Electricity

1. Electricity Ch 20-23

2. Static Electricity • Electrostatics= the study of electric charges that can be collected and held in one place • The buildup of electrons by an object • Examples? • Lightening • Getting shocked when you touch metal

3. Charged objects • All objects are made up of charged particles (atoms---remember those things called protons and electron?) • Like charges repel each other and opposite charges attract • Acquiring charge is a process of transferring electrons • Charge is conserved (it is not created or destroyed)

4. Moving charge • Insulator=material through which a charge will not move EASILY • Conductor=material that allows charges to move easily • Must have charges that are free to move • Air is typically an insulator….but a charge can move through it in certain circumstances • Think about lightening!

5. Forces on Charged bodies • Charges exert forces on other charges over a distance • The closer together two charges are the stronger the force is • The force can cause the objects to be repelled (like charges) or attracted (opposite charges)

6. Charging an object • Charging by Conduction: • The process of charging a neutral body by touching it with a charged body • Charging by Induction: • The process of charging an object without touching it, which can be accomplished by bringing a charged object close to a neutral object, causing a separation of charges, then separating the object to be charged, trapping opposite but equal charges

7. Charging an object • Grounding= connecting a body to Earth to eliminate excess charge, can be used as a source of electrons • Connections to ground limit the build-up of static electricity when handling flammable products or when repairing electronic devices. • Lightning protection systems are special grounding systems designed to safely conduct the extremely high voltage currents associated with lightning strikes.

8. Coulomb’s law • The force between two charges varies directly with the product of their charge and inversely with the square of the distance between them • F= K (qAqB) r2 K= Coulomb’s constant=9.0 x 109 Nm2/C2 q=charge (measured in Coulombs)

9. Problem • Sphere A (charge of 6.0 μC) is located near sphere B (charge of -3.0 μC). What is the force of sphere B on sphere A if B is 4.0 cm to the right of A? • ANS: 101N to the right (unlike charges)

10. Forces!!!! A B • The force of charge A on B is equal to the force of B on A, just opposite in direction • Same idea as when we had forces acting on each other last semester

11. Why should you care? • Uses of electrostatic forces in real life • Collect soot in smokestacks to decrease pollution • Paint cars • Photocopy machines

12. Electric Fields – Ch 21 • An electric field is the field that exists around any charged object • Produces forces that can do work, transferring energy from the field to another charged object • The interaction is between an object and the field at its location

13. Strength of Electric fields • E=(F on q’)/(q’) • Electric field=the force on a positive test charge divided by the strength of the test charge • Measured in N/C • The direction of an electric field is the direction of the force on a positive test charge

14. Electric field strength • An electric field is measured using a positive test charge of 3.0 x10-6 C. This test charge experiences a force of 0.12 N at an angle of 15 degrees north of east. What are the magnitude and direction of the electric field strength at the location of the test charge? • ANS: 4.0x104 N/C at 15 degrees north of east

15. Picturing the Electric field

16. Picturing the electric field • Lines of force are drawn perpendicularly away from a positively charged object • The strength of the electric field is indicated by the spacing between the lines • The field is strong where lines are close together and weak where they are far apart

17. Electric potential • The difference in electrical potential is the ratio of the work needed to move a charge to the strength of that charge • ∆V=W on q’ q’ • Electrical potential is measured in joules per Coulomb • One Joule/Coulomb=1 Volt

18. Electrical potential • Whenever the electric potential difference between two or more positions is zero, those positions are said to be at equipotential • Only differences of potential energy can be measured…the same is true of electrical potential • The electrical potential difference from point A to point B is defined as ∆V= VB-VA • Electrical potential difference is measured with a voltmeter

19. Electric potential • Electric Potential Difference in a Uniform Field • ∆V=Ed • E=product of electric field intensity • D=distance moved by a charge • A uniform electric force and field can be made by placing two large, flat conducting plates parallel to each other • The electric potential increases in the direction opposite the electric field direction

20. Millikan's oil drop experiment • Used to measure the charge of an electron • Oil drops were charged by friction with an atomizer and sprayed into the air • A few of the drops fell into the hole of the apparatus and then an electrical potential difference is placed across the two plates • The EPD between the plates was adjusted to suspend the drop…now the force of gravity and the upward force of the electric field are equal in magnitude…which is determined by the EPD of the plates

21. Millikan's oil drop experiment • Another method was used to find the weight of the drop using the relationship mg, which is too tiny to measure with conventional methods • Using the measured terminal velocity to calculate mg and knowing E, the charge, q, can be calculated!!!!

22. Sharing of Charge • Read pgs 575 -577 • Bring Questions Tomorrow!

23. Storing charges • The device used for storing a charge is called a capacitor • Capacitance is the ratio of charge on one plate to potential difference • C=q/∆V • Capacitance is measured in farads, F • Capacitors are designed to have specific capacitances • All capacitors are made up to two conductors that are separated by an insulator • The conductors are equal and opposite in charge

24. Current and Circuits Ch 22-23

25. Current And Electricity- Ch 22 • Remember- when two conducting spheres touch charges flow from the sphere at a higher potential to the one at a lower potential • A flow of charged particles is an electric current • Conventional Current= a flow of positive charges that move from high potential to lower potential • The flow stops when the potential difference is zero

26. Current • Once the potential difference between two points is zero the flow can be maintained by pumping charged particles back to the original point • This requires an external energy source • Batteries are composed of multiple galvanic cells • Galvanic Cells (a type of dry cell) converts chemical energy to electrical energy

27. Electric circuits • Any closed loop or conducting path allowing electric charges to flow is called an electric circuit • Circuits include a charge pump, which increases the potential energy of the charges flowing from point 1 to point 2 , and a device that reduces the potential energy of the charges flowing from point 2 to point 1 • The potential energy lost by the charges moving through the device is usually converted into some other form of energy

28. Circuits • A charge pump creates the flow of charged particles that make up a current • Charges cannot be created or destroyed but they CAN be separated • The total amount of charge in the circuit does not change

29. Rates of Charge flow • Remember- Power measure the rate at which energy is transferred (measured in watts) • The rate of flow of electric charge, called electric current, is measured in coulombs per second • Electric current is represented by I, so I=q/t • 1Coloumb/sec=1 ampere • Power delivered to an electric device • P=IV • I=current • V=voltage

30. Practice problem • Pg 594 • A 6.0V battery delivers a 0.50 A current to an electric motor connected across its terminals. • Draw the circuit. • What power is delivered to the motor? • If the motor runs for 5.0 min, how much electric energy is delivered?

31. Resistance • Ohm’s Law= current is directly proportional to the potential difference • Resistance=the property determining how much current will flow • R=V/I • R=resistance • V=electric potential difference • I=current

32. Resistance • How to change resistance • Length • Cross-sectional area • Temperature • Material

33. Diagramming Circuits • Chart on pg 597

34. Electric Energy • P=I2R • P=V2/R • Thermal Energy • E=Pt • E=I2Rt • E=(V2/R)t

35. Superconductor • A material with zero resistance • There is no restriction of current in superconductors, so there is no potential difference, V, across them • Practical uses are MRI magnets

36. Chapter 23-Circuits • A simple circuit contains the minimum things needed to have a functioning electric circuit. A simple circuit requires three (3) things: • A source of electrical potential difference or voltage. • A conductive path • An electrical resistance (resistor) which is loosely defined as any object that uses electricity to do work. (a light bulb, electric motor, heating element, speaker, etc.)

37. Series circuit • A circuit in which all current travels through each device • Current is the same throughout the circuit

38. Series circuit • The increase in voltage provided by the energy source is equal to the sum of voltage drops (potential difference) across the various resistors • Vsource = VA + VB • Equivalent resistance= the sum of all the individual resistances in a series circuit • R= RA + RB + …

39. Series Circuits • Remember ----- I=V/R • Current in a series circuit can be found using the voltage of the source and the equivalent resistance • I= Vsource/R

40. Practice Problem • Pg 621 • Two resistors, 47.0Ω and 82.0Ω, are connected in a series circuit across a 45.0 V battery. • What is the current in the circuit? • What is the voltage drop across each resistor? • If the 47.0 Ω resistor is replaced by a 39.0 Ω resistor, will the current increase, decrease, or stay the same? • What is the new voltage drop across the 82.0 Ω resistor?

41. Parallel circuits • A circuit in which there are several current paths • The resistors are connected in parallel, both ends of paths are connected

42. Parallel circuit • Equivalent resistance in a parallel circuit • 1 = 1 + 1 + 1 + … R RA RB RC • Placing two or more resistors in parallel always decreases the equivalent resistance of a circuit because the new resistor provides an additional path for current • This increases the total current while the potential difference remains the same

43. Practice problem • Pg 625 • There resistors, 60Ω, 30Ω, and 20Ω, are combined in parallel across a 90 V battery • Find the current through each branch of the circuit • Find the equivalent resistance of the circuit • Find the current through the battery

44. Application of circuits • Combined series-parallel circuits

45. Practice problem • Pg 630