straight line through the origin

{a} Pure metals at constanttemperature

I cc V- Hence the resistance V/l is constant.An electrical conductor which obeys Ohm's taw istermed an ohmic condudor.Resistance of metal is due to collisions between the freeelectrons and the metal ions in the lattice.lf the temperature of the conductor is kept constant,the lattice ion vibrations will remain the same,frequency of collision will be constant and theresistance will remain the same.

When temDerature increasesvibration of the atoms become largernumber of free electrons remains more or less thesame but the free electrons coljide more frequentlywith the atomsresults in more collision and hence resistance increases(bl Filament lamp

Curve concave downwards

At very low p.d, the characteristic is a straight line, butas the p.d, across a filament lamp increases. currenti4creases. This implies more electrons are flowing perunit time, and colliding more frequently with the latticeions-Energi transferred from the electrons results in thelattice ions vibrate with !4gg! amplitude, causing thefilament to lgqllp to over 1OO0"C, and the lamp getsbrighter.At the same time, as electrons collide more with thevibrating ions, the resistance of the filament wireincreases.

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(c)

voltage{=200V)

decreasing

Curve concave upwards

(dl Diodefpforward-biasedvoltage

A p-n iunction diode is a semiconductor which allowscurrent to pass easily in one direction, but not in theopposite direction. 5o, its resistance changes with thecurrent direction. This special property makes it usefLrlas a rectifier for changing a.c. to d.c.lf the: p.d. is applied in the direction where currentflows, it is said to be fuMlllhbfCd. A silicon dioderequires a p.d. around LZ_y to conduct.Once the applaed p.d, is above 0,7 V, the current risesvery quickly with the p.d., and the diode has very !!!a!!resistance.lf the p.d. is applied in the oppodte direction asdirection flows, it is said to be IgyglgqbiggClL almostlglq current flowt and the diode has very higbresistance.lf reverse-biased p.d. is too high {= 200 V), the diodemay h&a-k_-dpw! and the current becomes infinite,resulting in a short circuit.

Thermistor is o semi-conductor derire *hosi norne othe contrdction of "thermolly sensitive resistot".As larcer p.d is opplied, the current through thethelmistor rises ond couses the semi-concluctor to hedtap.

Whe n t em pef dtu re incr e ose s. vibrotion of the otoms become lotger ctnd resistonceincredses due to more callisions.. At the same time, the otoms rcleose morc mobileelectrons which fom port of the current. When thecurrent increoses, the rotio Vl decrease- resulting in odecreose in its resistonce.. The effect on rcsistonce due to the lorge incteose in thenumber of mobile electrons (charge cctrriers) is muchmore sionificont thon thot of the increose in otomicvibrotion.. Hence the overoll resistance decreoses.

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lnternal Resistanceln practice, some of the electrical energy in emfinternal resistance ofthe source itself.Unless specifically stated, internal resistancequantitative questions.

within the source due toneSligible when solving

on the voltmeter is not zero!

sources is dissipatedis considered to be

Terminal p.d. is defined as (emf - p.d. across internalresistance)

Note: lnternal resistance is "activated" whenever a current flows through the emf source-i.e. when no current flows through the emf source, there is no potential difference across

the internal resistance.When the swltch is open, no current flows in thecircuit. Hence there is no p,d. across the internal

ion, readingtrlQuestion: Across which two points in the above circuit will the voltmeter reading be zero?(Refer to examples on next page)

Maximum PowerTheorem (no need to derive -iust have to recall and applythis theorem)The theorem states that the maximum power is delivered to the load when the resistance ofthe load (R) is equal to the internal resistance of the source (r).

resistance,ln this situatInstead,

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More circuiis

Since no current flows in the circuit, there is !qpotential difference across the 2.5 O resistor.Voltmeterreading=0V

Voltmeterreading=6V

Since no current flows in thepotential difference across theVoltmeterreading=6V

circuit, there is !qinternal resistance-

Question: What are the respectiv voltmeter readings when the switch is closed instead?(Answer:5V0V,5V)

Physics - Summaries - Current Electricity

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