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Drouin Secondary College
VCE PHYSICS
Topic Electricity
VCE PHYSICS
UNIT 2
ELECTRICITY
TOPIC NOTES
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
VCE Physics
Unit 2
ELECTRICITY
Unit Outline
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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.
Chapter 1
The Basics
1.0
Electric Charge
Atoms consist of a nucleus, containing _______________ and
neutrons, with ________________circulating around it.
Electric Charge is a property of some atomic particles.
Which ones ?
________________and ________________
These two particles carry an equal and opposite electric charge.
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VCE PHYSICS
Topic Electricity
This charge is the smallest known amount of charge that exists
independently.
This charge is called the “________________
_______________”
The charge carried by the Proton is DEFINED to be ____________.
The charge carried by the Electron is DEFINED to be ___________.
The UNIT of Electric Charge is the ________________, 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.
_________________________________________________________
Questions
1. How many electrons make up 1 Coulomb of charge?
2. How many electrons in 7.5 C of charge ?
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1.2
VCE PHYSICS
Topic Electricity
Electric Current
When electric charges are made to move or flow an ELECTRIC
__________________(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 ___________________ often shortened
to Amp (Symbol A)
Mathematically:
I = Q/t
Where:
I = Current in Amps
Q = Charge in Coulombs
t = Time is Seconds
Electric current has the property of starting ________________ a
circuit is complete and stopping immediately a circuit is
_____________.
Once the current is flowing it stays the __________ all around the
circuit.
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VCE PHYSICS
Topic Electricity
Questions
3. Calculate the current flowing if 3.57 Coulomb of charge passes a
point in 1.25 sec.
4. If 5.62 A of current flows through a wire in 0.68 sec.
(a) How much charge has been moved ?
(b) How many electrons were needed to transport the charge in (a) ?
5. If a current of 125 A resulted from the movement of 225 C of
charge, for how long did the current flow ?
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1.3
VCE PHYSICS
Topic Electricity
Conventional versus Electron Current
In ____________ ____________ (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.
This means the current carriers must be
____________ 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.
BUT, we know that a current is a stream of _______________
(negative particles), which must travel in the other direction.
What’s going on ?
It is a quirk of history that the current direction is shown this way.
Electric ____________were discovered before the ___________.
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Topic Electricity
It was thought that the charge carriers were positive and the
current must flow this way.
Questions
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
1.4
Potential Difference
For a current to flow around a circuit a “____________
__________” is needed.
This driving force is the difference in _________________ between
the start and the end of the circuit.
Potential difference is best understood using the water analogy:
The larger the current you want the greater the Potential difference
Difference you require.
A short drop between storage and tap gives low water pressure = a
low potential difference.
Low output from the tap = _______
_______________.
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Topic Electricity
SO A SMALL POTENTIAL DIFFERENCE CAN ONLY DRIVE A
SMALL CURRENT.
A large drop between storage and tap gives high pressure = Large
P.D.
High output at the tap = _______ ______________
SO A LARGE POTENTIAL DIFFERENCE CAN DRIVE A LARGE
CURRENT.
Strictly, potential difference is DEFINED as a measure of the
___________ given to the charge carriers (the electrons) for them
to complete their job, that is, to travel once around the circuit.
Mathematically:
V = E/Q
Where:
V = Voltage measured in Volts (V)
E = Electrical Potential Energy in Joules (J)
Q = Electrical Charge in Coulombs (C)
This means that by passing through a Potential difference 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)
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Topic Electricity
The Battery is now increased to 24 V.
How many Joules of energy will each coulomb now pick up ?
Answer: _______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 is
Potential Difference
One further potential difference, EMF (Electro __________ Force),
while still measured in volts, is slightly different and cannot be
grouped with the other terms.
Questions
7. One coulomb of charge passing through a battery picks up 15 J of energy.
What potential difference did the charge pass through ?
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 ?
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Drouin Secondary College
Topic Electricity
(b) How much energy (in total) has the battery supplied to the charge passing
through it
1.6
Electrical Energy
The Charge Carriers in a circuit obtain their energy from a power
source or ____________ ______________.
The amount of energy the charge carriers pick up depends upon
the size of the potential difference difference through which they
are forced to travel.
Since energy transferred = work done, another way of defining
electrical energy is by the work done on the charge (Q) in passing
through a Potential difference (V)
An external wire connected to a battery will have electrons flowing
through it.
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.
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VCE PHYSICS
Topic Electricity
The work done on the electrons increases their electrical energy
and gives them enough energy to do another trip around the
external circuit.
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)
The energy picked up by the charge carriers is used up in
“______________________” whatever device is connected to the
external circuit.
Questions
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 ?
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 ?
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Drouin Secondary College
Topic Electricity
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.
1.8
Electric Power
Electric Power is DEFINED as the Time Rate Of
__________Transfer or the Time Rate Of Doing __________.
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 Ohm’s Law (See Chapter 2.) The Power formula can be
rewritten as: P = VI = I2R = V2/R
_________________________________________________________
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VCE PHYSICS
Topic Electricity
Questions
12. Calculate the power consumed by an electric drill operating at 240 V and 7.5
A.
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) ?
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.
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.
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Topic Electricity
1.9 Common Electrical Symbols
V
A
G
Questions
16. Identify the numbered components in the circuits below
(a)
(b)
1
V
A
3
2
4
1
2
3
5
4
6
5
G
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Drouin Secondary College
Topic Electricity
1.10 Series and Parallel
Electrical components can only be connected together in one of two
ways:
(a)
Series – where components are connected end to end
Series
(b)
Parallel - where components are connected side by side.
Parallel
1.11
A Typical Electric Circuit
An electric circuit contains a number of components, typically:
• A Power Supply
• Connecting _________
• Resistive Elements
• ____________
Circuit diagrams are usually drawn in an organized manner with
connecting wires drawn as straight ________ and the whole
diagram generally square or rectangular in shape.
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Topic Electricity
Resistive Element
Voltmeter
V
Ammeter
A
Connecting Wires
Power Supply
This power supply is a D.C. Supply (a Battery), and it drives the
current in __________direction only.
Connecting wires are drawn as straight lines with right angle
bends.
They are always regarded as _____________ conductors having no
resistance.
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.
The Voltmeter measures the potential difference drop across the
resistive element. It is connected in parallel. It has a very high
internal ___________________ which diverts very little current
from the main circuit.
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VCE PHYSICS
Topic Electricity
The Ammeter measures current flow in the main circuit. It is
connected in series. It has virtually no ___________ resistance so
as not to interfere with the current in the main circuit.
Questions
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 doesn’t 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
Chapter 2
2.0
Resistance
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VCE PHYSICS
Topic Electricity
All materials fall into one of three categories as far as their
electrical conductivity is concerned.
They are either :
1. _______________
2. Semiconductors, or
3. Insulators
ALL materials exhibit some _______________________ 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.
This opposition is called ELECTRICAL ______________________.
The amount of resistance depends on a number of factors:
1. The ______________ of the material.
2. The cross sectional area of the material.
3. The nature of the material, measured by Resistivity
Mathematically:
R = ρL/A
Where
R = Resistance in Ohms (Ω)
ρ = Resistivity in Ohm.Metres (Ω.m)
L = Length in Metres (m)
A = Cross Sectional Area in (m2)
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VCE PHYSICS
Topic Electricity
Length =
L
Wires 1 & 2 are made from the same
material (ρ is the same for each), and are
A1
Wire 1
the same length (L is also the same).
Wire 1 has twice the cross sectional area
A2
of Wire 2.
Wire 2
Wire 1 has ____ the resistance of Wire 2
Questions
20. Nichrome wire is sometimes used to make the heating elements in electric
kettles. It has a resistivity of 6.8 x 103 Ω.m. Calculate the resistance of a piece of
nichrome wire of length 1.2 m and cross sectional area 2 x 10-4 m2
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
2.1
Resistors
Resistors are conductors whose resistance to current flow has
been increased.
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VCE PHYSICS
Topic Electricity
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. Resistor is a generic term representing a
whole family of conductors such as toaster elements, light bulb
filaments, bar radiators and kettle elements.
They are represented on circuit diagrams as either,
There are only two ways to join resistors together
IN SERIES:
The resistors are connected _____ ____ _____ with only _______
path for the current to flow. The more resistors the ____________
the overall resistance
IN PARALLEL
The resistors are connected ______ ____ ________ with more than
one path for the current to flow. The more resistors the _________
the overall resistance
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2.2
Topic Electricity
Resistors in Series
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 = ____ = _____ = ____
The sum of the potential differences across the resistors is equal
to the potential difference of the supply
Thus VS = ____+ _____ + ______
Questions
24 Ω
11
15 Ω
11 Ω
21.
12
2.9
1.8
v3
2
I = 0.12 A
6.0
(a) Calculate the equivalent resistance that could replace the resistors in the
circuit.
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Drouin Secondary College
Topic Electricity
(b) Determine the value of V3
(c) Determine the values of I1 and I2
2.3
Resistors in Parallel
When connected side by side, this combination of resistors (called
a _____________ 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 ___________ 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.
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Drouin Secondary College
Topic Electricity
Thus I = ___ + ___ + ____
Each arm of the parallel network gets the full supply potential
difference.
Thus VS = ____ = _____ = _____
I = 12 mA
22.
1 kΩ
I1
v3
2
3 kΩ
v2
I = 1.0 mA 2
12 kΩ
v1
I = 17 mA
12V
(a) What single resistor could be used to replace the 3 resistors in the circuit above
?
(b) Determine the values of V1, V2, and V3
(c) Determine the value of I1
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Drouin Secondary College
Topic Electricity
50 Ω
23.
10 Ω
v1
50 Ω
15 Ω
24 Ω
3V
100 Ω
10 V
I = 0.5 A
25 V
(a) What single value resistor could be used to replace the network shown
above ?
(b) What is the potential difference across and the current through the 24 Ω
resistor ?
2.4
Ohm’s Law
Conductors which obey Ohm’s Law are called __________
_________________.
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
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 Ohm’s Law
Ohm’s Law stated mathematically is:
V = IR
Note the graph passes through the
V = P.D. in Volts (V)
I = Current in Amps (A)
R = Resistance in Ohms (Ω)
origin (0,0) as it must, since if both V and I are zero, resistance is a
meaningless term.
When expressed graphically, by plotting V against I, Ohm’s Law
produces a _____________ line graph with a slope equal to
resistance (R)
V
I
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Drouin Secondary College
Topic Electricity
Questions
24. A current of 2.5 mA is flowing through a resistor of 47 kΩ. What is the
potential difference drop across the resistor ?
25. A 12 V battery is driving a current through an 20 Ω resistor, what is the size of
the current flowing ?
26. A resistor has a 48 V potential difference drop across it and a 2.4 A current
flowing through it. What is it’s resistance ?
27. What are the readings on meters V and A ?
R = 1.2 kΩ
V
A
VP = 12 V
500 Ω
v2
1 kΩ
A2
500 Ω
1.5 kΩ
28 (a) Determine the value of
the current measured by
ammeter A1 (express your
answer in mA)
2.4 kΩ
V1
1 kΩ
v3
A1
25 V
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Drouin Secondary College
VCE PHYSICS
Topic Electricity
(b) Determine the value of the potential differences measured by voltmeters V1,
V2 and V3.
(c) Determine the current measured by ammeter A2
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 ______ path for the current.
The current immediately increases to its maximum.
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
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).
Chapter 3
3.0
Non Ohmic Devices
Conductors which do not follow Ohm’s 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 _______ _______.
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
V
I
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Drouin Secondary College
Topic Electricity
3.1
Non Ohmics in Series
Two non ohmic conductors with “Characteristic Curves” , as
shown are connected in series (end to end).
The potential difference across device 1 is 6.0 V.
6.0 V
Device 2
Device 1
Questions
29. What is:
(a)
the current through Device 2,
(b)
the potential difference across Device 2,
(c)
potential difference of the battery powering the circuit ?
Device 1
Device 2
Voltage (V)
Voltage (V)
8
8
6
6
4
4
2
Current (I)
1
2
2
Current (I)
1
2
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Drouin Secondary College
3.2
Topic Electricity
Non Ohmics in Parallel
Questions
30. The current through Device 1 is 1.5 A.
1.5 A
Device 1
Device2
What is:
(a) the potential difference across Device 2
(b) the current through Device 2
(c) the potential difference of the battery
(d) the total current drawn from the battery ?
Device
Voltage1
(V)
Device
2
Voltage
(V)
8
8
6
6
4
4
2
2
0
1
Current
2 (I)
0
1
2
Current
(I)
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Drouin Secondary College
4.0
Topic Electricity
Chapter 4
Cells & Batteries
Electrical Cells (as opposed to plant and animal cells) are devices
which perform two functions:
1. Charge ________________.
2. Charge ________________.
Charge Separation is the process of separating positive and
negative charges to produce a POTENTIAL DIFFERENCE or
____________ 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.
Batteries have a _____________ ability to separate and energise
charge, they eventually go “flat”.
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.
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Topic Electricity
They rely on the power generation company to ____________ and
_______________ 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.
4.2
Electromotive Force (EMF)
Electromotive Force (EMF) is not a true force in the Newton’s Laws
sense, but it is a term used to describe the OPEN ______________
POTENTIAL DIFFERENCE of a cell, battery or power supply.
“Open Circuit” means that no ______________ external circuit is
connected to the battery or power supply and thus no __________
is being drawn.
With the circuit complete, a current is flowing and the potential
difference across the power supply equals the potential difference
across the resistor.
I
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Drouin Secondary College
VCE PHYSICS
Topic Electricity
With the switch open the current __________ flowing, the potential
difference across the resistor falls to zero and the potential
difference reading across the power supply ____________. The
Potential difference reading now is the _______ of the supply
I=0
Questions
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 energisation.
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
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4.3
VCE PHYSICS
Topic Electricity
Internal Resistance
The reason the potential difference of the power supply falls when
a current is drawn from it is the “___________ ___________” 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 (Ω).
The larger the current drawn from the supply, the greater the
“_________” (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 ___________ circuit.
A cell, battery or power supply can be represented as a pure EMF
in series with a resistor, r, (representing the internal resistance).
With no external circuit connected (i.e. a so called “_____
_______” situation), no current is drawn from the supply, and the
voltmeter reading V1 will equal ε, the EMF of the supply.
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Drouin Secondary College
Topic Electricity
Power Supply
r
ε
V
The power supply now has an external circuit connected.
This draws a _______________ from the supply.
This current also flows through the _____________resistance r.
This causes a potential difference drop = Ir across that resistor.
The potential difference measured by V2 will now be less (by an
amount Ir) than the EMF (ε) of the power supply.
Mathematically: V2 = ε - Ir
V2
2
r
I
ε
R
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Drouin Secondary College
VCE PHYSICS
Topic Electricity
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.
Voltage
(V2)
Current (I)
This method allows you to calculate the internal resistance of the
power supply, cell or battery.
Question
34. A battery or power supply can be regarded as
A: A pure potential difference source in parallel with a resistance
B: A pure potential difference alone
C: A pure potential difference 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 drop across the battery terminals in measured at 8.6 V.
What is the internal resistance of the battery ?
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Drouin Secondary College
Topic Electricity
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.
V
2
I
I
2
Voltage (V)
(volts)
0.10
0.25
0.45
0.60
0.70
0.85
Current (I)
(milliamps)
120
100
70.0
50.0
35.0
15.0
R
(a) Plot this data onto the graph page
(b) Determine the EMF of the battery
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
(c) Calculate the internal resistance of the battery
_________________________________________________________
4.5
A Flat Battery
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 ___________ 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 (________________ and__________________) and the battery
is said to be _______.
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 potential difference across the battery now will now
give a more realistic picture of the battery’s condition.
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
Questions
37. Explain why is not sufficient to simply measure the EMF of a battery to check if
it is “flat” ?
Chapter 5
Fuses and Stuff
5.0
Fuses
Fuses are primarily __________ Devices placed in circuits to limit
the current flow to a certain (predetermined) value.
Limiting the current in this way reduces the chance of _______
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
________, 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”.
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
Modern Meter Boxes have resettable fuses called “___________
_____________” instead of the old style porcelain former with its
separate thin wire fuse.
5.1
Residual Current Interrupt
Increasingly, Meter Boxes contain Residual Current Interrupt
Devices (RCI), commonly called “___________ ___________” and
Surge Arrestors.
Both are safety devices.
The RCI is designed to protect ______________ while surge
arrestors protect electrical ______________.
THE RCI AND THE TOASTER
Active Wire
Neutral Wire
G.P.O. or
Power point
RCI
Coils
Earth
To understand the operation of the RCI you need to know that a
current in a wire causes a _______________ field around that wire.
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
The strength of the magnetic field depends upon the _______ of
the current.
The RCI operates using two ________ 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
____________. 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, with some passing down the
__________ wire, due to a short circuit in the toaster, the RCI
reacts by opening a switch in the active wire, cutting off the
current.
Active Wire
Neutral Wire
RCI Breaks
Circuit
RCI
Coils
Unequal
Currents
Earth
The RCI will respond in approx ________sec (less than a
heartbeat)
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Drouin Secondary College
VCE PHYSICS
Topic Electricity
Questions
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. RCI’s 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
5.2
Switches
Switches break circuits by moving ___________ apart.
In the domestic situation switches are always placed in the
_________ Line.
This is especially important for General Purpose Outlets (GPO’s)
more often called wall sockets or _________ _________.
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 _________ even with the switch “off”
There are large numbers and types of switches in use. They can be
Mechanical, Electromechanical or Electronic.
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Drouin Secondary College
VCE PHYSICS
Topic Electricity
Questions
40. Identify each of the following switches.
5.3
Earthing
The Earth is a giant “__________” for electricity, it will soak up
electric charge. The name given to the process of connecting a
circuit to the Earth is called “____________” 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 (potential difference) of _______ volts.
The Earth Wire provides a “__________ ______” 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.
Earthing, as used in domestic wiring, is just another safety feature.
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
Meter Box
Active
Neutral
Main
Fuse
Main
Switch
Electricity
Meter
Fuses
Neutral
Bar
Earth
Earth Stake
Power
point
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 potential difference at which death occurred
was ________ V AC
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
Domestic electricity in Australia is supplied at _______ V AC, at
______ 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.
Current (mA)
1
3
10
20
50
90
150
200
500
Effect on Body
Able to be felt – slight tingling
Easily felt – distinct muscle contraction
Instantly painful – muscle cramp
Instant muscle paralysis – can’t let go
Severe shock – knocked from feet
Breathing disturbed – burning noticeable
Breathing extremely affected
Death likely
Breathing stops – death inevitable
Chapter 6
Electricity Consumption
6.0
Power Consumption
In general, POWER is defined as the time rate of doing WORK or
the time rate of __________conversion.
Mathematically:
P = W/t = E/t
Rearranging the equation we wet:
Where
P = Power (Watts)
W = Work (Joules)
E = Energy (Joules)
t = time (secs)
E = P.t
so 1 Joule = 1 Watt.sec
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Drouin Secondary College
VCE PHYSICS
Topic Electricity
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 ____________ ________. (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.
Questions
41. An electricity bill indicates the household used 117.5 kWh of electricity in a
week. How many megajoules were used ?
42. What was the power consumption of the home (in W) ?
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VCE PHYSICS
Drouin Secondary College
6.1
Topic Electricity
The Kilowatt Hour
A 100 W (0.1 kW) incandescent light globe which runs for 1 hour
consumes 0.1 x 1 = ______ kWh of electricity.
A 1500 W (1.5 kW) electric kettle which boils water in 5 minutes
consumes 1.5 x 5/60 = _______ kWh of electricity.
A 2000 W (2 kW) oven operating for 3 hours consumes 2 x 3 =
_______ kWh of electricity.
Domestic electricity costs between 12 cents and 20 cents a kWh
Questions
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.
6.2
Load Curves
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VCE PHYSICS
Drouin Secondary College
Topic Electricity
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”
12 Midnight
6 am
12 noon
6 pm
12 Midnight
100 %
Hot Day
Summer
75 %
Cold Day
Winter
50 %
Mild Day
Spring
44. Why does the demand on a hot summer day exceed the demand on
a cold winter’s day ?
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
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