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7/29/2019 ch2 electricitt
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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, F to 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, I in Ampere, t in 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 µC (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 CurrentOhm’s 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 conductor A conductor that obeys Ohm’s Law.
I
V 0
A
V
I
Conductor
Switch
Rheostat Circuit used to find the
relationship between current
I and potential difference V
for a conductor.
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Electric Current
Non-ohmic conductor A conductor that does not obey Ohm’s Law.
I
V 0
I
V 0
I
V 0Dilute 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 V across the
conductor to the current I in the conductor.
V
I Resistance, R =
The unit of resistance is the ohm (Ω).
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
A Resistance, 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
V 1 V 2 V 3V
V 1 = IR1
V 2 = IR2
V 3 = IR3
When resistors are connected in series:a. Same current I is in all the resistors
b. Potential difference,
c. V = V 1 + V 2 + V 3
d. Effective resistance,
R = R1 + R2 + R3
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Parallel Circuit
I R1 I 1
R2 I 2
R3 I 3
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 = I 1 + I 2 + I 3
d. Effective resistance,
b. Current in the resistors,
V R1 I 1 =V
R2 I 2 =
V R3
I 3 =
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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
R I
E = V + Ir = I ( R + r )
E
V
R + r R
r R = = 1 +
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Electrical EnergyThe potential difference V across 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
= I 2 Rt I = V/R
V 2t
RE =
substitutions
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Electrical Power
Electrical power, P =
Energy dissipated
Time, t
V 2
R P =
= I 2 R 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.
V 2
P Resistance 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