Chapter 17 Table of Contents Section 1 Electric Potential Electrical Energy and Current Table of Contents Section 1 Electric Potential Section 2 Capacitance Section 3 Current and Resistance Section 4 Electric Power

Objectives: Section 1 Electric Potential Chapter 17 Objectives: Section 1 Electric Potential Distinguish between electrical potential energy, electric potential, and potential difference. Solve problems involving electrical energy and potential difference. Describe the energy conversions that occur in a battery.

Chapter 17 Electrical Potential Section 1 Electric Potential Work is done to separate charges, just as work is done to lift an object. Separated charges possess potential energy, as does an object elevated above a reference point. Bodies with different concentrations of electrons have a difference in potential between them and are a source of potential energy. The difference in potential depends on the concentration of electrons and is measured in volts. Flow of electrons + + - - Flow of electrons - - - Flow of electrons + + +

Electrical Potential Energy Section 1 Electric Potential Chapter 17 Electrical Potential Energy Electrical potential energy is potential energy associated with a charge due to its position in an electric field. Electrical potential energy is a component of mechanical energy. ME = KE + PEgrav + PEelastic + PEelectric PEelectric = –qEd electrical potential energy = – (charge)  (electric field strength)  (displacement from the reference point in the direction of the field) (-) sign indicates that the PE increases for a negative charge.

Electrical Potential Energy Section 1 Electric Potential Chapter 17 Electrical Potential Energy

Chapter 17 Electric Potential Section 1 Electric Potential Electric Potential equals 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. The electric potential associated with a charge is the electric potential energy divided by the charge: http://www.physicsclassroom.com/Class/circuits/u9l1b4.gif

Chapter 17 Electrical Potential Section 1 Electric Potential PEelectric = –qEd

Electric Potential Difference Section 1 Electric Potential Chapter 17 Electric Potential Difference Potential Difference equals the work that must be performed against electric forces to move a charge between the two points in question, divided by the charge. Potential difference is a change in electric potential. J/C or volts (V)

Electric Potential Difference Section 1 Electric Potential Chapter 17 Electric Potential Difference Units for potential difference are J/C or volts (V). A 1.5 V battery transfers 1.5 J energy/C of charge. Every coulomb of charge that moves will do 1.5 J of work. The D cell battery has more coulombs of charge with which to do work so it will last longer. In a 12 V car battery every coulomb will do 12 J of work. In a 120 V electrical outlet every coulomb will do 120 J of work.

Section 1 Electric Potential Chapter 17 Potential Difference

Potential Difference, continued Section 1 Electric Potential Chapter 17 Potential Difference, continued The potential difference in a uniform field varies with the displacement from a reference point. Displacement is parallel to the field. Potential Difference in a Uniform Electric Field ∆V = –Ed potential difference = –(magnitude of the electric field  displacement)

Chapter 17 Sample Problem Potential Energy and Potential Difference Section 1 Electric Potential Chapter 17 Sample Problem Potential Energy and Potential Difference 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. Find the charge on the moving particle. What is the potential difference between the two locations?

Sample Problem, continued Section 1 Electric Potential Chapter 17 Sample Problem, continued Potential Energy and Potential Difference Given: ∆PEelectric = –6.9  10–19 J d = 0.020 m E = 215 N/C Unknown: q = ? ∆V = ? Know: PEelectric = –qEd ∆V = –Ed

Sample Problem, continued Section 1 Electric Potential Chapter 17 Sample Problem, continued Potential Energy and Potential Difference Use the equation for the change in electrical potential energy. PEelectric = –qEd Rearrange to solve for q, and insert values.

Sample Problem, continued Section 1 Electric Potential Chapter 17 Sample Problem, continued Potential Energy and Potential Difference The potential difference is the magnitude of E times the displacement.

Chapter 17 Batteries Section 1 Electric Potential Batteries do work to move charges. In a voltaic pile, electrons move from one metal to the other through an electrolyte (ions in solution). The ions react with the metals, causing an electrochemical reaction, that transfers electrons. The two types of metals in a voltaic pile are called electrodes. One metal reacts more strongly than the other, which leaves an electrical potential difference (voltage) between the two types of metals. One metal becomes positively charged (the positive electrode) and the other becomes negatively charged (the negative electrode). This causes electrons to move. http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p063.shtml?from=Home

Chapter 17 Batteries Section 1 Electric Potential Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes are zinc and carbon, with an acidic paste between them that serves as the electrolyte. Alkaline battery - Alkaline chemistry is used in common Duracell and Energizer batteries, the electrodes are zinc and manganese-oxide, with an alkaline electrolyte. Lithium-iodide battery - Lithium-iodide chemistry is used in pacemakers and hearing aides because of their long life. Lead-acid battery - Lead-acid chemistry is used in automobiles, the electrodes are made of lead and lead-oxide with a strong acidic electrolyte (rechargeable). Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with potassium-hydroxide as the electrolyte (rechargeable). Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable). Lithium-ion battery - With a very good power-to-weight ratio, this is often found in high-end laptop computers and cell phones (rechargeable). Zinc-air battery - This battery is lightweight and rechargeable. Zinc-mercury oxide battery - This is often used in hearing-aids. Silver-zinc battery - This is used in aeronautical applications because the power-to-weight ratio is good.

Objectives: Section 2 Capacitance Chapter 17 Objectives: Section 2 Capacitance Relate capacitance to the storage of electrical potential energy in the form of separated charges. Calculate the capacitance of various devices. Calculate the energy stored in a capacitor.

Capacitors and Charge Storage Section 2 Capacitance Chapter 17 Capacitors and Charge Storage A capacitor is a device that is used to store electrical potential energy. Capacitance is the ability of a conductor to store energy in the form of electrically separated charges. The SI units for capacitance is the farad, F, which equals a coulomb per volt (C/V) Typical capacitances: 1mF to 1pF

Capacitors and Charge Storage Section 2 Capacitance Chapter 17 Capacitors and Charge Storage Capacitance is the ratio of charge to potential difference. Capacitance depends on the size and shape of a capacitor. Capacitance for a Parallel-Plate Capacitor in a Vacuum

Section 2 Capacitance Chapter 17 Capacitance

Capacitors and Charge Storage Section 2 Capacitance Chapter 17 Capacitors and Charge Storage The material between a capacitor’s plates can change its capacitance. The effect of a dielectric is to reduce the strength of the electric field in a capacitor. A dielectric is an insulating material. Earth has an extremely large capacitance.

Capacitors in Keyboards Section 2 Capacitance Chapter 17 Capacitors in Keyboards

Parallel-Plate Capacitor Section 2 Capacitance Chapter 17 Parallel-Plate Capacitor

Chapter 17 Energy and Capacitors Section 2 Capacitance Chapter 17 Energy and Capacitors The potential energy stored in a charged capacitor depends on the charge and the potential difference between the capacitor’s two plates.

Chapter 17 Sample Problem Capacitance Section 2 Capacitance Chapter 17 Sample Problem Capacitance 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: Q = 36 µC = 3.6  10–5 C ∆V = 12 V Unknown: C = ? PEelectric = ?

Sample Problem, continued Section 2 Capacitance Chapter 17 Sample Problem, continued Capacitance To determine the capacitance, use the definition of capacitance.

Sample Problem, continued Section 2 Capacitance Chapter 17 Sample Problem, continued Capacitance To determine the potential energy, use the alternative form of the equation for the potential energy of a charged capacitor:

Objectives Section 3 Current and Resistance Chapter 17 Objectives Section 3 Current and Resistance Describe the basic properties of electric current, and solve problems relating current, charge, and time. Calculate resistance, current, and potential difference by using the definition of resistance. Distinguish between ohmic and non-ohmic materials, and learn what factors affect resistance.

Current and Charge Movement Section 3 Current and Resistance Chapter 17 Current and Charge Movement Electric current is the rate at which electric charges pass through a given area. Current is the movement of electric charges. The large copper wire easily allows electrons to move and transfer charge so the leaves quickly collapse. The thin nichrome wire has fewer free electrons to easily transfer charge so the leaves gradually collapse.

Current and Charge Movement Section 3 Current and Resistance Chapter 17 Current and Charge Movement Electric current is the rate at which electric charges pass through a given area. Current is the movement of electric charges. Unit: Ampere (A); 1 A = 1 C/s By convention, the direction of current is opposite the movement of (-) charges.

Current and Charge Movement Section 3 Current and Resistance Chapter 17 Current and Charge Movement Charge carriers can be (+), (-), or both. Most common charge carriers are electrons in metals (copper wire) or ions in solutions (electrolytes). Current associated with an electron’s change in direction or change in electric field is propagated almost immediately so that when you flip a switch the device responds immediately. Electrons do not move from one part of the conductor (wire) to the device that quickly. It is the change in the electric field that moves quickly.

Section 3 Current and Resistance Chapter 17 Conventional Current

Chapter 17 Drift Velocity Section 3 Current and Resistance Chapter 17 Drift Velocity Drift velocity is the the net velocity of a charge carrier moving in an electric field. Drift speeds are relatively small because of the many collisions that occur when an electron moves through a conductor.

Section 3 Current and Resistance Chapter 17 Drift Velocity

Ohm’s Law: DV = IR Chapter 17 Resistance to Current Section 3 Current and Resistance Chapter 17 Resistance to Current Resistance is the opposition to electric current by a material or device. Resistance is the opposition to the motion of electrons. The SI units for resistance is the ohm (Ω) and is equal to one volt per ampere. Resistance Ohm’s Law: DV = IR

Chapter 17 Resistance to Current Section 3 Current and Resistance For many materials resistance is constant over a range of potential differences. These materials obey Ohm’s Law and are called ohmic materials. Ohm’s low does not hold for all materials. Such materials are called non-ohmic. Diodes are non-ohmic semiconducting devices used in circuits to control the direction of current by providing low resistance to flow in one direction and high resistance to flow in the other direction. Resistance depends on length, cross-sectional area, temperature, and material.

Factors that Affect Resistance Section 3 Current and Resistance Chapter 17 Factors that Affect Resistance

Chapter 17 Resistance to Current Section 3 Current and Resistance Resistors are used to control the amount of current in a conductor. Human Body: Salt water and perspiration lower the body's resistance. Rdry ~ 500 000 W; R salt H2O ~ 100 W 0.01 A current is imperceptible to causing a slight tingle. 0.15 A current can be fatal. GVR (galvanic skin response) sensors are used in lie detectors. Potentiometers have variable resistance.

Objectives Section 4 Electric Power Chapter 17 Objectives Section 4 Electric Power Differentiate between direct current and alternating current. Relate electric power to the rate at which electrical energy is converted to other forms of energy. Calculate electric power and the cost of running electrical appliances.

Sources and Types of Current Section 4 Electric Power Chapter 17 Sources and Types of Current Batteries and generators supply energy to charge carriers. Batteries – chemical Generators – mechanical (A wire loop is rotated in a magnetic field.) Wisconsin Valley Improvement Company - How Generators Work Current can be direct or alternating. In direct current, charges move in a single direction. In alternating current, the direction of charge movement continually alternates. 60 Hz supplied to homes

Chapter 17 Energy Transfer Section 4 Electric Power Batteries D Batteries A to B no change in potential energy B to C the potential energy drops to zero across as it is converted to light and heat C to D no change in potential energy D to A the battery does work and increases the PEelectrical by QDV where DV is the potential difference across the two terminals. B C A D B C Energy

Electric power = current  potential difference Section 4 Electric Power Chapter 17 Energy Transfer Electric power: rate of conversion of electrical energy (J/s) to other forms of energy rate at which charge carriers do work rate at which PEelectric is converted to other forms of energy Electric power P = I∆V (C/s)(J/C) Electric power = current  potential difference 1W = 1J/s (One joule of electrical energy is converted to another form of energy in one second.) P = W/t and W=DPE DV = DPE/q so DPE = qDV P = qDV/t and q/t = I P = IDV

P = IDV Chapter 17 Energy Transfer Section 4 Electric Power P = W/t and W=DPE DV = DPE/q so DPE = qDV P = qDV/t and q/t = I P = IDV P = IDV

Section 4 Electric Power Chapter 17 Energy Transfer

Relating Kilowatt-Hours to Joules Section 4 Electric Power Chapter 17 Relating Kilowatt-Hours to Joules

Chapter 17 Energy Transfer Section 4 Electric Power Power dissipated by a resistor (V= IR) I2R is called joule heating or I2R loss. Eloss= Pt = I2Rt (Loss due to resistance in the long wires.) Electrical energy is transferred at high potential differences to minimize energy loss. P = IDV so for same power use high DV and small I. Transformers: 765 000 V stepped down to 4000 V stepped down to 120 V

Chapter 17 Energy Consumption Section 4 Electric Power Electric companies measure energy consumption in kilowatt-hours. 1kW-h = 1000W-h = 1000J/s x h = 1000J/s x h x 3600s/h = 3 600 000 J How much does it cost to watch an entire World Series (21 h) on a 90.0 W black and white television set? My electric bill charged me $215.50 for 2201 kW-h last month. (Use that information.)

Chapter 17 Energy Consumption Section 4 Electric Power How much does it cost to watch an entire World Series (21 h) on a 90.0 W black and white television set? My electric bill charged me $215.15 for 2201 kW-h last month. (Use that information.) $215.5/2201 kW–h = $0.10/kW-h 21 h x 3600 s/h x 90 J/s = 6 804 000 J 1 kW-h/3 600 000 J x 6 804 000 J = 1.89 kW-h 1.89 kW-h x 0.10/ kW–h = $ 0.19

Energy Transfer, continued Section 4 Electric Power Chapter 17 Energy Transfer, continued Electric companies measure energy consumed in kilowatt-hours. 1kW-h = 3 600 000 J

Chapter 17 Multiple Choice Standardized Test Prep 1. What changes would take place if the electron moved from point A to point B in the uniform electric field? A. The electron’s electrical potential energy would increase; its electric potential would increase. B. The electron’s electrical potential energy would increase; its electric potential would decrease. C. The electron’s electrical potential energy would decrease; its electric potential would decrease. D. Neither the electron’s electrical potential energy nor its electric potential would change.

Section 2 Capacitance Chapter 17 Charging a Capacitor

A Capacitor With a Dielectric Section 2 Capacitance Chapter 17 A Capacitor With a Dielectric

Factors That Affect Resistance Section 2 Capacitance Chapter 17 Factors That Affect Resistance