Unit 3 Electricity
Ch. 9
Electric Charge
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There are two kinds of electric charge:
positive and negative.
A substance that is has no charge is called
neutral.
When 2 neutral substances are rubbed
together; one substance becomes positively
charged and the other negatively charged.
Electric Charge
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Charged objects attract neutral objects
including liquids and gases.
Objects with like charges repel each other.
Objects with opposite charges attract each
other.
This constancy of behaviour is called the Law
of Electric Charges.
Charging by Friction
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There are 3 ways that charges can be
transferred from neutral objects: friction,
induction and conduction.
Charging by friction involves 2 substances
rubbing together and transferring electrons
from one object to another.
Charging in this way has many of the same
effects as static electricity.
Charging by Friction
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Different substances hold onto their
electrons better than others.
You can determine what kind of charge an
object will obtain when rubbed with a
substance by using a list like the one in
Table 1 on page 275.
This is called an electrostatic series.
Transferring charges by contact
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This transfer of charges by contact is called
conduction.
The shock that occurs during this transfer
may be painful due to the very rapid transfer
of these electric charges.
Insulators
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Insulators are very useful substances.
Since electric charges cannot pass through
them they are used to protect us from
electric shock.
If electrical wires and appliances were not
covered with insulators, they would be
extremely dangerous.
Conductors
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No matter how hard you polish a metal
surface, it will never build up a static charge.
This is because it is a conductor.
A conductor is a substance in which
electrons can move freely from one atom to
another.
If a conductor becomes negatively charged,
the extra electrons move freely along the
conductor and eventually these charges are
lost.
Induction
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This is the third way in which an object can
become charged.
The other 2 types of charging involve the
transfer of charges by physical contact.
Induction involves the movement of charges
within an object.
When a neutral particle approaches a
charged object, the electrons within the
neutral object rearrange themselves.
Ch. 10
Electricity and Electric Circuits
The movement of
electric charges
from one place to
another is called an
electric current.
 Electric current can
be controlled. We
do this by using an
electric circuit.
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Parts of an Electric Circuit
1.
2.
3.
Source of Electrical Energy – can come
from a battery, a solar cell, a plug which
gets it power from a generating station,
etc.
Electrical Load: This is the name given to
anything that converts electrical energy
into whatever form of energy we need.
Electric Circuit Control Device: This is
simply the part of the circuit that controls
the flow of current through the circuit.
This includes switches, timers, etc.
Parts of an Electric Circuit
4.
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Connectors: These are the substances or
wires that carry the electricity through the
circuit.
The words that are used to describe a
circuit can be confusing.
A closed circuit is one where the electricity
is flowing through it powering the devices
connected to it. Power switch is turned on.
An open circuit is when there is no
electricity is running through the circuit.
Power switch is turned off.
Symbols for Circuit diagrams
One Cell
Switch
Voltmeter
Resistor
Motor
Heater
Lamp
Ammeter
Electric Potential (Voltage)
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The energy each electron has is called the
electric potential of the electron.
This is commonly referred to as Voltage.
The SI unit used to measure electric
potential is the volt (V).
Voltage is measured by a device called a
voltmeter.
The higher the voltage, the more electrons
leave the cell and flow through the circuit.
Positive
Terminal
Negative
Terminal
Cells in Series and Parallel
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Circuits can be connected in one of 2 ways;
In series or parallel.
A dry cell can only achieve a maximum
voltage of about 2 volts. If we want to
achieve higher voltage we have to connect
many dry cells together.
A 9 volt battery has six miniature dry cells in
them.
As we connect the dry cells together in
series, the voltage compounds with each cell
you add to the series.
Parallel Cells
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This arrangement of cells is used to provide
more energy to a device than one battery
can supply.
We connect 2 cells side by side, in parallel,
and connect the positive terminals and the
negative terminals together.
This will provide twice as many electrons to
the device but the electric potential will be
the same as though only one cell was
present.
Ohm’s Law
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Ohm’s Law states that: The potential
difference between 2 points on a conductor is
proportional (directly related) to the electric
current flowing through the conductor.
This describes the voltage drop caused by
resistance of the conductor.
Potential difference = Electric current x Electric resistance
(voltage drop)
V=IxR
Potential difference is measured in Volts (V),
Current in amperes (A), & Resistance in ohm’s
(Ω)
Types of Electric Circuits
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There are 2 types of circuits: Series circuits
and parallel circuits.
Series Circuits:
Have you ever had a string of Christmas
lights that when you removed one bulb, or a
bulb burned out the whole string of lights
stops working?
This is because these lights are in a series
circuit.
A series circuit has the current flowing on
only one pathway in the circuit.
The Parallel Circuit
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In this circuit, the current has more than one
pathway (circuit) to travel to multiple loads
on the circuit.
Each of these circuits to the loads is called a
branch circuit.
Because each load is connected to its own
branch circuit, it does not affect the other
loads.
This means that if you remove on bulb from
the string of Christmas lights, the rest will still
stay lit.
Ch. 11
Calculating the amount of
electrical energy
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Electrical Energy = Voltage Drop x Electric
Current x Time Interval
This can be written by using the following
symbols:
E
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= V x I x ∆t
E = Electrical energy in Joules
V = Voltage Drop in Volts
I = Current in Amperes (A)
∆t = time interval in seconds (s)
11.4 - Rate at Which Energy Is Used
 Electrical Power is a measure of the
rate at which electrical energy is
being used.
 The symbol for electrical power is P,
and it is measured in Watts (W).
 Power for appliances is often
measured in kilowatts (kW) in order
to make it easier to understand. 1
kW = ___________ W
Calculating Electrical Power
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Electrical power is calculated by using the
amount of electrical energy used in a period
of time.
Electrical Power = Electric Energy
Time Interval
In symbols, this formula is: P = E
∆t
This formula is rarely used in practice because
you have to measure both the energy and the
time interval to be able to solve the formula.
Calculating Electrical Power
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This formula is the one typically used since
we already know how to calculate voltage
and current.
P
=VxI
Ch. 12
12.2 Distributing Electricity Safely
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There are 3 wires that run from the power
pole to the back of your house. 2 Live wires
and one neutral wire.
The neutral wire does not carry current, it is
connected to the ground through the
plumbing or by a metal stake.
The live wires go through the meter on the
back of the house before it enters the
home.
It then runs through a main breaker switch
and into the distribution panel.
12.2 Distributing Electricity Safely
A
circuit breaker controls the
amount of current running through
the circuit. If too much runs
through the circuit then it will shut
the power off to the circuit.
 The basic circuit breaker consists of
a simple switch, connected to
either a bimetallic strip or an
electromagnet.
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A fuse is a very simple circuit breaker that can
only be used until it is blown. Once it has
been blown, it must be replaced.
A fuse is just a thin wire, enclosed in a casing,
that plugs into the circuit. When a circuit is
closed, all charge flows through the fuse wire.
The fuse is designed to disintegrate when it
heats up above a certain level -- if the current
climbs too high, it burns up the wire.
Destroying the fuse opens the circuit before
the excess current can damage the building
wiring.
12.5 – Electrical Energy Use in the Home
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Electrical energy use in the home is measure
in kilowatt hours (kWh).
We can rearrange the formula we used in Ch.
11 to calculate the energy that a house or
appliance uses.
Since E = V x I x ∆t and P = V x I we can
calculate energy by using the following
formula: E = P x ∆t
E = Energy in kilowatt hours (kWh)
P = Power in kilowatts (kW)
∆t = time interval in hours (h)
12.5 – Electrical Energy Use in the Home
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We can also calculate the cost of the
electrical energy that is used.
Cost = Electrical Energy (kWh) x Rate (cost /
kWh)
Cost = E x Rate
12.6 Reading a Power Meter
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Stand directly in front of the meter so that you
can clearly see the location of each pointer.
If the pointer is between two numbers, read
the number the pointer has just passed, always
the lowest number.
If the pointer is between 9 and 0, always read
9.
If the pointer appears to be exactly on a
number, read the next lowest number unless
the pointer to its right has passed zero. Since
the first dial has no dial to its right, the number
must be read independently.
What is the reading of the example
electric meter shown below?
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The pointer on the far left dial has passed 8, and is between 8 and
9. Read the smaller number which the pointer just passed, which is
8.
The pointer on the next dial looks like it is right on the 4. But, the
dial to its right has not passed zero. So, you would read this dial as
3.
The next dial has passed 8, and is between 8 and 9. Again, read the
smaller number which the pointer has just passed, which is 8.
The second pointer from the right has just passed 9, and is
between 9 and 0. Read this as 9.
The pointer on the far right is directly on number 5. Read this as 5.
The answer is 83895.