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Form 5
Physics
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The study of matter
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Chapter 2:
Electricity
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Physics: Chapter 2
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Objectives:
(what you will learn)
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1)
electric fields & charge flow
2)
electric current & potential difference
3)
series & parallel circuits
4)
electromotive force & internal resistance
5)
electrical energy & power
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Electric Fields
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Electric field: region where a charged body
experiences 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
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direction = motion of a free positive charge
electric field pattern
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Positive point charge
Negative point charge
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Electric Fields
Electric lines of force
Between a positive
and a negative
point charge
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Between two
positive point
charges
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Electric Fields
Electric field between two
parallel metal plates that
are oppositely charged.
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Electric field between
two opposite charges.
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Electric Fields
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Experiments to show existence of electric fields.
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F
F
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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.
Positive ions
Negative ions
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–
Candle flame spreading
sideways between 2 plates due
to attraction between oppositely
charged ions and metal plates.
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Electric Fields
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Electric fields cause charges to move.
Net movement of charges = electric current
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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 and
protons 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.
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Current, I
electrons
So, the charge carriers (electrons) move in the
opposite direction to the current.
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Electric Charge
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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Electric charge, Q = It
units Q in Coulomb, I in Ampere, t in second
C=As
Electric current = Rate of flow of electric charge
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I=
Q
t
, t = time
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Potential Difference
Potential difference (V) between 2 points in
an electric field = work done (W) in moving 1
coulomb of charge (Q) between the 2 points.
W
Work done
V=
=
Q
Charge
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Potential difference between 2 points
A
Moving 1 coulomb of charge
Unit of potential difference:
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Volt (V) =
J
C
= J C-1
B
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Electric Current
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Ohm’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
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I
= constant
Ohmic conductor
A conductor that obeys Ohm’s Law.
0
V
Switch
I
A
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Rheostat
Conductor
V
Circuit used to find the
relationship between current
I and potential difference V
for a conductor.
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Electric Current
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Non-ohmic conductor
A conductor that does not obey Ohm’s Law.
Examples
I
I
I
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0
V
Dilute sulphuric acid
0
V
Filament lamp
V
0
Junction diode
A circuit element is non-ohmic if the graph of
current versus voltage is nonlinear.
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A filament lamp is a non-ohmic conductor since its
resistivity, like most materials, varies with
temperature. As the filament gets hot, the
resistance increases quickly.
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Resistance
The resistance, R of a conductor is defined as the
ratio of the potential difference V across the
conductor to the current I in the conductor.
V
Resistance, R =
I
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The unit of resistance is the ohm (Ω).
I
conductor
I
V
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Potential difference, V = IR
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Resistance
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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
Based on a constant temperature:
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pl
Resistance, R =
A
R
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R
T/oC
0
Metal
0
T/oC
Semi-conductor
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Series Circuit
When resistors are connected in series:
a. Same current I is in all the resistors
b. Potential difference,
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V1 = IR1
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V2 = IR2
V3 = IR3
c. V = V1 + V2 + V3
d. Effective resistance,
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R = R1 + R2 + R3
I
R1
R2
R3
V1
V2
V
V3
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Parallel Circuit
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When resistors are connected in parallel:
a. Same potential differences across all resistors, V
b. Current in the resistors,
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I1
=
I2
=
I3
=
V
R1
V
R2
V
R3
I
R1
I1
R2
I2
R3
I3
c. I = I1 + I2 + I3
d. Effective resistance,
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1
1
1
1
+
=
+
R
R2
R3
R1
V
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Electromotive Force
Electromotive 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
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Using a high resistance voltmeter
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E = 1.5 V
Potential difference V < e.m.f. E
because work is done to drive a
charge through a cell with internal
resistance, r.
E = V + Ir = I(R + r)
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E
r
R+r
=1+
=
R
R
V
r
I
V
R
V
I
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Electrical Energy
The 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.
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From volt, V = J C-1 =
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Energy dissipated, E
Charge, Q
substitutions
Energy dissipated, E = QV
= IVt
= I2Rt
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V2t
E=
R
Q = It
V = IR
I = V/R
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Electrical Power
Electrical power, P =
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Energy dissipated
Time, t
E = IVt
= IV
V = IR
substitutions
= I2R
I = V/R
V2
P=
R
Power rating of an electrical appliance is the power
consumed by it when the stated voltage is applied.
V2
Resistance of the appliance, R =
P
1 unit of electrical energy consumed = 1 kW h
= (1000 Js-1)(3600 s) = 3.6 x 106 J
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Cost of electrical energy = units x cost per unit
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Summary
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What you have learned:
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1.
Electric fields & charge flow
2.
Electric current & potential difference
3.
Series & parallel circuits
4.
Electromotive force & internal resistance
5.
Electrical energy & power
Thank You