S.MORRIS 2006

1. ELECTRICAL CIRCUITS S.MORRIS 2006 More free powerpoints at www.worldofteaching.com

2. The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.

3. What is an electric current? An electric current is a flow of microscopic particles called electrons flowing through wires and components. - + In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell.

4. simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. wires cell lamp switch To make the circuit, these components are connected together with metal connecting wires.

5. simple circuits When the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around. If there were any breaks in the circuit, the current could not flow.

6. circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires

7. Circuit Symbols: Battery Resistor Light-bulb Switch Wire

8. Three general types of circuits: Closed Circuit - There is a complete loop with wires going from one side of the battery through a resistor(s) to the other side of the battery. Open Circuit - There is not a complete loop. Short Circuit - There is a complete loop, but it does not contain any resistors. Only Working Circuit

9. Chapter 22: Current and Resistance • Most modern day appliances require electricity to operate. • The electricity generates a current which runs through the appliance. • The current is made out of charged particles • Current is defined as the rate at which charge flows through a surface area perpendicular to the direction of flow. • For example: if the charges are flowing through a wire, the area is the cross-sectional area of the wire

10. CURRENT Where: I = current (amps, A) Q = change in charge or how much charge flows (Coulombs, C) t = time (seconds)

11. Example: The amount of charge that passes through the filament of a certain lightbulb in 2.00 sec is 1.67 C. Find (a) the current in the bulb and (b) the number of electrons that pass through the filament in 5.00 sec. (b) First find the total charge in 5.00 sec. Then find how many electrons that is (remember, 1 electron = 1.6x10-19C)

12. Electric Current Flow In real circuits it is electrons (-charges) that are flowing. We call this kind of current “electron current”. However, for simplicity we assume that all charge carriers are positive. We draw current arrows in the direction that such charges would move….that is, positive to negative.

13. I was positive I was right about this. Historically this is because Ben Franklin guessed wrong about the charge carriers in a metal being positive.

14. For an electric current to flow between two points, two conditions must be met. 1. There must be a conducting path between the points along which the charges can move. WHY? The conducting material allows the charges to move. 2. There must be a difference of electric potential (volts) between the two points. WHY? The potential difference makes the charges want to move.

15. What does potential have to do with it? Water doesn’t flow in the pipe when both ends are at the same height. That is to say, water will not flow when both ends of the pipe are at the same potential (height).

16. But tip the pipe, increase the potential of one end so there is a difference in potential across the ends of the pipe, and water will flow. And so it is with electric current flow.

17. + - voltage You get current to flow through a conductor by applying a voltage across it. This voltage creates an electric field within the conductor. This electric field in turn creates a force on all the charges within the conductor. If charges are free to move, as in a conductor, they do. In solids these charges are always electrons. Current flow is the measure of electrons moving within a conductor. Amperes, or amps, measures the amount of current that flows. Amps is named after a 19th-century French physicist, Andre-Marie Ampere, and abbreviated A.

18. An ampere is a fairly large amount of current: 0.1 A flowing between your hands across your heart will kill you. (Fortunately, your body has fairly high resistance so it takes a substantial voltage to drive that much current.)

19. Voltage Voltage, or electric potential, is measured in volts, named after a physicist named Volta, and abbreviated V. Most small batteries (size AAA, AA, C, D) are 1.5 V; there is the familiar box-like 'transistor' 9 V battery, and car batteries are 12 V. In contrast, high-voltage lines have many thousands of volts between them.

20. A common misconception about electricity is the source of electrons. Myth: Electrons come from the power utility company. Some people think that the electrical outlets in the walls of their homes are a source of electrons. They think that electrons flow from the power utility through the power lines and into the wall outlets of their homes.

21. So when you are jolted by an electric shock, the electrons making up the current in your body originate in your body. Electrons do not come out of the wire and through your body and into the ground; energy does. The energy simply causes free electrons in your body to vibrate in unison. Small vibrations tingle; large vibrations can be fatal.

22. A resistor is anything that electricity cannot travel through easily. When electricity is forced through a resistor, often the energy is changed into another form of energy, such as light or heat. The reason a light bulb glows is that the electricity is forced through tungsten, which is a resistor. The energy is released as light and heat.

23. Electrical Resistance Resistance is sort of like the friction associated with moving charge or electrical current. • Good conductors, like most metals, allow current to flow without much loss. • Poor conductors, like most non-metals, impede the flow of current to a great extent. • Superconductors like very cold niobium-tin, are special substances that allow current to flow with essentially zero loss; • Semiconductors, like silicon, are either good or poor conductors depending on certain conditions.

24. Resistance The resistance of a conductor that obeys Ohm’s law depends upon four factors: • The material of which it is composed; the ability to carry an electric current varies more than almost any other physical property of matter. 2. Its length (L): The longer the conductor, the greater its resistance. 3. Its temperature . Resistance generally increases as temperature increases. 4. Its cross-sectional area (A). The thicker the conductor the less its resistance.

25. It’s much easier to travel on an open highway than a rocky trail.

26. Also, the longer & hotter the trip, the harder it is. It’s much easier to travel on an open highway than a rocky trail.

27. The filament in a 60 W light bulb has a larger resistance than the filament in a 100 W light bulb. If both filaments are made out of tungsten, how do their length’s & cross-sectional areas compare? 60 W (large resistance) = long & thin filament 100 W (small resistance) = short & thick filament

28. You are wiring up a circuit and want the lowest resistance possible. What type of wire should you choose? Rubber or Copper Thick or Thin Lots of extra wire or just enough to get the job done. Run the circuit at a high temperature or a low temperature

29. Ohm’s Law Resistance quantifies how much current you get across something per volt applied. Resistance has units of Volts/Amp, which get another name, ohms, represented by the Greek letter omega Ω   . R = Resistance () I = Current (A) V = Voltage (V) or

30. How much voltage difference does it take for .25amps to flow through a resistance of 8.0 Ω ? 1) 1.0 V 2) 2.0 V 3) 4.0 V 4) 8.0 V 5) 32 V

31. POWER P = IV But V = IR so…. P = I2R

32. What is the current flow to a 1.0x104W hairdryer plugged into a 120V socket? 1) 83A 2) 10.A 3) 12. A 4) 16A 5) 20.A

33. Ohm’s Law Equations: V = IR P = IV P = (V/R)V = V2/R P = V2/R

34. Lets say you want to protect your new speakers by putting a fuse in line with the speakers. Assume you have 100.W speakers and that they are 8.0 ohms. What size (amps) fuse should you use so that it just blows at 100. Watts? P = I2R 100W = I2 8 I2 = 100/8 = 12.5 I = 3.5 amp fuse

35. Two light bulbs operate from 120V, but one has a power rating of 25W, and the other has a power rating of 100W. Which bulb has the higher resistance? 1) 25W bulb 2) 100W bulb 3) Both the same 4) Depends on the bulb

36. Thermal Energy The sole purpose of some appliances is to produce heat (electric stove, toaster, hair dryer). These appliances have a resistor in them which turns electrical energy into thermal energy (or heat) E = P*t = I2R*t = (V2/R)*t E = Thermal Energy (J) P = Power (W) t = time (sec) I = Current (A) R = Resistance (W) V = Voltage (V) P = I2R P = V2/R

37. There are two ways to put resistors into a circuit. 1. Resistors can be in series OR 2. Resistors can be in parallel

38. Resistors in Series • Resistors are considered to be in series if the current must go through all of the resistors in order. • The current (amps) through all resistors in series is the same. • The voltage across resistors in series may be different • The rate of electron flow (or current) is determined by which resistor? The resistor with the largest amount of ohms.

39. Pop Quiz • Draw a circuit with a 4.0, 6.0 and 10.0 ohm resistor in series connected to a 24 V battery. Calculate the voltage across and current through each. • Calculate the energy used and current drawn by a 200-W window fan when plugged into a 110-V outlet and left on for an 8-hr time period. • Calculate the energy used and the current drawn by a 3400-W air conditioner that is wired to a 220-V circuit and left on for an 8-hr time period.

40. R1 R2 R3 Combining (adding) Resistors Series Resistors Itotal = I1 = I2 = I3 Req = Rtotal = R1 + R2 + R3 Voltage is calculated with Ohm’s Law Q Amps

41. Example 1: A circuit has three 8.0 W, 5.0 W and a 12 W resistors in series along with a 24 V battery. • Draw the circuit. • Calculate the total resistance of the circuit. • Calculate the total current through the circuit. • What is the current through each resistor? • Calculate the voltage across each resistor.

42. Resistors in Parallel • Resistors are considered to be in parallel if the current is shared between multiple resistors. • The current (amps) through all resistors in parallel may be different. • The voltage across all parallel resistors is the same. • Will a resistor with a large resistance have more or less current through it then a resistor with a small resistance? The resistor with a large resistance will have a smaller current than the resistor with the smaller resistance.

43. Combining (adding) Resistors Parallel Resistors Current is calculated with Ohm’s Law Vtotal = V1 = V2 = V3

44. Example 2: A circuit has three resistors: 8.0 W, 5.0 W and a 12 W resistors in parallel along with a 24 V battery. • Draw the circuit. • Calculate the total resistance of the circuit. • Calculate the total current through the circuit. • What is the voltage across each resistor? • Calculate the current across each resistor.

45. Electrical Outlets • Electrical outlets provide electric potential (or the voltage) for any appliance plugged in to it. • In the United States ALL outlets provide 120 V (in Europe it is 240 V)

46. Light bulbs are made to be the only appliance plugged into a socket. • The power rating of a light bulb (25 W or 100 W…) is as if that bulb was the only bulb plugged in to a 120 V power source. • The resistance of a light bulb is calculated by knowing the power rating and the voltage (120 V) • Current and actual voltage used by a light bulb depends on the circuit.

47. Example 3: What will the power output be if an American-made 45 W light bulb is plugged in to a 310 V power source? • Using 120 V, calculate the resistance of the light bulb. • Using the resistance and the voltage of the new source, calculate the new power

48. As more identical resistors R are added to the parallel circuit shown, the total resistance between points P and Q … • Increases • Remains the same • Decreases R P Q …

49. As more identical resistors R are added to the parallel circuit shown, the total resistance between points P and Q … 1. Increases 2. Remains the same 3. decreases R P Q … Q

50. When one bulb is unscrewed, the other bulb will remain lit in which circuit… • I • II • Both • Neither Circuit II Circuit I