Chap 17 currentE 12 S

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Current electricity

Conventional current and electron flow Conventional current and electron flow

a. state that current is a rate of flow of charge and that it is measured in

amperes

b. distinguish between conventional current and electron flow

c. recall and apply the relationship charge = current x time to new

situations or to solve related problems

Electromotive force Electromotive force

d. define electromotive force (e.m.f.) as the work done by a source in

driving a unit charge around a complete circuit

e. calculate the total e.m.f. where several sources are arranged in series f. state that the e.m.f. of a source and the potential difference (p.d.)

across a circuit component is measured in volts

 Potential Difference_{Potential Difference}

g. define the p.d. across a component in a circuit as the work done to drive

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Current electricity ‘08

Resistance

Resistance

h. state the definition that resistance = p.d. / current

i. apply the relationship R = V/I to new situations or to solve related

problems

j. describe an experiment to determine the resistance of a metallic

conductor using a voltmeter and an ammeter, and make the necessary calculations

k. recall and apply the formulae for the effective resistance of a number of

resistors in series and in parallel to new situations or to solve related problems

l. recall and apply the relationship of the proportionality between resistance

and the length and cross-sectional area of a wire to new situations or to solve related problems

m. state Ohm’s Law

n. describe the effect of temperature increase on the resistance of a metallic

conductor

o. sketch and interpret the I/V characteristic graphs for a metallic conductor

at constant temperature, for a filament lamp and for a semiconductor diode

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Circuits

 Closed circuit (pg. 334)_{Closed circuit (pg. 334)}

• a circuit in which there is an a circuit in which there is an

unbroken conducting path round unbroken conducting path round which charge can flow continuously. which charge can flow continuously.

 Open circuit_{Open circuit}

• a circuit in which which there is a a circuit in which which there is a break at one or more points in the break at one or more points in the conducting path so that there is no conducting path so that there is no

current anywhere in the circuit. current anywhere in the circuit.

 Short circuit_{Short circuit}

• a circuit acts as a bypass for the a circuit acts as a bypass for the electric current by providing an electric current by providing an

easier or shorter path for it to flow easier or shorter path for it to flow

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Electric Current



Flow direction (P)

•

Conventional current is opposite to the flow of

negative charge

•

Conventional current flows from +ve terminal to

-ve terminal, while electrons flows from -ve

terminal to +ve terminal.

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 Electric currentElectric current

• Defined as the rate of flow of electric charge through a given Defined as the rate of flow of electric charge through a given cross section of a conductor

cross section of a conductor

• Amount of charge passing a given point in one secondAmount of charge passing a given point in one second

I = Q / t

I = Q / t or or Q = I tQ = I t

Where: I is the current (A)

Q is the amount of charge (C)

T is time (s)

• E.g. 17.1 & 17.2, pg. 332_{E.g. 17.1 & 17.2, pg. 332}

 Questions:_Questions:

• when a current of 3 A flows, how many coulombs of charge are _{when a current of 3 A flows, how many coulombs of charge are}

flowing per second?

• If 8 C of charge flows through a lamp in 2 s, what is the current?_{If 8 C of charge flows through a lamp in 2 s, what is the current?}

• How many electrons flows through the same lamp in 2 s?_{How many electrons flows through the same lamp in 2 s?}

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Measuring Current

• Measured with an ammeter, connected in series in the circuit._{Measured with an ammeter, connected in series in the circuit.}

• Current (conventional) flows into the ammeter from the positive _{Current (conventional) flows into the ammeter from the positive}

terminal (red), and flows out by the …..

• Accuracy and range _{Accuracy and range}

 In the circuit shown, the readings taken from all the ammeters will be the same.

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Electromotive Force



A source of energy is required to move the electrons round a

circuit. e.g. a cell.



E.m.f. is a measure of electrical potential energy which the

charges gain as they pass through the cells.



As charges flow round a circuit, they lose their P.E.,

transforming P.E. into other forms of energy.



The electromotive force (e.m.f.)

•

the e.m.f.of a cell is defined as the work done by the cell in

driving a unit charge round a

complete circuit

•



= work done / charge

= W / Q

where where



= emf, (Volts, V) = emf, (Volts, V)

W = work done, (Joule, J) W = work done, (Joule, J)

Q = positive charge, (Coulomb, C) Q = positive charge, (Coulomb, C)

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Measuring emf

 _{The emf of a cell can be measured by connecting}_{The emf of a cell can be measured by connecting} _a_a

voltmeter directly across the terminals of a cell.

voltmeter directly across the terminals of a cell.  Current should flow into the positive terminal and_{Current should flow into the positive terminal and}

out from the negative terminal.

out from the negative terminal.  Accuracy and range._{Accuracy and range.}

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 Potential difference between 2 points_{Potential difference between 2 points}

• Defined as the amount of electrical energy converted to other _{Defined as the amount of electrical energy converted to other}

forms of energy when one coulomb of positive charge passes

between the two points

• If there is no potential difference, charges will not move and _{If there is no potential difference, charges will not move and}

there will not be a current flow.

• When moving from higher potential to lower potential, electrical _{When moving from higher potential to lower potential, electrical}

energy is converted to light or heat energy.

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• E.g. 17.3 & 17.4, pg. 338_{E.g. 17.3 & 17.4, pg. 338}

• Question:_Question:

– 5 coulomb of charge requires 30 J of energy to move across a _{5 coulomb of charge requires 30 J of energy to move across a}

resistor, what is the p.d. across the resistor?

resistor, what is the p.d. across the resistor?  _{Potential difference across a component}_{Potential difference across a component}

• Potential difference = work done / charge_{Potential difference = work done / charge}

V = W / Q

• Units involved :_{Units involved :} volt(V) = Joule(J) / coulomb(C)volt(V) = Joule(J) / coulomb(C)

• SI unit is volt (V): 1 volt is the potential difference across a _{SI unit is volt (V): 1 volt is the potential difference across a}

component such that 1 joule of work is done in taking 1 coulomb

of charge from one point to another.

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Potential Difference

• A voltmeter is used to measure voltage _{A voltmeter is used to measure voltage}

across a component.

• Voltmeter must be connected in parallel _{Voltmeter must be connected in parallel}

to the component across which the p.d. is

being measured.

• sum of emf of all the cell must be equal to _{sum of emf of all the cell must be equal to}

the sum of p.d. difference across all the

components in the circuit.

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Discussion



Ever wonder why birds do not get electrocuted

when standing on a power cable?



Why do many cows die during a thunder storm even

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Resistance

• A property of a conductor that restricts the movement of free _{A property of a conductor that restricts the movement of free}

electrons in the conductor.

• Causes the electrons give up the potential energy they carry from _{Causes the electrons give up the potential energy they carry from}

the battery.

• Conductors with high resistance converts a lot of electrical energy _{Conductors with high resistance converts a lot of electrical energy}

to other forms of energy.

• good conductor has low resistance._{good conductor has low resistance.}

 Definition_Definition

• resistance of a conductor is defined as the ratio of potential _{resistance of a conductor is defined as the ratio of potential}

difference across the conductor and the current flowing in it

resistance

resistance = volt / current= volt / current

R (R () ) = V (V) / I (A)= V (V) / I (A)

• E.g. 17.5, pg. 340_{E.g. 17.5, pg. 340}

• Try Q.2b, pg. 348Try

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Resistor

 a resistor is a component in a circuit that resist the flow of electricity._{a resistor is a component in a circuit that resist the flow of electricity.}

 _{A variable resistor (Rheostat):}_{A variable resistor (Rheostat):}

• The resistance of this resistor can be changed._{The resistance of this resistor can be changed.}

• Use to control the size of the current in a circuit._{Use to control the size of the current in a circuit.}

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Resistance



Experiment to find resistance of a metallic conductor

• (Pg. 341)_{(Pg. 341)}

• Connect the circuit as shown_{Connect the circuit as shown}

• set the rheostat to maximum resistance_{set the rheostat to maximum resistance}

• close circuit and adjust rheostat until a _{close circuit and adjust rheostat until a} small current is shown on the ammeter. small current is shown on the ammeter.

• record the ammeter and voltmeter reading_{record the ammeter and voltmeter reading}

• adjust the rheostat again and repeat the measurement with suitable _{adjust the rheostat again and repeat the measurement with suitable} values of I.

values of I.

• tabulate results for 5 sets of readings. _{tabulate results for 5 sets of readings.}

• Plot a graph of V against I _{Plot a graph of V against I}

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Ohm’s Law



Ohm’s law states that the:

•

current passing through a conductor is directly

proportional to the potential difference across its

ends, provided that the physical conditions and

temperature remains constant.

I I  V V

or

or V / I = constantV / I = constant



Look at Eg. 17.6, pg. 343

v

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V / I graph for various conductors

Metallic conductor Filament lamp diode

Obeys Ohm’s law

(V/I constant) Yes No No

V/I ratio when

current reverse No change No change

Very high when current flow in –ve

to +ve direction

Special characteristics

V/I constant when physical conditions

are constant

Resistance increase when I

increase

Current increases very rapidly when

V>0.6V

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Resistivity

 Factors affecting resistance of a given _{Factors affecting resistance of a given}

conductor:

• length of the conductor, _{length of the conductor,}

R

R  ℓℓ (1)(1)

• the cross-sectional area of conductor, _{the cross-sectional area of conductor,}

A

R

R  1/ A 1/ A (2)(2)

• the type of material_{the type of material}

• the temperature of the conductor_{the temperature of the conductor}

 _{Combining (1) & (2)}_{Combining (1) & (2)}

R

R  ℓ / A ℓ / A or

or R = R =  ℓℓ / A / A, , where

where  is a property of the material for is a property of the material for the conductor called resistivity.

the conductor called resistivity.

 SI unit for _{SI unit for} is _is m._m.

 Resistivity table ( for info)_{Resistivity table ( for info)}

• low resistivity implies the material has _{low resistivity implies the material has}

low resistance low resistance Area A length l Material

Material Resistivity Resistivity _m_m //

silver silver copper copper aluminium aluminium tungsten tungsten iron iron manganin manganin constantan constantan mercury mercury

1.6 x 10

1.6 x 10-8-8

1.7 x 10

1.7 x 10-8-8

2.8 x 10

2.8 x 10-8-8

5.5 x 10

5.5 x 10-8-8

9.8 x 10

9.8 x 10-8-8

44 x 10

44 x 10-8-8

49 x 10

49 x 10-8-8

96 x 10

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Resistivity

 _{E.g. 17.7, pg. 346}_{E.g. 17.7, pg. 346}

 Try Q6 , pg. 349

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Summary

Current = charge / time

• conventional current conventional current direction

direction • electric flowelectric flow

Current, I

Current, I Potential difference, VPotential difference, V ElectromotiveElectromotive force,

force, 

depends on depends on is driven by

is driven by is driven byis driven by

is is

in in

• lengthlength

• cross-sectional areacross-sectional area

• resistivityresistivity

•temperaturetemperature

Resistance = V/I

flows as flows as relate to relate to Electricity Electricity

• sum of e.m.f.’s sum of e.m.f.’s for cells in series

for cells in series • less than sum of less than sum of e.m.f.’s for cells

e.m.f.’s for cells

in parallel in parallel Effective resistance Effective resistance Arrangement of Arrangement of resistors: resistors: • in series in series

• in parallelin parallel

depends on depends on is

is