Download Engineering Science EAB_S_127_Ch1

Engineering Science EAB_S_127
Electricity Chapter 1
Electrical Energy
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Energy cannot be created or destroyed, however, it may
be converted from one form into another.
In the next four lectures we are going to investigate
electrical energy and its application, from basic concepts
to electric circuits.
A cell is a device that can generate electricity, more
precisely, it is a device that converts stored chemical
energy into electrical energy
An electrolyte causes EMF
or Voltage to appear across
the terminals of the cell
Conventional Current
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In reality the flow of current relates to the movement of
charged particles (i.e. electrons) which are in fact negatively
charged through conductive material (e.g. metal wires)
However, historically scientists have considered the flow of
current from high to low potential (voltage)
This is considered “Conventional Current” and most
scientists and engineers use this and not “electron flow”.
Charge and Voltage
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Cells have two principle parameters, the Charge stored,
Q and the terminal Voltage, V.
Charge is measured in Coulombs [C]
Voltage is measured in Volts [V]
Voltage is the Energy Stored per Coulomb of Charge
W
V
Q
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Where W = Energy Stored in Joules [ J ] and Q = Charge
Example: A cell uses 1500 Joules of energy to generate
1000 Coulombs of charge, what is its voltage?
Current and Charge
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The smallest charge is a single electron which has 1.6x10-19
Coulombs
The rate of flow of electrical charge is termed ‘Current’
Q
I
t
Where Q = Charge [C] and t = time [s]
Current is measured in Amps [A]
Example: If 1000C of electrons travel through a wire in
100s, what is the current in A and mA?
Resistance
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Resistance is the property of a material to “resist” the
flow of current
Conductors have low resistivity per unit area
Insulators have high resistivity per unit area
The flow of current through a resistive material causes a
potential difference (or voltage) to develop across it
Fixed external resistors are very useful circuit
components and are made from materials with a known
resistivity per unit area
Voltage Divider Example
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Given that V = 10 and the voltage at VB = 3 what are the
voltages VAB and VBC?
-
+
I
V=10V
VBC
C
VC
VAB
A
B
VB
V
B
VA
Figure for Question 1.2 A cell and two resistors in series
Electrical Power
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Electrical power, P, is given by the amount of electrical
energy converted (or absorbed) per unit time in Watts [W]
Hence
W VQ
P
t

t
 VI
Where W = Energy [ J] absorbed, t = time [s], Q = Charge
[C] and I = Current [A]
Example: A DC motor consumes 2000J of electrical energy
per second when it is in use. Find:
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a) the power consumed by the motor
b) given that the motor requires 200V to operate deduce the
electric current flowing through the motor.
Internal Resistance of Cells
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All the materials inside cells have some resistance
The resistance inside a cell is called its “internal resistance”,
this is denoted by r, and causes a voltage loss when
loaded by an external resistance
V
VL
-
+
+
-
r
-
VR
+
External resistor, R
I
Internal Resistance and Voltage Drop
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Example: A cell V has an internal voltage 1.8 V and the
lost voltage VL is 0.3 V (dropped across its internal
resistance). What is the terminal voltage V across an
external resistor?
V =1.8V
VL=0.3V
-
+
+
-
r
VR
-
+
External resistor, R
I
Connecting Cells in Series
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In order for cells to be connected in series the positive
terminal of Cell 1 must connect to the negative terminal of
Cell 2
Cell 1
VC
-
Cell 2
+
VB
-
VBC
+
VA
VAB
Figure 1.4 Two cells in series
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The voltage across Cell 1 VBC = 15 V, and the voltage across
Cell 2 is VAB = 15 V. If we set VC = 0, then the voltage across
two cells VAC is?
Connecting Cells in Parallel
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In practice two (or more) cells with the same voltage can
be connected in parallel
The voltage across the connected cells is the same as that
across the any of them
The current available from the connected cells is then
multiplied by the number of cells (as the internal
resistance is effectively reduced)