Download Electrical Energy and Current

ELECTRICAL
ENERGY AND
CURRENT
PHYSICS 1-2
MR. CHUMBLEY
PHYSICS : CHAPTERS 17 AND 18
SECTION 1
P. 580 - 587
ELECTRIC
POTENTIAL
ELECTRICAL
POTENTIAL ENERGY
o When two electrically charged objects interact, there exists an
electrical force between them
o Similar to gravity, there is a potential energy associated with this
force
o Electrical potential energy is the potential energy associated
with a charge due to its position in an electric field
o Electrical potential energy is an integral part the total mechanical
energy
Σ𝑀𝐸 = 𝐾𝐸 + 𝑃𝐸𝑔𝑟𝑎𝑣𝑖𝑡𝑦 + 𝑃𝐸𝑒𝑙𝑎𝑠𝑡𝑖𝑐 + 𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
ELECTRICAL
POTENTIAL ENERGY
o One way to look at electrical potential energy is to look at what
happens to a positive charge in a uniform electric field
o As the charge moves through the electric field, there is a
change in the potential energy
o The change in electrical potential energy is dependent on the
charge, the strength of the electric field, and the displacement
the charge moves
∆𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = −𝑞𝐸𝑑
ELECTRICAL
POTENTIAL ENERGY
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-
ELECTRIC POTENTIAL
o When a charge is moved against an electrical field, work must
be done to increase the charge’s electrical potential energy
o Electric potential is the work that must be performed against
electric forces to move a charge from a reference point to the
point in question, divided by the charge
𝑉=
𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
𝑞
o Electric potential is independent of the charge
POTENTIAL
DIFFERENCE
o
Potential difference is the change in electric potential
o
Potential difference is the work that must be performed against electric
forces to move a charge between two points in question, divided by the
charge
∆𝑉 =
∆𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
𝑞
o The SI unit for potential difference is the volt (V)
o One volt is equivalent to one joule per coulomb
o (1 V = 1 J / 1 C)
o The reason potential difference is valuable is that the reference points
are arbitrary, so only changes in electric potential are significant
POTENTIAL
DIFFERENCE
o The expression for potential difference and electrical potential
energy can be combined to express the potential difference in a
uniform electric field
∆𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = −𝑞𝐸𝑑
∆𝑉 =
∆𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
𝑞
∆(−𝑞𝐸𝑑)
∆𝑉 =
𝑞
∆𝑉 = −𝐸𝑑
o This shows that as a charge moves through an electric field, the
amount potential difference is independent of the charge because
it is work per unit of charge
SAMPLE PROBLEM 17A
A charge moves a distance of 2.0 cm in the direction of a uniform electric
field whose magnitude is 215 N/C. As the charge moves its electrical
potential energy decreases by 6.9 × 10-19 J.
A) Find the charge on the moving particle.
B) Find the potential difference between the two locations.
Given:
• ∆𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = -6.9 × 10-19 J
• d = 0.020 m
• E = 215 N/C
Unknown:
•q=?
• ΔV = ?
HOMEWORK!
Page 585 - Practice A
#1-3
SECTION 2
P. 588 - 593
CAPACITANCE
CAPACITOR
o A capacitor is a device used to store electrical potential energy
o Capacitance is the ability of a conductor to store energy in the
form of electrically separated charges
𝐶=
𝑄
∆𝑉
o Capacitance is measured in the farad, F which is equivalent to
one coulomb per volt
ENERGY AND
CAPACITORS
o Charged capacitors are a store of electrical potential energy
o When a capacitor is discharged, work done to move charge
through a circuit
o The amount of potential energy stored in a charged capacitor
is given by the following expression
𝑃𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
1
= 𝑄∆𝑉
2
SAMPLE PROBLEM 17B
A capacitor, connected to a 12 V battery holds 36 μC of charge on each
plate. What is the capacitance of the capacitor? How much electrical
potential energy is stored in the capacitor?
Given:
• ∆V = 12 V
• Q = 36 μC = 36 × 10-6 C
Unknown:
•C=?
• PEelectric = ?
HOMEWORK!
Page 593 - Practice B
#1-3
SECTION 3
P. 594 - 602
CURRENT AND
RESISTANCE
CHARGE MOVEMENT
o While many practical applications exist using static electricity, the most
integral part of electricity in daily life is moving electric charge, called
current
o Current exists whenever there is a net movement of electric charge
o Electric current is the rate at which electric charges pass through a
given area
𝐼=
∆𝑄
∆𝑡
o The SI unit for current is the ampere (A)
o One ampere is equal to one coulomb of charge passing through a
given area in one second
CONVENTIONAL
CURRENT
o Moving charges can be positive, negative, or a combination of
both
o In common conductors, current is a result of the motion of
negative charge
o In particle accelerators current results from protons being set in
motion
o In gases and certain dissolved salts, current is a result of
positive and negative charges moving in opposite directions
CONVENTIONAL
CURRENT
o Charges in motion are
called charge carriers
o Conventional current is
defined by the rate of flow
of positive charge
o Negative charge carriers
have a current in the
direction opposite their
motion
SAMPLE PROBLEM 17C
The current in a light bulb is 0.835 A. How long does it take for a total
charge of 1.67 C to pass through the filament of the bulb?
Given:
• I = 0.835 A
• ∆Q = 1.67 C
Unknown:
• ∆t = ?
HOMEWORK!
Page 595 - Practice C
#1-5
RESISTANCE TO
CURRENT
o So far the only factor we’ve discussed that affects the current
in an electric circuit is the potential difference
o When there is an electric current, there exists opposition to the
flow of electric charge
o Resistance is the opposition presented to electric current by a
material or device
OHM’S LAW
o Looking at resistance quantitatively, there exists a relationship among
resistance, potential difference, and current within an electric circuit
o Georg Ohm was among the first to show that for most materials
resistance is constant over a wide range of applied potential differences
o This is known as Ohm’s law, and it is represented mathematically as
∆𝑉
𝑅=
𝐼
o
A more common way Ohm’s law is expressed is ∆𝑉 = 𝐼𝑅
o
The SI unit of resistance is the ohm (Ω)
o
One ohm is equal to one volt per ampere
SAMPLE PROBLEM 17D
The resistance of a steam iron is 19.0 Ω. What is the current in the iron
when it is connected across a potential difference of 120 V?
Given:
• R = 19.0 Ω
• ∆V = 120 V
Unknown:
•I=?
HOMEWORK!
Page 601 - Practice D
#1-6
CHAPTER 18 SECTION1
P. 628 – 633
SCHEMATIC
DIAGRAMS
AND CIRCUITS
SCHEMATIC DIAGRAM
o A schematic diagram is a representation of a circuit that uses
lines to represent wires and different symbols to represent
components
o Often referred to as a circuit diagram
o The circuit components that you will be expected to know are
on the Symbols Sheet
ELECTRIC CIRCUITS
o An electric circuit is a set of electrical components connected
such that they provide one or more complete path for the
movement of charges
o Whenever components dissipate the energy in the circuit,
those components are called the load
o A closed circuit is a circuit that contains a complete path for
charge to move
o An open circuit is a circuit that does not contain a complete
path for charge to move, so no charge is moving
HOMEWORK!
Page 633 – Formative Assessment
#1-3
CHAPTER 18 SECTION 2
P. 635 – 644
RESISTORS IN
SERIES AND
PARALLEL
MULTIPLE RESISTORS
o While some circuits consist of a single power source with a single
component, many circuits have multiple components
o Circuits in series are described as two or more components of a
circuit that provide a single path for current
o Circuits in parallel are described as two or more components of a
circuit that provide separate conducting paths for current because
the components are connected across common joints or junctions
o While the resistance of individual resistors is useful, the total
resistance of the whole circuit, called the equivalent resistance,
is more useful
RESISTORS IN SERIES
o Series circuits are connected in
such a way that the current
passes through each component
o As a result, the current in each
resistor is the same
o However, the potential difference
across each resistor is different
o The equivalent resistance of
resistors in series is greater than
any individual resistance
RESISTORS IN
PARALLEL
o Parallel circuits are connected
in such a way that the
potential difference across
each component is the same
o As a result, the current
through each resistor is
different
o The equivalent resistance of
resistors in parallel is less than
the smallest resistance
SERIES AND
PARALLEL
Series
Parallel
current
𝐼𝑇 = 𝐼1 = 𝐼2 = 𝐼3 …
𝐼𝑇 = 𝐼1 + 𝐼2 + 𝐼3 …
potential difference
∆𝑉𝑇 = ∆𝑉1 + ∆𝑉2 + ∆𝑉3 …
∆𝑉𝑇 = ∆𝑉1 = ∆𝑉2 = ∆𝑉3 …
equivalent
resistance
𝑅𝑇 = 𝑅1 + 𝑅2 + 𝑅3 …
1
1
1
1
=
+
+
…
𝑅𝑇
𝑅1 𝑅2 𝑅3
schematic diagram
SAMPLE PROBLEM 18A
A 9.0 V battery is connected in series to four light bulbs with resistances of
2.0Ω, 4.0 Ω, 5.0 Ω, and 7.0 Ω. Find the equivalent resistance and the
current of the circuit.
Given:
• ∆V = 9.0 V
• R1 = 2.0 Ω
• R2 = 4.0 Ω
• R3 = 5.0 Ω
• R4 = 7.0 Ω
Unknown:
• RT = ?
• IT = ?
SAMPLE PROBLEM 18B
A 9.0 V battery is connected in parallel to four light bulbs with resistances of
2.0Ω, 4.0 Ω, 5.0 Ω, and 7.0 Ω. Find the equivalent resistance and the
current of the circuit.
Given:
• ∆V = 9.0 V
• R1 = 2.0 Ω
• R2 = 4.0 Ω
• R3 = 5.0 Ω
• R4 = 7.0 Ω
Unknown:
• RT = ?
• IT = ?
HOMEWORK!
Page 638 – Practice A
#1, 2, 4
P. 643 – Practice B
#1 – 3
CHAPTER 17 SECTION 4
P. 604 – 609
ELECTRIC
POWER
SOURCES AND TYPES
OF CURRENT
o Electric current can come from a variety of sources, but each
source is a source of potential difference
o Batteries and cells convert chemical energy into electric
potential energy
o Generators convert mechanical energy into electrical energy
o In direct current (DC), the charges move in only one
direction
o In alternating current (AC), the terminals of the source of
potential difference are constantly changing sign, resulting
in no net motion of charge
ELECTRIC POWER
o As electrical energy moves through a circuit, energy is
consumed by components with resistance
o Electric power is the rate at which electrical energy is
converted to nonelectrical forms of energy
P = I∆V
o The SI unit of electric power is the same as it is for mechanical
power, the watt (W)
o One watt is equal to one joule of energy per one second
SAMPLE PROBLEM 17E (p. 607)
An electric space heater is connected across a 120 V outlet. The heater
dissipates 1320 W of power in the form of electromagnetic radiation and
heat. Calculate the resistance of the heater.
Given:
• ∆V = 120 V
• P = 1320 W
Unknown:
•R=?
POWER IN EVERYDAY
LIFE
o Most electrical appliances and devices are identified by their
power rating
o This rating is often referred to as the wattage of the device
o Electric companies do not calculate energy consumption in terms
of power, but in terms of actual energy used
o To do this, they use a unit of kilowatt-hours (kWh)
o One kWh is equal to consuming 3.6 × 106 J
o When electricity is transported long distances, energy is lost
o To minimize the loss, electricity is transported at very high
voltages (~750,000 V) and very low currents and reduced
down at different stages
HOMEWORK!
Page 607 – Practice 17E
#1 – 5