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Journal #
1. Identify the parts of the atom
and their charges.
2. When an atom loses electrons,
it becomes positive/negative
(CIRCLE ONE).
3. When an atom gains electrons,
it becomes positive/negative
(CIRCLE ONE).
4. An object that is positively
charged has more/less
(CIRCLE ONE) electrons than
protons.
5. An object that is negatively
charged has more/less
(CIRCLE ONE) electrons than
protons.
electron (-) proton (+)
neutron (0)
Journal # 1. Draw sketches of a charged electroscope and a
discharged electroscope.
2. What is the purpose of an electroscope?
3. Can an electroscope determine the charge of an
object (whether positive or negative)?
1. See diagram
2. The purpose of an
electroscope is to detect
charge.
3. No, an electroscope can
only detect charge. It
does not determine
whether the charge is
positive or negative.
2/19/2013 3
• In 1752, Benjamin Franklin flew a kite with
a key attached to the string. As a
thunderstorm approached, the loose
threads of the kite string began to stand up
and repel one another, and when Franklin
brought his knuckle close to the key, he
experienced a spark.
• Electrostatics is the study of electric
charges that can be collected and held in
one place.
Introduction to Electricity
2/19/2013 4
Section 20.1: Electric Charges
• With the addition of energy, the outer
electrons can be removed from
atoms.
• An atom who loses electrons has an
overall positive charge, and
consequently, any matter made of
these electron-deficient atoms is
positively charged.
2/19/2013 5
• A material that allows charges to
move about easily is called an electric
conductor. – Most metals, salt water, and acidic solutions are
good conductors.
• A material through which a charge will
not move easily is called an electric
insulator. – Glass, dry wood, most plastics, cloth, and dry air are
all good insulators and poor conductors.
Conductors versus Insulators
2/19/2013 6
Section 20.2: Electric Charges
• There are two kinds of electric charges:
positive and negative.
• Like charges repel; opposite charges
attract.
2/19/2013 7
• An electroscope consists of a metal
knob connected by a metal stem to
two thin, lightweight pieces of metal
foil, called leaves, and it detects
charge. It CANNOT determine what
type of charge.
The Electroscope
2/19/2013 8
• Charging a neutral body by touching
it with a charged body is called
charging by conduction.
• Charging without actually touching
the body is called charging by
induction.
Conduction versus Induction
2/19/2013 9
Section 20.2- Coulomb’s Law
• Coulomb discovered that charges exert forces
on other charges at a distance and that he force
is stronger when the charges are closer
together.
• The SI standard unit of charge is called the
coulomb (C).
• The charge on a single electron is 1.60×10−19 C.
The magnitude of the charge of a single electron
is called the elementary charge. There is no
charge less than this and all charges are
multiples of this number.
2/19/2013 10
Section 21.1-Electric Fields • The electric field extends around any charged
particle and can produce a force capable of
doing work.
• The field is invisible to the eye, so to represent it
on paper we use electric field lines.
– The field is strong where the lines are close together.
It is weaker where the lines are spaced farther apart.
– Force lines in an electric field are drawn away from a
positive charge and toward a negative charge.
2/19/2013 11
Journal #
Complete the table below.
The charge of an electron is 1.60 x 10-19 C.
Object # of Electrons Charge
A 1.5 x 107 deficient electrons
B 4.3 x 108 excess electrons
C 2.7 x 1010 deficient electrons
D 3.1 x 103 deficient electrons
E 7.8 x 1028 excess electrons
2/19/2013 12
Journal #
Object # of Electrons Charge
A 1.5 x 107 deficient electrons +2.4 x 10-12 C
B 4.3 x 108 excess electrons -6.9 x 10-11 C
C 2.7 x 1010 deficient electrons +4.3 x 10-9 C
D 3.1 x 103 deficient electrons +5.0 x 10-16 C
E 7.8 x 1028 excess electrons -1.2 x 1010 C
Complete the table below.
The charge of an electron is 1.60 x 10-19 C.
2/19/2013 13
• A person touching the
terminal of a Van de
Graaff machine is
charged electrically.
• The charges on the
person’s hairs repel each
other, causing the hairs
to follow the field lines as
shown in the animation.
Van De Graaff Generator
2/19/2013 14
Van de Graaff Generator Demo
• What happens when a conductor is
brought near to the dome?
• What effect does the generator have on a
fluorescent light bulb? Why?
Charging by CONDUCTION Charging by INDUCTION
Picture
To Touch or Not to Touch
What happens to the initial charge of the
charged object?
What is the final charge of the neutral object being charged
compared to the original charged
object?
Charging by CONDUCTION Charging by INDUCTION
Picture
To Touch or Not to Touch Touch No Touch
What happens to the initial charge of the
charged object? Loses a little of the initial charge Keeps all of the initial charge
What is the final charge of the neutral object being charged
compared to the original charged
object?
Same as the initial charge Opposite of the initial charge
2/19/2013 17
Chapter 21 Continued
• Electric potential difference, ΔV, is the change
in potential energy from one place to another.
Sometimes, the electric potential difference is
simply called the voltage because it is measured
in the unit of volts (V).
• The term volt (and voltage) comes from the
name of the inventor of the battery, Alessandro
Volta, an Italian physicist.
2/19/2013 18
• A battery converts
chemical potential energy to
electric energy.
• A common battery is made
from multiple layers of
different metals surrounded
by an electrolyte (either
acidic or alkaline) solution.
• FYI… there are many types
of batteries that use
different chemistry to
produce a potential
difference.
Batteries
2/19/2013 19
• The rate of flow of electric charge, called
electric current, is measured in amperes, A.
• Power, which is measured in watts, W,
measures the rate at which energy is
transferred.
• The property determining how much current
will flow is called resistance. The resistance
of the conductor, R, is measured in ohms (Ω).
Current, Power, and Resistance
2/19/2013 20
Current and Circuits
• Electric circuits provide the means to transfer
large quantities of energy over great distances
with little loss.
• When two conductors touch, charges flow from
the conductor at a higher potential to the one at
a lower potential.
• The flow continues until there is no potential
difference between the two spheres.
2/19/2013 21
P = IV V=IR (Ohm’s Law)
• P = power
• I = current
• V = potential difference aka voltage
• R = resistance
Important Formulas
Resistance
• A resistor is a device designed to
have a specific resistance.
• In wires, resistance increases
under the following conditions:
– Longer wires
– Thinner wires
– Higher temperatures
2/19/2013 Template copyright 2005 www.brainybetty.com 23
Diagram Symbols
2/19/2013 Template copyright 2005 www.brainybetty.com 24
Practice Question
A 30.0-V battery is connected to a
10.0 Ω resistor.
A. Draw the circuit.
B. What is the current in the circuit?
2/19/2013 Template copyright 2005 www.brainybetty.com 25
Practice Question
• V = 30.0 V
• R = 10 Ω
• I = ?
A
30.0V
10.0Ω
2/19/2013 Template copyright 2005 www.brainybetty.com 26
Practice Question
R
V I
10.0
V 30.0 I
I = 3.00 A
Journal #
Complete the table below.
Quantity Definition Symbol Unit
Current
Resistance
Potential Difference
Power
Journal #
Complete the table below.
Quantity Definition Symbol Unit
Current the flow rate of electrons I amperes (A)
Resistance a property that determines how
much current will flow R ohms (Ω)
Potential Difference the change in potential energy
required for charge to flow V volts (V)
Power the rate at which energy is
transferred P watts (W)
Types of Circuits
• Series circuit (single path)
• Parallel circuit (poly path)
2/19/2013
Series Circuit • A circuit in which all current travels
through each device, is called a
series circuit.
2/19/2013
Series Connection
A connection with
only one current path
is called a series
connection. An
ammeter is
connected to a circuit
in series.
2/19/2013
Parallel Circuit • A circuit in which there are several
current paths is called a parallel
circuit.
2/19/2013
Parallel Connection
When a voltmeter is
connected across
another component, it is
called a parallel
connection because the
circuit component and
the voltmeter are
aligned parallel to each
other in the circuit, as
diagrammed in the
figure.
2/19/2013 34
Series: • Current is the same at all
point in the circuit.
• Voltage drops each time
the current passes through
a resistor until it reaches
zero volts.
• same current and different
voltage
• If one bulb goes out, all
others in series with it will
go out.
Parallel: • Current depends on the
resistance and will always
flow down the path of least
resistance. The total current
prior to the branches is the
sum of the current in each
of the branches.
• Voltage is the same in each
branch.
• same voltage and different
current
• If one bulb goes out, the
others remain on.
2/19/2013
Series:
• Resistance can be
found 2 ways:
Parallel:
• Equivalence
resistance must be
found for the parallel
branches by using
the following
method:
R V
I
Rtotal R1 R2 ...
1
Rtotal
1
R1
1
R2
...
Useful when you don’t have a
calculator… I’ll show you a faster
way with a calculator.
Practice Question #1
1. What type of circuit does the diagram represent?
2. What is the equivalence resistance for the circuit?
3. What is the total current in the circuit?
4. What is the voltage drop across each resistor?
2. Finding Equivalence
Resistance Use the formula
Rtotal = R1 + R2 + R3
Rtotal = 2Ω + 3Ω + 4Ω
Rtotal = 9Ω
3. Finding Total Current
Use the formula
V = IR so I = VTotal/R
I = 12V/9Ω
I = 1.3A
4. Finding Voltage Drops
Use the formula
V = IR and use each individual resistance and total current so
V1 = IR1 = (1.3A)(2Ω) = 2.7V
V2 = IR2 = (1.3A)(3Ω) = 4V
V3 = IR3 = (1.3A)(4Ω) = 5.3V Check:
V1 + V2 + V3 = 12V
2.7V+ 4V + 5.3V = 12V
Practice Question #2
1. What type of circuit does the diagram represent?
2. What is the equivalence resistance for the circuit?
3. What is the total current leaving the generator?
4. What is the current through each resistor?
Use the formula,
1
Rtotal
1
R1
1
R2
1
R3
in a calculator... same as
Rtotal (R1
1 R2
1 R3
1)1
Rtotal (241 91 61)1
Rtotal 3.13
2. Finding Equivalence
Resistance
Use the formula,
?
V I Rtotal
V
Rtotal I
120V
3.13 38.3A
Notice how we
have used the
equivalence
resistance from
the previous
question
3. Finding Total Current
Use the formula three times, once for each resistor,
?
?
?
I V
R1
120V
24 5.0A
4. Finding Current in Each
Branch
I V
R2
120V
913.3A
I V
R3
120V
6 20A
The sum
of the
three is
equal to
the total
found
earlier
SERIES PARALLEL
Total Current
stays constant
Itotal = I1 = I2
adds up
Itotal = I1 + I2
Total Voltage
adds up
Vtotal = V1 + V2
stays constant
Vtotal = V1 = V2
Total Resistance
increases
Rtotal = R1 + R2
decreases
Rtotal = (R1-1 + R2
-1)-1
2/19/2013
Journal #
1. What type of circuit does the diagram below represent?
2. If R1 = 3.0Ω, R2 = 4.0Ω, and R3 = 5.0Ω, what is the
equivalent resistance of the circuit?
3. If the electric potential difference of the battery is 12 V,
what is the voltage across each resistor?
4. What is the current flowing in each branch?
R1 = 6.0 Ω, R2 = 4.0 Ω, R3 = 3.0 Ω, and V = 12V
Journal #
1. In which circuit is the current constant?
2. In which circuit is the voltage drop across
each resistor the same?
3. In which circuit is the current the greatest?
A B
2/19/2013
Journal #
1. How many batteries are drawn in the circuit?
2. What is the equivalent resistance in this circuit?
3. What is the voltage drop across each resistor?
4. What is the current in each resistor in the
circuit?
5. What is the power dissipated by each resistor?
R1 = R2 = R3 = 4.0 Ω
V = 12V