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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 Ξ£ππΈ = πΎπΈ + ππΈππππ£ππ‘π¦ + ππΈππππ π‘ππ + ππΈπππππ‘πππ 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 βππΈπππππ‘πππ = βππΈπ ELECTRICAL POTENTIAL ENERGY + + + + + + + + + + + + + - ELECTRIC POTENTIAL o When a charge is moved against an electrical field, work must be done to increase the chargeβs 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 π= ππΈπππππ‘πππ π 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 βπ = βππΈπππππ‘πππ π 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 βππΈπππππ‘πππ = βππΈπ βπ = βππΈπππππ‘πππ π β(βππΈπ) βπ = π βπ = βπΈπ 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: β’ βππΈπππππ‘πππ = -6.9 × 10-19 J β’ d = 0.020 m β’ E = 215 N/C Unknown: β’q=? β’ ΞV = ? 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 πΆ= π βπ 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 ππΈπππππ‘πππ 1 = πβπ 2 SAMPLE PROBLEM 17B A capacitor, connected to a 12 V battery holds 36 ΞΌC of charge on each plate. What is the capacitance of the capacitor? How much electrical potential energy is stored in the capacitor? Given: β’ βV = 12 V β’ Q = 36 ΞΌC = 36 × 10-6 C Unknown: β’C=? β’ 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 πΌ= βπ βπ‘ 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: β’ I = 0.835 A β’ βQ = 1.67 C Unknown: β’ βt = ? HOMEWORK! Page 595 - Practice C #1-5 RESISTANCE TO CURRENT o So far the only factor weβve 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 OHMβS 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 Ohmβs law, and it is represented mathematically as βπ π = πΌ o A more common way Ohmβs law is expressed is βπ = πΌπ o The SI unit of resistance is the ohm (Ξ©) o One ohm is equal to one volt per ampere SAMPLE PROBLEM 17D The resistance of a steam iron is 19.0 Ξ©. What is the current in the iron when it is connected across a potential difference of 120 V? Given: β’ R = 19.0 Ξ© β’ βV = 120 V Unknown: β’I=? HOMEWORK! Page 601 - Practice D #1-6 CHAPTER 18 SECTION1 P. 628 β 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 β Formative Assessment #1-3 CHAPTER 18 SECTION 2 P. 635 β 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 πΌπ = πΌ1 = πΌ2 = πΌ3 β¦ πΌπ = πΌ1 + πΌ2 + πΌ3 β¦ potential difference βππ = βπ1 + βπ2 + βπ3 β¦ βππ = βπ1 = βπ2 = βπ3 β¦ equivalent resistance π π = π 1 + π 2 + π 3 β¦ 1 1 1 1 = + + β¦ π π π 1 π 2 π 3 schematic diagram SAMPLE PROBLEM 18A A 9.0 V battery is connected in series to four light bulbs with resistances of 2.0Ξ©, 4.0 Ξ©, 5.0 Ξ©, and 7.0 Ξ©. Find the equivalent resistance and the current of the circuit. Given: β’ βV = 9.0 V β’ R1 = 2.0 Ξ© β’ R2 = 4.0 Ξ© β’ R3 = 5.0 Ξ© β’ R4 = 7.0 Ξ© Unknown: β’ RT = ? β’ IT = ? SAMPLE PROBLEM 18B A 9.0 V battery is connected in parallel to four light bulbs with resistances of 2.0Ξ©, 4.0 Ξ©, 5.0 Ξ©, and 7.0 Ξ©. Find the equivalent resistance and the current of the circuit. Given: β’ βV = 9.0 V β’ R1 = 2.0 Ξ© β’ R2 = 4.0 Ξ© β’ R3 = 5.0 Ξ© β’ R4 = 7.0 Ξ© Unknown: β’ RT = ? β’ IT = ? HOMEWORK! Page 638 β Practice A #1, 2, 4 P. 643 β Practice B #1 β 3 CHAPTER 17 SECTION 4 P. 604 β 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 = IβV 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: β’ βV = 120 V β’ P = 1320 W Unknown: β’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 β Practice 17E #1 β 5