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Chapter 2:

Electricity

Form 5

1

Physics

Next >

The study of matter

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Objectives:(what you will learn)

1) electric fields & charge flow

2) electric current & potential difference

3) series & parallel circuits

4) electromotive force & internal resistance

5) electrical energy & power

Physics: Chapter 2

2

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Electric Fields

Electric field: region where a charged bodyexperiences a force

It is shown by a field pattern that are lines of forces.

line of force = path of a test charge in the field

direction = motion of a free positive charge

+

Positive point charge

Negative point charge

electric field pattern

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Electric Fields

Between a positive

and a negative

point charge

Between two

positive point

charges

Electric lines of force

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Electric Fields

Electric field between twoparallel metal plates that

are oppositely charged.

Electric field between

two opposite charges.

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Electric Fields

+

+

+ F F

Ball coated with conductor

hangs vertically in the centre

because it is neutral.

Ball oscillating between 2

plates, after it touches one side

causing a force,Fto repel the

ball due to like charges.

+

Negative ions

Positive ions

Candle flame spreading

sideways between 2 plates due

to attraction between oppositelycharged ions and metal plates.

Experiments to show existence of electric fields.

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Electric Fields

Electric fields cause charges to move.Net movement of charges = electric current

In the late 1700s scientists chose the direction of electric

current to be the direction in which positive charges move in

an electric field. They did not know that electrons andprotons were the negative and positive charge particles, and

that the electron moved much more easily.

In a copper wire, the outer electrons of the copper

atom move relative to the nucleus of the atom.

+ -Current,I electrons

So, the charge carriers (electrons) move in the

opposite direction to the current.

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Electric Charge

8

Electric current = Rate of flow of electric charge

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Next >Electric charge, Q =It

units Q in Coulomb,Iin Ampere, tin second

I=

Q

t , t=time

C = A s

Basic unit of electric charge = Coulomb (C)Charge of a proton or electron = 1.60 10-19 C

A Coulomb of charge is a lot, at 6.25 x 1018 electrons

most objects have charges in the C (10-6 C) range.

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Potential Difference

V=

W

Q

Work done

Charge=

Potential difference (V) between 2 points inan electric field = work done (W) in moving 1

coulomb of charge (Q) between the 2 points.

Unit of potential difference:

Volt (V) = = J C-1

J

C

A BMoving 1 coulomb of charge

Potential difference between 2 points

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Electric CurrentOhms Law

The current (I) in a conductor is directly

proportional to the potential difference (V) across

the conductor if the temperature is constant.

V

I = constant

Ohmic conductorA conductor that obeys Ohms Law.

I

V0

A

V

I

Conductor

Switch

Rheostat Circuit used to find the

relationship between current

Iand potential difference V

for a conductor.

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Electric Current

Non-ohmic conductorA conductor that does not obey Ohms Law.

I

V0

I

V0

I

V0Dilute sulphuric acid Filament lamp Junction diode

Examples

A circuit element is non-ohmic if the graph of

current versus voltage is nonlinear.

A filament lamp is a non-ohmic conductor since its

resistivity, like most materials, varies with

temperature. As the filament gets hot, theresistance increases quickly.

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Resistance

The resistance,R of a conductor is defined as theratio of the potential difference Vacross the

conductor to the currentIin the conductor.

V

IResistance,R=

The unit of resistance is the ohm ().

conductor

V

I I

Potential difference, V=IR

H li ti T iti

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Resistance

Factors that affect the resistance of a conductor:a. length of wire, l

b. cross-sectional area,A

c. type of material with resistivity,p

d. temperature, T

pl

AResistance,R=

Based on a constant temperature:

R

T/o

C0 Metal Semi-conductor

R

0 T/o

C

H li ti T iti

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Series Circuit

I

R1 R2 R3

V1 V2 V3V

V1 =IR1

V2 =IR2

V3 =IR3

When resistors are connected in series:a. Same currentIis in all the resistors

b. Potential difference,

c. V= V1 + V2 + V3

d. Effective resistance,

R =R1 +R2 +R3

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Parallel Circuit

I R1 I1

R2 I2

R3 I3

V

When resistors are connected in parallel:a. Same potential differences across all resistors, V

1

R

1

R1

1

R3

1

R2= + +

c. I=I1 +I2 +I3

d. Effective resistance,

b. Current in the resistors,

VR1I1 =V

R2I2 =

VR3

I3 =

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Electromotive ForceElectromotive force (e.m.f.),E

Work done to drive a unit charge (1 C) around circuit

where the unit is

volt, V = J C-1

Using a high resistance voltmeter

Potential difference V< e.m.f.E

because work is done to drive a

charge through a cell with internalresistance, r.

E= 1.5 V

V

I

r

V

RI

E= V+Ir=I(R + r)

E

V

R + rR

rR= = 1 +

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Electrical EnergyThe potential difference Vacross a conductor is the

work done in moving a charge of 1 C across the

conductor. The work done is transformed into heat

which is dissipated from the conductor.

From volt, V= J C-1 = Energy dissipated, E

Charge, Q

Energy dissipated, E = QV Q = It

=IVt V= IR

=I2Rt I= V/R

V2t

RE =

substitutions

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Electrical Power

Electrical power,P=

Energy dissipated

Time, t

V2

RP=

= I2R I= V/R= IV V= IR

substitutions

E= IVt

Power rating of an electrical appliance is the power

consumed by it when the stated voltage is applied.

V2

PResistance of the appliance, R =

1 unit of electrical energy consumed = 1 kW h

= (1000 Js-1)(3600 s) = 3.6 x 106 J

Cost of electrical energy = units x cost per unit

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Summary

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What you have learned:

1. Electric fields & charge flow

Thank You

2. Electric current & potential difference

3. Series & parallel circuits

4. Electromotive force & internal resistance

5. Electrical energy & power

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