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Title: VCE Physics Unit 1


1
VCE Physics Unit 1
ELECTRICITY
2
Unit Outline
  • Apply the concepts of Charge (Q), Electric
    Current (I), Potential Difference (V), Energy (E)
    and Power (P), in electric circuits.
  • Analyse electrical circuits using the
    relationships I Q/t, V E/Q, P EIt VI, E
    VIt.
  • Model Resistance in Series and Parallel using,
  • Potential Difference versus Current (V-I) Graphs
  • Resistance as the Potential Difference to
    Current ratio, including V/I R constant
    for ohmic devices.
  • Equivalent effective resistance in arrangements
    in
  • series RT R1 R2 R3 ..
  • parallel 1/RT 1/R1 1/R2 ..
  • Model simple electrical circuits such as car and
    household AC electrical systems as simple direct
    current (DC) circuits.
  • Model household electricity connections as a
    simple circuit comprising fuses, switches,
    circuit breakers, loads and earth.
  • Identify causes, effects and treatment of
    electric shock in homes and relate these to
    approximate danger thresholds for current and
    time.
  • Investigate practically the operation of simple
    circuits containing resistors, including variable
    resistors, diodes and other nonohmic devices.
  • Convert energy values to kilowatt-hour (kWh)
  • Identify and apply safe and responsible when
    conducting investigations involving electrical
    equipment and power supplies.

3
Chapter 1
  • The Basics

4
1.0 Electric Charge
Atoms consist of a nucleus, containing protons
and neutrons, with electrons circulating around
it.
Electric Charge is a property of some atomic
particles. Which ones ?
Protons and Electrons. These two particles carry
an equal and opposite electric charge. This
charge is the smallest known amount of charge
that exists independently. This charge is called
the ELEMENTARY CHARGE
The UNIT of Electric Charge is the COULOMB,
Symbol (C).
Protons carry a charge of 1.6 x 10-19
Coulombs Electrons carry a charge of 1.6 x
10-19 Coulombs
If each electron (or proton) carries such a
small charge, a large number would be needed to
make up 1 Coulomb of charge.
The charge carried by the Proton is DEFINED to be
POSITIVE. The charge carried by the Electron is
DEFINED to be NEGATIVE.
5
Charge
1. How many electrons in 1 coulomb of charge ?
1 electron carries 1.6 x 10-19 C So 1 Coulomb of
charge will have 1/(1.6 x 10-19) electrons 6.25
x 1018 electrons
2. How many electrons in 7.5 C of charge ?
1C has 6.25 x 1018 electrons so 7.5 C will have
7.5 x 6.25 x 1018 4.69 x 1019 electrons
6
1.2 Electric Current
Mathematically I Q/t
When electric charges are made to move or flow an
ELECTRIC CURRENT (Symbol I) is said to
exist. The SIZE of the Current depends on the
number of Coulombs of Charge passing a given
point in a given Time. The Unit of Current is the
Ampere often shortened to Amp (Symbol A)
Where I Current in Amps Q Charge in
Coulombs t Time is Seconds
Electric current has the property of starting
immediately a circuit is complete and stopping
immediately a circuit is broken.
Once the current is flowing it stays the same all
around the circuit.
So, in 1 second 6.25 x 1018 electrons pass this
point
7
Current
3. Calculate the current flowing if 3.57 Coulomb
of charge passes a point in 1.25 sec.
I Q/t 3.57/1.25 2.86 Amp
  • 4. If 5.62 A of current flows through a wire in
    0.68 sec.
  • How much charge has been moved ?

I Q/t ? Q It (5.62)(0.68) 3.82 C
(b) How many electrons were needed to transport
the charge in (a) ?
If 1 C of charge is carried by a total of 6.25 x
1018 electrons, then 3.82 C is carried by (3.82)(
6.25 x 1018) 2.39 x 1019 electrons
5. If a current of 125 A resulted from the
movement of 225 C of charge, for how long did the
current flow ?
I Q/t ? t Q/I 225/125 1.8 sec
8
1.3 Conventional versus Electron Current
In Direct Current (DC) electric circuits, the
current always flows in one direction. On circuit
diagrams, it is ALWAYS shown as flowing from the
positive to negative terminal of the power source.
BUT, we know that a current is a stream of
electrons (negative particles), which must travel
in the other direction. Whats going on ?
It is a quirk of history that the current
direction is shown this way. Electric currents
were discovered before the electron. It was
thought that the charge carriers were positive
and the current must flow this way,
Never shown on circuit diagrams
This means the current carriers must be
positively charged because they will be repelled
(like charges repel) from the positive terminal,
and attracted (unlike charges attract) to the
negative terminal. Protons are the Positive
particles.
Wire connected to a Battery
Always shown on circuit diagrams
9
Conventional vs Electron Current
6. Currents shown on circuit diagrams A are
from the negative to the positive terminal of the
power supply and are called electron currents B
are from the positive to the negative terminal of
the power supply and are called conventional
currents C are from the positive to the negative
terminal of the power supply and are called
electron currents D are from the negative to the
positive terminal of the power supply and are
called conventional currents
10
1.4 Potential Difference
For a current to flow around a circuit a driving
force is needed. This driving force is the
difference in VOLTAGE between the start and the
end of the circuit. The larger the current you
want the greater the Potential Difference
(Voltage Difference) you require.
Potential Difference (P.D.) is best understood
using the water analogy
A short drop between storage and tap gives low
water pressure a low P.D.
Low output from the tap low current. SO A SMALL
P.D. CAN ONLY DRIVE A SMALL CURRENT.
A large drop between storage and tap gives high
pressure Large P.D.
High output at the tap high current SO A LARGE
P.D. CAN DRIVE A LARGE CURRENT.
Strictly, Potential Difference is DEFINED as a
measure of the energy given to the charge
carriers (the electrons) for them to complete
their job, that is, to travel once around the
circuit.
11
1.5 Potential Difference (2)
Where V P.D. measured in Volts (V) E
Electrical Potential Energy in Joules (J) Q
Electrical Charge in Coulombs (C)
Mathematically V E/Q
This means that by passing through a P.D. of 1
Volt, 1 Coulomb of charge picks up 1 J of energy,
or more simply 1 V 1 JC-1
In this case, each coulomb passing through the
battery will pick up 12 J of energy. (The energy
is used up in lighting the globe)
24 V
The Battery P.D. is now increased to 24 V. How
many Joules of energy will each coulomb now pick
up ?
Answer 24 J
There are many terms used in texts to describe
Voltage, some of these include Potential,
Potential Difference, Potential Drop, Voltage
Drop, Voltage Difference At this stage of your
studies you can take them all to mean the same
thing. The preferred term for the VCAA examiners
in Potential Difference
One further voltage, EMF (Electro Motive Force),
while still measured in volts, is slightly
different and cannot be grouped with the other
terms.
12
Voltage
7. One coulomb of charge passing through a
battery picks up 15 J of energy. What Potential
Difference did the charge pass through ?
V E/Q 15/1 15 V
8. An external circuit is connected to a 24 V
battery. If 6.5 C of charge passes through the
battery. (a) How much energy does each coulomb
of charge pick up in passing through the battery ?
Each Coulomb will pick up 24 J of energy in
passing through the battery
(b) How much energy (in total) has the battery
supplied to the charge passing through it
V E/Q ? E VQ (24)(6.5) 156 J
13
1.6 Electrical Energy
The Charge Carriers in a circuit obtain their
energy from a power source or power supply. The
amount of energy (E) the charge carriers pick up
depends upon the size of the voltage difference
through which they are forced to travel. Since
energy transferred work done, another way of
defining electrical energy (E) is by the work
done on the charge (Q) in passing through a
Voltage (V)
Mathematically E VQ and since
Q It Substituting we get E VIt
Where E Electrical Energy (J) V Voltage
(V) Q Charge (C) I Current (A) t Time (s)
An external wire connected to a battery will have
electrons flowing through it as shown.
In completing the circuit inside the battery, the
electrons need to flow from the positive to the
negative terminal.
They will not do this willingly and must be
forced through the battery. The work done on the
electrons increases their electrical energy and
gives them enough energy to do another trip
around the external circuit.
14
Electrical Energy
9. A current of 4.2 A is being driven around a
circuit by a Potential Difference of 87 V. If the
circuit is allowed to operate for 36 s, how much
energy has been transferred to the charge
carriers ?
E VIt (87)(4.2)(36) 1.32 x 104 J
10. A total of 1.2 x 103 J of electrical energy
has been transferred to the charge carriers in a
circuit driven by a 48V battery. If the circuit
is switched on for 12 minutes, how many mA
(milliamp) of current will have flowed during
this time ?
E VIt ? I E/Vt (1.2 x 103)/(48)(12 x 60)
0.035 A 35 mA
11. A circuit is switched on for 6.5 minutes in
that time 3.5 x 104 J of energy has been
transferred to the charge carriers. If the
current flowing was 11.3 amps, calculate the
Potential Difference of the power supply driving
that current.
E VIt ? V E/It (3.5 x 104)/(11.3)(6.5 x
60) 7.9 V
15
1.7 Electrical Energy (2)
The energy picked up by the charge carriers is
used up in driving whatever device is connected
to the external circuit. Here we have an
incandescent light globe as part of a circuit.
16
1.8 Electric Power
Electric Power is DEFINED as the Time Rate Of
Energy Transfer or the Time Rate Of Doing Work.
Mathematically P E/t And since E VIt
Substituting we get P VI
Where P Power (in Watts, W) E Electrical
Energy (J) V Voltage (V) I Current (A) t
Time (s)
Using Ohms Law (See Chapter 2.) The Power
formula can be rewritten as P VI I2R V2/R
17
Electrical Power
12. Calculate the power consumed by an electric
drill operating at 240 V and 7.5 A.
P VI (240)(7.5) 1800 W
13. An electric oven consumes 1.5 x 107 J of
energy while cooking a roast. If the roast took 2
hours to cook, at what power is the oven
operating (quote your answer in kW) ?
P E/t (1.5 x 107)/ (2 x 60 x 60) 2.1 x 103
W 2.1 kW
14. An electric kettle is rated at 3000 W. It is
fitted with a 15 Amp safety switch. If it is
connected to a 240 V supply will the safety
trip (switch off) ? Back up your answer with a
calculation.
No P VI ? I P/V 3000/240 12.5 A less
than the 15 A safety switch rating
15. The kettle mentioned in Q 14 is taken on a
world trip by its owner. In America (where the
mains supply operates at 110V) he plugs it into a
wall socket. Will the safety switch trip now ?
Back up your answer with a calculation.
Yes P VI ? I P/V 3000/110 27.3 A greater
than safety switch rating
18
1.9 Common Electrical Symbols
Single Cell
Battery
A.C. Power Supply
Earth or Ground
Crossed Wire not Joined
Fixed Resistor
Switch
Crossed Wires - Joined
Capacitor
Globe
Diode
Variable Resistor
LED
Voltmeter
Galvanometer
Ammeter
19
Electrical Components
16. Identify the numbered components in the
circuits below
(a)
(b)
(a) 1 Variable Resistor, 2 AC Supply, 3
Ammeter, 4 Lamp, 5 Earth, 6 Switch (b) 1
Voltmeter, 2 Battery, 3 Capacitor, 4
Galvanometer, 5 Fixed Resistor
20
1.10 Series and Parallel
Electrical components can only be connected
together in one of two ways Series where
components are connected end to end
Parallel - where components are connected side by
side.
21
1.11 A Typical Electric Circuit
Circuit diagrams are usually drawn in an
organized manner with connecting wires drawn as
straight lines and the whole diagram generally
square or rectangular in shape.
  • An electric circuit contains a number of
    components, typically
  • A Power Supply
  • Connecting Wires
  • Resistive Elements
  • Meters

The Voltmeter measures the voltage drop across
the resistive element. It is connected in
parallel It has a very high internal resistance
which diverts very little current from the main
circuit.
This represents the part of the circuit where
electrical energy is consumed. The resistive
element could be a light globe or heater or a
radio or a television.
This power supply is a D.C. Supply (a Battery),
and it drives the current in one direction only.
Connecting wires are drawn as straight lines with
right angle bends. They are always regarded as
pure conductors having no resistance.
The Ammeter measures current flow in the main
circuit. It is connected in series It has
virtually no internal resistance so as not to
interfere with the current in the main circuit
A
22
Meters
17. A Galvanometer (which is a very sensitive
ammeter) when included in a circuit should be
connected A In parallel B Across the power
supply C In series D Any way around, it doesnt
matter
18. In ideal circuits the wires used to connect
the circuit components together have A No
resistance B A small amount of resistance C A
large amount of resistance D An infinite amount
of resistance.
19. Voltmeters and Ammeters differ because A
Voltmeters have low internal resistance and are
connected in series while Ammeters have high
internal resistance and are connected in
parallel. B Voltmeters have high internal
resistance and are connected in parallel while
Ammeters have low internal resistance and are
connected in series. C Voltmeters have low
internal resistance and are connected in series
while Ammeters have high internal resistance and
are connected in parallel. D Voltmeters have
high internal resistance and are connected in
series while Ammeters also have high internal
resistance and are also connected in series
23
Chapter 2
  • Resistance

24
2.0 Resistance
All materials fall into one of three categories
as far as their electrical conductivity is
concerned.
ALL materials exhibit some opposition to currents
flowing through them. Conductors show just a
small amount of opposition. Semiconductors show
medium to high opposition. Insulators show very
high to extreme opposition.
  • They are either
  • Conductors
  • Semiconductors, or
  • Insulators
  • This opposition is called ELECTRICAL RESISTANCE.
  • The amount of resistance depends on a number of
    factors
  • The length of the material.
  • The cross sectional area of the material.
  • The nature of the material, measured by
    Resistivity

Mathematically R ?L/A
Where R Resistance in Ohms (?) ? Resistivity
in Ohm.Metres (?.m) L Length in Metres (m) A
Cross Sectional Area in (m2)
Wires 1 2 are made from the same material (? is
the same for each), and are the same length (L is
also the same). Wire 1 has twice the cross
sectional area of Wire 2. Wire 1 has ½ the
resistance of Wire 2
25
Resistance
20. Nichrome wire is sometimes used to make the
heating elements in electric kettles. It has a
resistivity of 6.8 x 103 O.m. Calculate the
resistance of a piece of nichrome wire of length
1.2 m and cross sectional area 2 x 10-4 m2
R ?L/A (6.8 x 103)(1.2)/(2.0 x 10-4) 4.1 x
107 O 41 MO
21. Two pieces of wire are made of the same
material and are of the same cross sectional
area. Wire 1 is 3 times as long as wire 2. A
Wire 1 has 3 times the resistance of Wire 2 B
Wire 2 has 2/3 the resistance of Wire 1 C Wire 1
has 1/3 the resistance of Wire 2 D Wire 1 has 6
times the resistance of Wire 2
26
2.1 Resistors
Resistors are conductors whose resistance to
current flow has been increased.
or
They are useful tools for demonstrating the
properties of Electric Circuits. Understanding
how these circuits work is an important life
skill you all need to develop.
There are only two ways to join resistors
together IN SERIES The resistors are connected
end to end with only one path for the current to
flow. The more resistors the greater the overall
resistance
Resistor is a generic term representing a whole
family of conductors such as toaster elements,
light bulb filaments, bar radiators and kettle
elements.
IN PARALLEL The resistors are connected side by
side with more than one path for the current to
flow. The more resistors the lower the overall
resistance
They are represented on circuit diagrams as
either,
27
2.2 Resistors in Series
Connected end to end, this combination of
resistors gives only 1 path for current flow.
The TOTAL RESISTANCE (RT) of this combination
equals the sum of resistances Thus, RT R1
R2 R3 In other words, the 3 resistors can be
replaced in the circuit with a single resistor of
size RT
Because there is only 1 path for the current to
flow, the current must be the same everywhere.
The current drawn from the power supply (I) is
equal to the currents through the resistors.
Thus I I1 I2 I3
The sum of the Potential Differences across the
resistors is equal to the Potential Difference of
the supply Thus VS V1 V2 V3
28
Resistors in Series
21.(a) Calculate the equivalent resistance that
could replace the resistors in the circuit.
In series resistances add. RT R1 R2 R3 24
15 11 50 O
(b) Determine the value of V3
In a series circuit VS V1 V2 V3 ? V3 VS
(V1 V2) 6.0 (2.9 1.8) 1.3 V
(c) Determine the values of I1 and I2
In a series circuit, current is the same
everywhere so I1 I2 I 0.12 A
29
2.3 Resistors in Parallel
When connected side by side, this combination of
resistors (called a parallel network) gives many
paths for current flow.
The TOTAL RESISTANCE (RT) is calculated
from 1/RT 1/R1 1/R2 1/R3. In other words
the three resistors can be replaced by a single
resistor of value RT.
The physical effect of this formula is that the
value of RT is always less than the lowest value
resistor in the parallel network.
The current has many paths to travel and the
total current drawn from the supply (I) is the
sum of the currents in each arm of the
network. Thus I I1 I2 I3
Each arm of the parallel network gets the full
supply Potential Difference. Thus VS V1 V2
V3
30
Resistors in Parallel
22. (a) What single resistor could be used to
replace the 3 resistors in the circuit ?
1/RE 1/R1 1/R2 1/R3 1/1000
1/3000 1/12000 RE 706 O
(b) Determine the values of V1, V2, and V3
In a parallel network VS V1 V2 V3 12 V
(c) Determine the value of I1
In a parallel network I I1 I2 I3
I1 17 (12 1)
4 mA
31
Resistors Combined
23. (a) What single value resistor could be used
to replace the network shown ?
Simplify parallel networks first. For 10 and 15,
RE (1/10 1/15)-1
6.0 O. For 50,50,100 RE (1/50 1/50
1/100)-1
20 O. Now RE 6 24 20
50 O
(b) What is the Potential Difference across and
the current through the 24 O resistor ?
V24O 25 (10 3) 12 V, I24O I 0.5 A
32
2.4 Ohms Law
Conductors which obey Ohms Law are called Ohmic
Conductors.
When expressed graphically, by plotting V against
I, Ohms Law produces a straight line graph with
a slope equal to resistance (R)
The relationship between, the potential
difference across, the current through, and the
resistance of, a conductor was discovered by
Georg Ohm and is known as Ohms Law
Ohms Law stated mathematically is V IR
Where V Potential Difference in Volts (V) I
Current in Amps (A) R Resistance in Ohms (?)
Note the graph passes through the origin (0,0) as
it must, since if both V and I are zero,
resistance is a meaningless term.
33
Ohms Law (1)
24. A current of 2.5 mA is flowing through a
resistor of 47 kO. What is the Potential Drop
drop across the resistor ?
V IR (2.5 x 10-3)(4.7 x 104) 117.5 V
25. A 12 V battery is driving a current through
an 20 O resistor, what is the size of the current
flowing ?
V IR ? I V/R 12/20 0.6 amp
26. A resistor has a 48 V potential difference
across it and a 2.4 A current flowing through
it. What is its resistance ?
V IR ? R V/I 48/2.4 20 O
34
Ohms Law (2)
27. What are the readings on meters V and A ?
V VR VSUPPLY 12 V I VR/R 12/(1.2 x
103) 0.01 A
28. (a) Determine the value of the current
measured by ammeter A1 (express your answer in mA)
Need to find equivalent resistance by simplifying
parallel networks. Simplify parallel networks
first. For 1.0k and 1.5k, RE (1/1000
1/1500)-1 600 O. For 500, 500, 1k RE
(1/500 1/500 1/1000)-1 200 O. Now
RE 600 2400 200 3200 O
3.2 k O Now I V/R 25/3200
0.0078 A
7.8 mA
35
Ohms Law (3)
(b) Determine the value of the potential
differences measured by voltmeters V1, V2 and V3.
V1 Voltage across 1k, 1.5k combination. RE
600 O, so V1 IRE (7.8 x 10-3)(600)
4.68 V V2 voltage across 2.4 kO
(7.8 x 10-3)(2.4 x 103) 18.72 V V3
Voltage across the 500, 500, 1k combination
IRE (7.8 x 10-3)(200) 1.56 V
(c) Determine the current measured by ammeter A2
A2 current through 500 O resistor V/R
1.56/500 3.12 x 10-3 A
36
2.5 Short Circuits
Short circuits occur when the Resistive parts of
a circuit are bypassed, effectively connecting
the positive terminal of the power supply
directly to the negative terminal providing a
resistance free path for the current.
The current immediately increases to its maximum.
This can be disastrous for the circuit causing
rapid heating and possibly a fire.
This situation is taken care of by the use of
fuses, circuit breakers, safety switches, or
residual current devices. (See chapter 5).
37
Chapter 3
  • Non Ohmic Devices

38
3.0 Non Ohmic Conductors
Conductors which do not follow Ohms Law are
called Non Ohmic Conductors Devices such as
diodes and transistors can be classed as non
ohmics, but the best known non ohmic is the
incandescent light globe. When a plot of
Potential Difference against Current is drawn, it
is not a straight line.
A Typical Characteristic Curve for an
Incandescent Light Globe
39
Non Ohmics - Series
29. Two non ohmic conductors with Characteristic
Curves as shown opposite are connected in series
in a circuit as shown. The voltage across device
1 is 6.0 V. (a) What is the current through
Device 2, (b) What is the voltage across Device
2, (c) What is the voltage of the battery
powering the circuit ?
  • Voltage across device 1 6.0 V so current
    through device 1 0.5 A (read from graph)
    because devices are in series current is same
    through both.
  • So I DEVICE 2 0.5
  • Voltage across device 2 4.0 V (read up from 0.5
    A on graph for device 2).
  • VSUPPLY Sum of voltage drops around the circuit
    6.0 4.0 10.0 V

40
Non Ohmics Parallel
30. Two devices with Characteristic Curves shown
are connected in parallel. The current through
Device 1 is 1.0 A. (a) What is the voltage across
Device 2 ? (b) What is the current through Device
2 ? (c) What is the voltage of the battery ? (d)
the total current drawn from the battery ?
  • If current through device 1 1.5 A, then voltage
    across it 8.0 V (read from graph). In parallel
    network voltage is same across each member
  • so VDEVICE 2 8.0 V
  • (b) If voltage across device 2 8.0 V, current
    2.0 A (read from graph).
  • (c) VSUPPLY VDEVICE 1 VDEVICE 2 8.0 V
  • (d) Total Current Sum of currents through each
    component 1.5 2.0 3.5 A

41
Chapter 4
  • Cells Batteries

42
4.0 Cells and Batteries
  • Electrical Cells (as opposed to plant and animal
    cells) are devices which perform two functions
  • Charge Separation.
  • Charge Energisation.

Charge Separation is the process of separating
positive and negative charges to produce a
POTENTIAL DIFFERENCE capable of driving a current
around an external circuit.
Charge Energisation is the process of providing
the separated charges with the ELECTRICAL ENERGY
they need to complete their journey around the
circuit connected to the cell.
A Single Cell
A group of Cells ie. a Battery
Batteries have a limited ability to separate and
energise charge, they eventually go flat. See
Slide 4.5
43
4.1 Power Supplies
Power Supplies, (as opposed to cells and
batteries) obtain their separated and energized
charges from the mains supply to which they are
connected, via the standard 3 pin plug.
They rely on the power generation company to
separate and energize the charge carriers at the
power station. The power station remains on
line at all times, so the power supply can
operate indefinitely, i.e., it does not go flat
like a battery.
In all other senses, power supplies behave in a
similar fashion to cells and batteries.
44
4.2 Electromotive Force (EMF)
Electromotive Force (EMF) is not a true force in
the Newtons Laws sense, but it is a term used to
describe the OPEN CIRCUIT VOLTAGE of a cell,
battery or power supply.
Open Circuit means that no complete external
circuit is connected to the battery or power
supply and thus no current is being drawn.
EMF is represented by the symbol (e). The Greek
letter epsilon.
Consider the circuit shown
With the circuit complete, a current is flowing
and the P.D. across the power supply equals the
P.D. across the resistor.
With the switch open the current stops flowing,
the voltage across the resistor falls to zero and
the voltage reading across the power supply
rises. The Voltage reading now is the EMF of the
supply
45
Cells and Batteries
31. The primary task of a battery or power supply
is to A Supply electrons and energise them B
Provide energy for charge carriers C Provide
charge separation and energization. D Separate
electrons from protons.
32. The EMF of a battery or power supply is A
The Potential Difference of the supply when a
current is being drawn. B The Potential
Difference of the supply when no current is being
drawn C The Potential Difference difference
between the positive and negative terminals when
they are short circuited. D The Potential
Difference difference between earth and the
positive terminal.
33. When a battery or power supply is switched
into an external circuit the Potential Difference
measured across the terminals of battery or power
supply will A Fall because a current is now
flowing B Rise because a current is now
flowing C Remain unchanged even through a
current is now flowing D None of these answers
46
4.3 Internal Resistance
  • The reason the Potential Difference of the power
    supply falls when a current is drawn from it is
    the Internal Resistance of the supply.
  • The internal resistance is
  • The price which must be paid for drawing a
    current from the supply.
  • A measurable quantity and, as with all
    resistance, is measured in Ohms (?).

A cell, battery or power supply can be
represented as a pure EMF in series with a
resistor, r, (representing the internal
resistance).
The larger the current drawn from the supply, the
greater the cost (in terms of energy wasted
inside the supply), because of the internal
resistance. This means less energy is available
for the charge carriers to flow around an
external circuit.
With no external circuit connected (i.e. a so
called no load situation), no current is drawn
from the supply, and the voltmeter reading V1
will equal e, the EMF of the supply.
47
4.4 Internal Resistance
The power supply now has an external circuit
connected.
This draws a current from the supply.
This current also flows through the internal
resistance r. This causes a potential difference
Ir across that resistor.
The voltage measured by V2 will now be less (by
an amount Ir) than the EMF (e) of the power
supply. Mathematically V2 e - Ir
By replacing the fixed resistor (R) in the
external circuit with a variable resistor,
and changing the value of the resistance, a set
of values for V2 and the corresponding current,
I, can be obtained. Plotting these values gives
the following.
This method allows you to calculate the internal
resistance of the power supply, cell or battery.
48
Cells and Internal Resistance
34. A battery or power supply can be regarded as
A A pure P.D. source in parallel with a
resistance B A pure P.D. alone C A pure P.D.
source in series with a resistor D A pure
resistance in parallel with an EMF
35. A battery has an EMF of 9.0 V. When connected
into a circuit drawing 25 mA the potential
difference across the battery terminals in
measured at 8.6 V. What is the internal
resistance of the battery ?
V e Ir r (e V)/I (9.0 8.6)/(2.5 x
10-2) 16 O
49
Internal Resistance
36. A group of students set out to study the
properties of a D cell battery. Using the
following circuit and varying the resistance of
the rheostat they collected the data shown.
Voltage (V) (volts) Current (I) (milliamps)
0.10 120
0.25 100
0.45 70.0
0.60 50.0
0.70 35.0
0.85 15.0
(b) EMF y intercept 0.95 V
(c) Internal resistance (r) negative of slope
-(0.85 0.25)/(15 x 10-3 100 x 10-3)
- (0.6)/(-75 x 10-3) 8O
50
4.5 A Flat Battery
In testing a battery with a multimeter, you
measure the EMF, which may seem fine, because you
are not drawing a current from it. To properly
test a battery it needs to be placed in a
resistive circuit of some kind so that a current
is drawn. Measuring P.D. across the battery now
will now give a more realistic picture of the
batterys condition.
In a cell or battery, the chemical processes used
to provide charge separation and energisation
become less efficient as current is drawn from
it. This shows up in an increase in the Internal
Resistance of the battery. The internal
resistance will continue to increase until the
battery is no longer able to provide sufficient
energy to perform its primary task (separation
and energisation) and the battery is said to be
flat.
51
Flat Battery
37. Explain why is not sufficient to simply
measure the EMF of a battery to check if it is
flat ?
EMF does not measure the internal resistance of
the battery and hence its ability (or otherwise)
to provide a current to external circuit.
52
Chapter 5
  • Fuses and Stuff

53
5.0 Fuses
Fuses are primarily Safety Devices placed in
circuits to limit the current flow to a certain
(predetermined) value. Limiting the current in
this way reduces the chance of fire caused by
overheating in a circuit carrying excessive
current. A Fuse is basically a short piece of
thin wire which, when too much current tries to
flow through it, overheats and then melts,
breaking the circuit.
In the electricity supply network fuses are
present throughout the system and at the domestic
or household end fuses are located in the Fuse
Box sometimes also called the Meter Box.
Modern Meter Boxes have resettable fuses called
Circuit Breakers instead of the old style
porcelain former with its separate thin wire fuse.
54
5.1 Residual Current Interrupt
Increasingly, Meter Boxes contain Residual
Current Interrupt Devices (RCI), commonly called
safety switches and Surge Arrestors. Both are
safety devices. The RCI is designed to protect
people while surge arrestors protect electrical
equipment.
To understand the operation of the RCI you need
to know that a current in a wire causes a
magnetic field around that wire. The strength of
the magnetic field depends upon the size of the
current.
THE RCI AND THE TOASTER
The RCI operates using two coils to monitor the
magnetic fields produced by the currents in both
the active and neutral wires. Under normal
conditions the Active and Neutral currents will
be equal. This means the induced currents in the
coils will also be equal and will cancel one
another out inside the RCI.
If the two A and N currents are different, as
shown with some passing down the earth wire, due
to a short circuit in the toaster, the RCI reacts
by opening a switch in the active wire, cutting
off the current.
The RCI will respond in approx 0.03sec (less than
a heartbeat)
55
Safety
38. Which one or more of the following act as
safety devices in electric circuits ? A
Fuses B Safety switches C Surge arrestors D
Short Circuits
39. RCIs monitor the currents in A The Neutral
and Earth Lines B The Active and Earth
Lines C The Active Line only D The Active and
Neutral Lines
56
5.2 Switches
Switches break circuits by moving contacts
apart. In the domestic situation switches are
always placed in the Active Line. This is
especially important for General Purpose Outlets
(GPOs) more often called wall sockets or power
points.
Opening the switch (turning it off) isolates
the power point from the supply. If the switch
was placed in the Neutral line the power point
would remain live even with the switch off
There are large numbers and types of switches in
use. They can be Mechanical, Electromechanical or
Electronic.
Sample Mechanical Switches are shown
SPST Single Pole, Single Throw
SPDT Single Pole, Double Throw
DPDT Double Pole, Double Throw
DPST Double Pole, Single Throw
57
Switches
40. Identify each type of switch
58
5.3 Earthing
The Earth is a giant sink for electricity, it
will soak up electric charge. The name given to
the process of connecting a circuit to the Earth
is called earthing and the physical connection
is via the Earth Wire.
To better understand earthing, an understanding
of domestic wiring is needed. Below is a sample
domestic wiring system showing one power point
only.
The Earth is also physically connected to the
neutral bar, holding it at Earth potential
(voltage) of 0 volts.
The Earth Wire provides a resistance free path
to earth for any current that leaks from the
active and/or neutral lines. Leaking current
will choose to use the no (or low) resistance
path to earth rather than the high resistance
path through a human.
The earth stake is a solid copper piece driven
about 2 m into the ground
Meter Box
Earthing, as used in domestic wiring, is just
another safety feature.
59
5.4 Electric Shock
Electricity is dangerous! We all know this, it
was drummed into us throughout our childhood. We
can all remember the reaction of adults the first
time they found us playing with electrical
sockets at home. But exactly how dangerous is
electricity and what does it do to our bodies ?
The lowest recorded voltage at which death
occurred was 32 V AC
Domestic electricity in Australia is supplied at
240 V AC, at 50 Hz. If you are exposed to this
supply for 0.5 sec and depending on the size of
the current, the following effects will be
experienced.
Remember that 1 mA 1/1000th of an Amp ( 1 mA
1 x 10-3 A)
1 Able to be felt slight tingling
3 Easily felt distinct muscle contraction
10 Instantly painful - Cramp type muscle
reaction
20 Instant muscle paralysis cant let go
50 Severe shock knocked from feet
90 Breathing disturbed - burning noticeable
150 Breathing extremely affected
200 Death likely
500 Breathing stops death inevitable
60
Chapter 6
  • Electricity Consumption

61
6.0 Power Consumption
In general, POWER is defined as the time rate of
doing WORK or the time rate of ENERGY
conversion. Mathematically P W/t E/t
Where P Power (Watts) W Work (Joules) E
Energy (Joules) t time (secs)
Rearranging the equation we wet E P.t
so 1 Joule 1 Watt.sec
The Joule is a very small unit, too small for the
Energy companies to use when it comes to sending
out the bills to customers, so electricity is
sold in units called kilowatt hours. (kWh). Have
a look at your own electricity bill at home !
1 kW 1000 W and 1 hour 3600 s So 1 kWh 1000
x 3600 J 3.6 x 106 J 3.6 MJ.
62
Electric Power
41. An electricity bill indicates the household
used 117.5 kWh of electricity in a week. How many
megajoules were used ?
1 kWh 3.6 MJ so 117.5 kWh (117.5)(3.6) 423
MJ
42. What was the power consumption of the home
(in Watts) ?
P E/t, U 423 MJ 4.23 x 108 J
t 1 week 7 x 24 x 60 x 60 sec 604800 s P
E/t (4.23 x 108)/(6.048 x 105) 699 W
63
6.1 The Kilowatt Hour
A 100 W (0.1 kW) incandescent light globe which
runs for 1 hour consumes 0.1 x 1 0.1 kWh of
electricity.
Cost to run _at_ 12c/kWh 1.2 cents
A 1500 W (1.5 kW) electric kettle which boils
water in 5 minutes consumes 1.5 x 5/60 0.125
kWh of electricity.
Cost to run _at_ 12c/kWh 1.5 cents
A 2000 W (2 kW) oven operating for 3 hours
consumes 2 x 3 6 kWh of electricity.
Cost to run at 12c/kWh 72 cents
Domestic electricity costs between 12 cents and
20 cents a kWh
64
Running Costs
43. If domestic electricity costs 13.5 c per kWh.
Calculate the cost of running (a) a 100 W light
globe run for 1 hr, (b) a 1500 W kettle run for 5
mins and (c) a 2 kW oven run for 3 hrs.
Light Globe (0.1)(13.5) 1.35 cents Kettle
(0.125)(13.5) 1.69 cents Stove (6.0)(13.5)
81 cents
65
6.2 Load Curves
  • The demand on the electricity supply is not
    constant
  • It varies from time to time during the day.
  • It varies from day to day during the week.
  • It varies from season to season during the year.
  • This variation is best displayed on a Load Curve

66
Load Curves
Questions 44. Why does the demand on a hot summer
day exceed the demand on a cold winters day ?
Hot Summer days means use of air conditioners,
the greatest consumers of electrical power of all
household appliances. In addition commercial air
conditioners must also work harder thus consuming
more electricity.
45. Blackouts, loss of supply often occur when
demand exceeds supply. At what times and on what
type of day are blackouts likely to occur ?
Just after noon and just after 6 pm on hot summer
days
67
Ollie Leitl 2008
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