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
Σ푀퐸 = 퐾퐸 + 푃퐸푔푟푎푣푖푡푦 + 푃퐸푒푙푎푠푡푖푐 + 푃퐸푒푙푒푐푡푟푖푐
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
∆푃퐸푒푙푒푐푡푟푖푐 = −푞퐸푑
ELECTRICAL POTENTIAL ENERGY
+ - + - + - + - + - + + - + - + - + - + - + - + - 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
푃퐸 푉 = 푒푙푒푐푡푟푖푐 푞 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
∆푃퐸 ∆푉 = 푒푙푒푐푡푟푖푐 푞 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 ∆푃퐸 ∆푃퐸 = −푞퐸푑 ∆푉 = 푒푙푒푐푡푟푖푐 푒푙푒푐푡푟푖푐 푞 ∆(−푞퐸푑) ∆푉 = 푞
∆푉 = −퐸푑 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:
-19 • ∆푃퐸푒푙푒푐푡푟푖푐 = -6.9 × 10 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
푄 퐶 = ∆푉 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
1 푃퐸 = 푄∆푉 푒푙푒푐푡푟푖푐 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
∆푄 퐼 = ∆푡 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 ∆푉 푅 = 퐼 o A more common way Ohm’s law is expressed is ∆푉 = 퐼푅 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
schematic diagram
current 퐼푇 = 퐼1 = 퐼2 = 퐼3 … 퐼푇 = 퐼1 + 퐼2 + 퐼3 …
potential difference ∆푉푇 = ∆푉1 + ∆푉2 + ∆푉3 … ∆푉푇 = ∆푉1 = ∆푉2 = ∆푉3 …
1 1 1 1 equivalent 푅푇 = 푅1 + 푅2 + 푅3 … = + + … resistance 푅푇 푅1 푅2 푅3
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