Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Preview Section 1 Electric PotentialElectric Potential Section 2 CapacitanceCapacitance Section 3 Current and ResistanceCurrent and Resistance Section 4 Electric PowerElectric Power

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company The student is expected to: TEKS 6B investigate examples of kinetic and potential energy and their transformations

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company What do you think? You may have purchased batteries for radios, watches, CD players, and other electronic devices. Batteries come in a variety of different sizes and voltages. You probably have 1.5 volt, 3 volt, and 12 volt batteries in your home. What do volts measure? Is the number of volts related to the size of the battery? How is a 3 volt battery different from a 1.5 volt battery?

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Electrical Potential Energy A uniform electric field exerts a force on a charged particle moving it from A to B. Will the particle shown gain or lose PE electric as it moves to the right? –Lose energy (because it is moving with the force, not against it) –Similar to a falling object losing PE g –  PE electric = W done = Fd = -qED

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Electrical Potential Energy  PE electric is positive if the charge is negative and moves with the field.  PE electric is positive if the charge is positive and moves against the field.

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Classroom Practice Problem A uniform electric field strength of 1.0 x 10 6 N/C exists between a cloud at a height of 1.5 km and the ground. A lightning bolt transfers 25 C of charge to the ground. What is the change in PE electric for this lightning bolt? Answer: x J of energy

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Gravitational Potential Difference Suppose a mass of 2.00 kg is moved from point A straight up to point B a distance of 3.00 m. Find the  PE g for the mass if g = 9.81 m/s 2. Repeat for a mass of 5.00 kg. –Answer: 58.9 J and 147 J What is the  PE g per kg for each? –Answer: 29.4 J/kg for both The change per kg does not depend on the mass. It depends only on points A and B and the field strength. There is an analogous concept for electrical potential energy, as shown on the next slide.

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Potential Difference Potential difference (  V) is the change in electrical potential energy per coulomb of charge between two points. –Depends on the electric field and on the initial and final positions –Does not depend on the amount of charge –SI unit: joules/coulomb (J/C) or Volts (V)

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Potential Difference The potential difference is calculated between two points, A and B. –The field must be uniform.

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Electrical Potential Energy

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Potential Difference Near a Point Charge The above  V determines the potential energy per coulomb at a point compared to a very distant point where V would equal zero. Potentials are scalars (+ or -) so the total potential at a point is the sum of the potentials from each charge.

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Superposition Principle and Electric Potential

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Batteries A battery maintains a constant potential difference between the terminals. –1.5 V (AAA, AA, C and D cell) or 9.0 V or 12 V (car) In 1.5 V batteries, the electrons use chemical energy to move from the positive to the negative terminal. –They gain 1.5 joules of energy per coulomb of charge When connected to a flashlight, the electrons move through the bulb and lose 1.5 joules of energy per coulomb of charge.

Electrical Energy and CurrentSection 1 © Houghton Mifflin Harcourt Publishing Company Now what do you think? You may have purchased batteries for radios, watches, CD players, and other electronic devices. Batteries come in a variety of different sizes and voltages. You probably have 1.5 volt, 3 volt, and 12 volt batteries in your home. –What do volts measure? –Is the number of volts related to the size of the battery? –How is a 3 volt battery different from a 1.5 volt battery?

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company The student is expected to: TEKS 6B investigate examples of kinetic and potential energy and their transformations

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company What do you think? If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates? If not, why not? If so, is the flow similar or different from that with the light bulb? Explain. The battery shown has a potential difference of 6.0 volts. It has just been connected to two metal plates separated by an air gap. There is no electrical connection between the two plates and air is a very poor conductor.

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Capacitors The two metal plates are electrically neutral before the switch is closed. What will happen when the switch is closed if the left plate is connected to the negative terminal of the battery? –Electrons will flow toward lower PE. From the battery to the left plate From the right plate to the battery

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Parallel Plate Capacitors Electrons build up on the left plate, giving it a net negative charge. The right plate has a net positive charge. –Capacitors can store charge or electrical PE.

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Capacitance

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Capacitance Capacitance measures the ability to store charge. SI unit: coulombs/volt (C/V) or farads (F) In what way(s) is a capacitor like a battery? In what way(s) is it different?

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Capacitance How would capacitance change if the metal plates had more surface area? –Capacitance would increase. How would it change if they were closer together? –Capacitance would increase.

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Capacitance  is a constant that is determined by the material between the plates (  0 refers to a vacuum). Combining the two equations for C yields:

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Parallel-Plate Capacitor

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Dielectrics The space between the plates is filled with a dielectric. –Rubber, waxed paper, air The dielectric increases the capacitance. –The induced charge on the dielectric allows more charge to build up on the plates.

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Capacitor Applications Connecting the two plates of a charged capacitor will discharge it. –Flash attachments on cameras use a charged capacitor to produce a rapid flow of charge. Some computer keyboards use capacitors under the keys to sense the pressure. –Pushing down on the key changes the capacitance, and circuits sense the change.

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Energy and Capacitors As the charge builds, it requires more and more work to add electrons to the plate due to the electrical repulsion. –The average work or PE stored in the capacitor is (1/2)Q  V. –Derive equivalent equations for PE electric by substituting: Q = C  V and  V = Q/C

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Classroom Practice Problem A 225  F is capacitor connected to a 6.00 V battery and charged. How much charge is stored on the capacitor? How much electrical potential energy is stored on the capacitor? –Answers: 1.35 x C, 4.05 x J

Electrical Energy and CurrentSection 2 © Houghton Mifflin Harcourt Publishing Company Now what do you think? If a light bulb replaced the two metal plates and the battery was connected, electrons would flow out of the negative and into the positive terminal. Will this also occur with the two metal plates? If not, why not? If so, is the flow similar or different from that with the light bulb? Explain.

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company The student is expected to: TEKS 5E characterize materials as conductors or insulators based on their electrical properties

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company What do you think? The term resistance is often used when describing components of electric circuits. What behavior of the components does this term describe? Do conductors have resistance? If so, are all conductors the same? Explain. What effect would increasing or decreasing the resistance in a circuit have on the circuit?

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Electric Current Electric current (I) is rate at which charges flow through an area. SI unit: coulombs/second (C/s) or amperes (A) –1 A = 6.25  electrons/second

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Conventional Current Conventional current (I) is defined as the flow of positive charge. –The flow of negative charge as shown would be equivalent to an equal amount of positive charge in the opposite direction. In conducting wires, I is opposite the direction of electron flow.

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Conventional Current

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Velocity of Electrons Through Wires When you turn on a wall switch for a light, electrons flow through the bulb. Which speed below do you believe most closely approximates that of the electrons? –The speed of light ( m/s) –1 000 m/s –10 m/s – m/s Why do you think so?

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Drift Velocity Electrons undergo collisions with atoms in the metal. –They “drift” through the wire. –Drift velocity for a copper wire with a current of 10 A is m/s. The E field moves through the wire near the speed of light, causing all electrons in the wire to move nearly instantly.

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Drift Velocity

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Resistance to Current Resistance is opposition to the flow of charge. –SI unit: volts/ampere (V/A) or ohms (  ) Ohm’s Law :  V = IR –Valid only for certain materials whose resistance is constant over a wide range of potential differences

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Classroom Practice Problems A typical 100 W light bulb has a current of 0.83 A. How much charge flows through the bulb filament in 1.0 h? How many electrons would flow through in the same time period? –Answers: 3.0  10 3 C, 1.9  electrons This same 100 watt bulb is connected across a 120 V potential difference. Find the resistance of the bulb. –Answer: 1.4  10 2 

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Resistance of a Wire On the next slide, predict the change necessary to increase the resistance of a piece of wire with respect to: –Length of wire –Cross sectional area or thickness of the wire –Type of wire –Temperature of the wire

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Factors that Affect Resistance

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Applications Resistors in a circuit can change the current. –Variable resistors (potentiometers) are used in dimmer switches and volume controls. –Resistors on circuit boards control the current to components. The human body’s resistance ranges from  (dry) to 100  (soaked with salt water). –Currents under 0.01 A cause tingling. –Currents greater than 0.15 A disrupt the heart’s electrical activity.

Electrical Energy and CurrentSection 3 © Houghton Mifflin Harcourt Publishing Company Now what do you think? The term resistance is often used when describing components of electric circuits. What behavior of the components does this term describe? Do conductors have resistance? If so, are all conductors the same? Explain. What effect would increasing or decreasing the resistance in a circuit have on the circuit?

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company The student is expected to: TEKS 6B investigate examples of kinetic and potential energy and their transformations

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company What do you think? Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets. What is electric power? Is electric power the same as the power discussed in the chapter “Work and Energy?” Do the utility companies bill your household for power, current, potential difference, energy, or something else? What do you think is meant by the terms alternating current (AC) and direct current (DC)? Which do you have in your home?

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Types of Current - Direct Batteries use chemical energy to give electrons potential energy. –Chemical energy is eventually depleted. Electrons always flow in one direction. –Called direct current (DC)

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Types of Current - Alternating Generators change mechanical energy into electrical energy. –Falling water or moving steam Electrons vibrate back and forth. –Terminals switch signs 60 times per second (60 Hz). –Called alternating current (AC) –AC is better for transferring electrical energy to your home.

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Energy Transfer Is the electrical potential energy gained, lost, or unchanged as the electrons flow through the following portions of the circuit shown: –A to B –B to C –C to D –D to A Explain your answers.

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Energy Transfer –A to B (unchanged) –B to C (lost in bulb) –C to D (unchanged) –D to A (gained in battery)

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Electric Power

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Electric Power Power is the rate of energy consumption (  PE/  t ). For electric power, this is equivalent to the equation shown below. –SI unit: joules/second (J/S) or watts (W) –Current (I) is measured in amperes (C/s). –Potential difference (  V) is measured in volts (J/C). Substitute using Ohm’s law (  V = IR) to write two other equations for electric power.

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Classroom Practice Problems A toaster is connected across a 120 V kitchen outlet. The power rating of the toaster is 925 W. –What current flows through the toaster? –What is the resistance of the toaster? –How much energy is consumed in 75.0 s? Answers: 7.7 A, 16 , 6.94  10 4 J

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Household Energy Consumption Power companies charge for energy, not power. –Energy consumption is measured in kilowatthours ( kwh). The joule is too small. –A kwh is one kilowatt of power for one hour. Examples of 1 kwh: –10 light bulbs of 100 W each on for 1 h –1 light bulb of 100 W on for 10 h 1 kwhr = J or 3.6 x 10 6 J

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Relating Kilowatt-Hours to Joules

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Electrical Energy Transfer Transfer of energy from power plants to your neighborhood must be done at high voltage and low current. –Power lost in electrical lines is significant. P = I 2 R Power lines are good conductors but they are very long. Since power companies can’t control the resistance (R), they control the current (I) by transferring at high voltage.

Electrical Energy and CurrentSection 4 © Houghton Mifflin Harcourt Publishing Company Now what do you think? Hair dryers, microwaves, stereos, and other appliances use electric power when plugged into your outlets. –What is electric power? Is electric power the same as the power discussed in the chapter “Work and Energy?” –Do the utility companies bill your household for power, current, potential difference, energy, or something else? –What do you think is meant by the terms alternating current (AC) and direct current (DC)? Which do you have in your home?