Electricity

1. Electricity

2. INTRO • Intro battery/wire/light bulb activity • Use multi meter to look at battery voltage.

3. 5.1.1 Define electric potential difference • 5.1.2 Determine the change in potential energy when a charge moves between two points at different potentials.

4. 5.1.1 Define electric potential difference • When two charges interact there is an electric force between them. • Potential energy from this is: Electrical Potential Energy • Similar to gravitational force • Type of mechanical energy • Metotal = KE + PEg + PEspring + PEelectric

5. Where does the “potential” come from • With potential gravity, an object can “potentially” fall from a height. • With electrical potential, a charge can “potentially” fall down an electric field. • A positive charge is placed in a uniform electric field and is displaced. • The new position will have a ∆PE

6. Where does the “potential” come from • With potential gravity, an object can “potentially” fall from a height. • With electrical potential, a charge can “potentially” fall down an electric field. • A positive charge is placed in a uniform electric field and is displaced. • The new position will have a ∆PE

7. Electric Potential Energy Difference between two points in an electric field is equal to the work done in moving a charge from one position to a position of higher electrical potential energy in an electric field. • See Video • Make Drawing

8. Equation • ∆PEelectric = -q Ed • q = charge • E = electric field strength • d = displacement from reference point • (-) = inidcates that the electrical potential energy will INCREASE if the charge is NEGATIVE • Unit is a Joule(J)

9. Potential Difference • A measure of the difference in the electrical potential energy between two positions in space. • AKA “voltage” • Describes a change in energy per unit charge • Unit is volt (V) • Most batteries are 1.5V • Similar idea to a water slide ***Draw diagram***

10. Equation • ∆V = ∆PEelectric / q • ∆V = voltage • ∆PEelectric= electric potential energy • q = charge

11. Equation • Combine the PEelectric equations with the ∆V and you get…. • ∆V = -Ed ***show substitution*** New Equation: ∆V = -Ed

12. Last one • ∆V = kc q/r • kc = Coulomb constant (8.99 x 109Nm2/C2) • q = charge • r = distance separating the two charges

13. 5.1.4 Solve problems involving electric potential difference. • Pg 599, Practice A 1) 6.4 x 10-19 2) -750V 3) (charge of electron = 1.6x10-19) 2.3 x 10-16J • Section Review pg 601

14. CAPACITANCE!!! • 5.1.12 - Relate capacitance to the storage of electrical potential energy in the form of separated charges. • 5.1.13 - Calculate the capacitance of various devices. • 5.1.14 - Calculate the energy stored in a capacitor.

15. Capacitor • Device that stores electrical potential energy. • Uses • tuning the frequency of radios • Eliminating sparking in automobile ignition systems • Storing energy in electronic flash units

16. Capacitor • If, energized or charged has energy • Energized by connecting the plates to the terminals of a battery. • Charge transfer between the plates stops when the potential difference between the plates is equal to the pd of the battery.

17. Capacitance • Is the ability of a conductor to store energy in the form of electrically separated charges. • Is the ratio of charge to potential difference • So… • Capacitance = magnitude of charge/potential difference • C = Q/∆V • SI unit is farad (F)

18. Capacitance • Depends on the size and shape of the capacitor. • Many capacitors have parallel plates with no material between them • The equation shows that as the area of increases the capacitance increases • C = εA/d • ε = permittivity of a vacuum (8.85 x10-12C2/Nm2) • A = area of one of the plates • d = distance between the plates

19. Dielectric • The space between isn’t always empty • Dielectric – insulating material that fills the space between two capacitor’s plates • Ex – air, rubber, glass, waxed paper • Capacitance increases with use of dielectric. • Molecules in a dielectric can align with the field of the pates • Reduces the charge on the capacitor plates. • Q= C∆V

20. The space between isn’t always empty • Dielectric – insulating material that fills the space between two capacitor’s plates • Ex – air, rubber, glass, waxed paper • Capacitance increases with use of dielectric. • Molecules in a dielectric can align with the field of the pates • Reduces the charge on the capacitor plates.

21. Discharge • Is the opposite of charging • After the two plates are charged they will stay charged until… • Connected with a conductive material. • Charges move back from one plate to the other until both are balanced and uncharged. • This is because this is the lowest potential energy.

22. Discharge • Examples: • Flash in camera • Keyboards • Size of capacitors video

23. Stored Energy • A charged capacitor stores electrical potential energy. • If each plate is neutral, then it takes almost no work to move charges in the circuit. • Once the plates start to build up a charge, a potential difference develops

24. Stored Energy • This requires more and more work to be done to move the charge through the pd. • This builds the electric potential energy. • The work done on these charges is the transfer of energy.

25. Stored Energy • This energy can be calculated by: • Electric potential energy = ½ (charge on plate)(final potential difference) • PEelectric = ½ Q ∆V • By substitution this can become • PEelectric= Q2/2C • C= capacitance

26. Example • A capacitor, connected to a 12V 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? • Identify your variables. Identify your equations. Watch your units. • Answer: C = 3.0 x 10-6μF, PE = 2.2 x 10-4 J

27. Practice • Pg 607 Practice B #1-4 1) a) 4.8 x 10-5 C, b) 4.5 x 10-6 J 2) a) 4.8 x 10-6 F, b) 5.4 x 10-6 J 3) a) 9.00V b) 5.0 x 10-12 C 4) 1.13 x 108 m2, ???

28. 5.1.5 - Define electric current.

29. CURRENT • So far static electricity (charges in electric fields) • Static electricity isn’t very useful • Moving electricity IS!!

30. CURRENT • The movement of electric charge • Could be positive or negative charges • Used everywhere • Lights, radios, TV, air conditioners, Cars, computers • Bodies, discovered in mid 1700s. • Luigi Galvani experimented next to a dissected frog • Current transmit messages between muscles and brain

31. CURRENT • More specifically…. • Imagine a cross section of wire • Electrons are now moving through that wire • Current is the rate these charges move

32. CURRENT • Current is the ratio of the amount of charge to the time interval • ∆q is amount of charge • ∆t is the time it takes these charges to pass a point

33. CURRENT • Current is the ratio of the amount of charge to the time interval • ∆q is amount of charge • ∆t is the time it takes these charges to pass a point • I = ∆q/∆t • I = current • Unit is Ampere, A

34. CURRENT ***Special note*** • Current is the OPPOSITE direction of the movement of the negative charges.

35. Example • The current in a light bulb is 0.835A. How long does it take for a total charge of 1.67C to pass through the filament of the bulb? • Identify what you know. Identify what you want to know. Identify your correct equation. • Answer: 2.00s

36. Practice • Pg 609 Practice C #2-5 • Answers: 1) 400s 2) 1.00s 3) 600s 4) 20C 5) a) 2.6 x 10-3A, b) 1.6 x 1017 electrons c) 5.1 x 10-3A

37. Drift Velocity • When you flip the switch how fast do the lights come on? Instantaneously? • Does this mean the electrons are moving at the speed of light? • Not quite.

38. Drift Velocity • When you turn on the light the electron motion near the switch changes the electric field there. • That change in the electric field then propagates throughout the wire at nearly the speed of light. • The charges move much more slowly.

39. Drift Velocity • When these charges do move do they move in a straight line? • Not quite. Sorry.  • Remember that electrons in a metallic bond jump from nucleus to nucleus. • These jumps are random and sporadic.

40. Drift Velocity • What causes these crazy movements? • As they move the individual atoms are vibrating. (see Brownian Movement) • In these collisions energy is transferred, which causes more vibration and an increase in temperature. (think friction)

41. Drift Velocity • Electrons want to accelerate in this electrical field but can’t because of these collisions. • Eventually things will balance out. • Drift velocity is the net velocity of these individual movements.

42. Drift Velocity • So how fast does an electron really move??? • In a wire with current 10A, the drift speed would be 2.46 x 10-4m/s. • Meaning it would take about 68 minutes for it to travel 1 meter.

43. Resistance 5.1.6 – Define resistance 5.1.7 – Apply the equation for resistance 5.1.8 – State and apply Ohm’s Law

44. Resistance • What is a conductor? • What is an insulator? • What is a resistor? • Is there a difference between an insulator and a resistor? • Is there a difference between a conductor and a resistor?

45. Resistance • A resistor is a conductor. • It’s just not a good conductor. • Opposition to the motion of charge through a conductor is RESISTANCE. • For many materials… resistance is constant over a wide rang of applied potential differences.

47. Resistance • Earlier we learned that electrons do not move in straight-lines through conductors. • This is because of collisions with the metal atoms. • Anything that affect the number of collisions will also affect the materials resistance. • Primarily 4 factors. • Length • Cross-sectional Area • Material • Temperature

48. Resistance • Two are purely geometrical factors. • Length – the longer the wire the more collisions which means more resistance. • Cross-Sectional Area – a thinner wire physically restricts the path an electron can take, increasing the resistance.

49. Resistance • Material – different material have different structures of atoms. Some are more conductive than others. • Examples…. • Temperature – Temperature is a measure of how much an object’s atoms are vibrating. More vibrations means it’s harder for atoms to travel, which means more resistance.

50. Resistance • The relationship between resistance, voltage and current can be expressed by saying resistance is the ratio of potential difference to current. • OR R = ∆V / I • SI unit for resistance is ohm, Ω