What is electricity? Charges that could be either positive or negative and that they could be transferred from one object to another.

Transcription

1 Electricity

2 What is electricity? Charges that could be either positive or negative and that they could be transferred from one object to another.

3 What is electrical charge Protons carry positive charges Electrons carry negative charges Electrical charge is a property of protons and electrons. A negatively charged body contains fewer protons than electrons. A positively charged body contains fewer electrons than protons.

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5 How to measure charge The unit: the coulomb (C) 1 C = 6.25 X1018 electrons or protons 1 proton or 1 electron contains X C of charge This is called the elementary charge

6 Attraction and Repulsion Opposites attract and likes repel! Attraction and repulsion between charges is a force

7 The Law of Conservation of Charge Charges are never created or destroyed, just transferred from one place to another.

8 Objects that are electrically neutral These are objects that contain the same amount of positive and negative charges Protons = Electrons

9 Charging an object The transfer of electrons from one object to another requires energy. Transferring electrons from one atom to another causes an imbalance of the positive and negative charges in an object. As a result, the object can become electrically charged.

10 Static electricity Electrical charges at rest. Usually with insulated charges.

11 Conductors A substance that permits the free flow of electrical charges. Usually metals and electrolytic solutions Electrolytic solutions are solutions in which ions are dissolved. Ex: salt dissolved in water

12 Insulators Substances that block the free flow of charges When an insulator is charged, the charges in the object do not move. Mostly nonmetals Other substances include: wood, plastic, glass, paper, ceramics, rubber, silk and air.

13 Semiconductors Are only conductors in certain conditions. Metalloids and Carbon are examples of semiconductors.

14 Charging an Object Friction: 2 neutral bodies are rubbed together and electrons are transferred from the atoms of one body to the atoms of the other body. The objects become oppositely charged.

15 Friction Triboelectric series: list of substances that can take on a negative or positive charge through friction. More likely to capture electrons (become negatively charged) More likely to give up electrons (become positively charged) Plastic Sulphur Gold Nickel, copper Hard rubber Wood, yellow amber, resin Cotton Paper Silk Lead Wool/ Fur Glass

16 Friction

17 Charging by Conduction Putting an uncharged object in contact with another object that is already charged. The result is two objects of the same charge The charge will flow through the uncharged object (conductor) Both objects will now have a weaker charge than the original single charged object.

18 Conduction

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20 Charging by Induction There is no direct contact between the objects. When the objects are brought close to one another the neutral object becomes partially charged. Electrons within the object move so that opposite charges are brought close together Like charges move as far from each other as possible.

21 Induction- with an insulator

22 Induction with a conductor

23 EST-SE Electrical Fields

24 When two charged objects come together Any electrically charged body placed near another charged body is subjected to an electrical force. This is the force of attraction or repulsion between objects.

25 EST Coulomb s Law The magnitude of the force of one particle over the other depends on their charges and the distance separating them. The greater the charge, the greater the electric force The greater the distance, the weaker the electric force

26 What is an electrical field? The area of space in which the electrical force of a charged body can act on another charged body. It is actually invisible A field explains how a force can act over a certain distance.

27 How to draw field lines around a single charge Field lines point towards a negative charge Field lines point away from a positive charge.

28 What would the field lines look like between these two metal plates?

29 EST Coulomb s Law Fe = kq1q2 r2 Fe = electric force Q1 = charge of the first particle Q2 = charge of the second particle R = the distance between the particles Let s work through the example on page 149!

30 p. 172 # 9 Take a few minutes to complete the problem.

31 Dynamic Electricity STATIC ELECTRICITY DYNAMIC ELECTRICITY Describes all phenomena related to electrical charges in motion.

32 Current The number of charges that flow past a given point in an electrical circuit every second. The symbol for current is I. The unit for current is the ampere or amp (A). 1A = 1C/s

33 Calculating current I = q/t Example: The data sheet for a car headlight indicates that the light requires a current of 15A. What is the charge needed for one minute?

34 Calculating current I = 15A Q =? T = 1 min = 60 sec I = Q/T

35 An ammeter A device for measuring the current intensity. It acts as a checkpoint that counts the number of charges that flow past a given point in a circuit in one second. Ammeters must be placed in the path that the charge will take.

36 Understanding your ammeter readings 1000 ma = 1 A 50mA = 500mA =

37 A Voltmeter An instrument to measure potential difference. A checkpoint for measuring the energy each of the charges transfers to a circuit element. Voltmeters must be placed at the points where the circuit enters or exits an element (light bulb, resistor, etc.)

38 Placing an Ammeter and Voltmeter

39 Drawing Circuit Diagrams Dynamic electricity is the phenomenon of electrical charges in motion. Circuits: closed loops that allow the free flow of charges. There cannot be a break in the circuit for electrons to flow freely.

40 FLOWING CHARGES When a battery is attached to a circuit, the freely moving electrons can flow. Electrons flow freely in and out of the battery. Since electrons are negative, they flow out from the negative terminal of the battery and towards the positive terminal of the battery.

41 Conventional Current Because scientists did not know this when they discovered electric current, by convention we say that current flows from the positive to the negative terminal. (Conventional current direction) This type of flowing current is called direct current. Batteries provide the energy for electrons to flow.

42 Plugging into a socket When a socket is attached to a circuit, the electrons move back and forth. This type of current is called alternating current. The socket = the source of alternating current. Symbol:

43 Drawing more symbols: Circuit wires are made of conductors such as copper (a metal). More symbols for drawing circuits: Wire Battery Switch Light bulb

44 Resistors Resistors: transform electrical energy into heat. Resistors reduce/hinder the flow of current Symbol: In comparison, light bulbs transform electrical energy into light and heat.

45 Build one with a resistor and one without

46 Protection: Fuses and Breakers Protection fuses/breakers: If electrons flow too quickly, the fuse or breaker actually breaks the circuit. The filament inside melts or the switch needs to be closed again. Symbol:

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48 Potential Difference The amount of energy transferred between two points in an electrical circuit. The greater the potential difference, the more energy transferred between two points. The unit of measurement is the volt (V) 1V = 1J/C *The amount of energy transferred per coulomb of charge.

49 Calculating Potential Difference V = E/Q (energy/coulomb) Example: The electrical circuits in our homes usually supply a potential difference of 120V. What amount of energy is provided by a charge of 200 C?

50 Calculating Potential Difference V = 120V E =? Q = 200C V= E/Q

51 A Voltmeter An instrument to measure potential difference. A checkpoint for measuring the energy each of the charges transfers to a circuit element. Voltmeters must be placed at the points where the circuit enters or exits an element (light bulb, resistor, etc.)

52 Placing an Ammeter and Voltmeter

53 Resistance Resistance is a force that hinders the flow of electric current. Resistors transform electrical energy into another form of energy (thermal, mechanical, etc.)

54 Factors Increasing the Resistance Nature of the material in the circuit: A poor conductor will increase resistance to flow. Length: The longer the element or wire, the greater the resistance to flow. Diameter: a smaller diameter resists current more than a larger one. Temperature: The hotter the element the more the resistance.

55 What if you want a good conductor? Short, Fat, and Cold! You want a good conducting metal with these characteristics. This scenario presents the least resistance to flow.

56 Calculating Resistance The symbol for resistance is R. The unit for resistance is the ohm (Ω) 1Ω= 1V/1A To calculate resistance, you need Ohm s Law.

57 Ohm s Law V = IR The law states that for a given resistance, the potential difference in an electrical circuit is directly proportional the current intensity. Try p. 173 # 16

58 Electrical Power The amount of energy transformed in a given amount of time. The amount of work an electrical device can perform per second. The symbol for electrical power is P. The unit is the watt (W). 1W = 1J/s J= joule s = second

59 Power Rating

60 Calculating Electrical Power P = E/t P = electrical power in watts E = Energy or work (joules) t = time in seconds Example: If a light bulb consumes 2400J of energy in 60 seconds. What is the power of this light bulb?

61 Calculating Power in terms of Potential Difference P = VI P = electrical power in watts V= potential difference in volts I = current intensity in amps Example: If a light bulb has a potential difference of 10 V and a current of 4A what is the power of this light bulb?

62 Let s look at why there are 2 formulas. 1W = 1V X 1A = 1 J/s 1V = 1A =

63 Power and Energy What is the relationship between power and electrical energy? Remember... Electrical Power indicates the amount of energy a device can transform in a certain period of time. Electrical Energy is the amount of energy a device ACTUALLY uses over an ACTUAL period of time.

64 Power and Energy We can calculate Electrical Energy three different ways! Energy= P( in Watts) x t in seconds Energy= P (in kw) x t (in hours) Energy= P(in Watts) x t(in hours) We can calculate Electrical Energy three different ways! Energy= P( in Watts) x t in seconds Energy= P (in kw) x t (in hours) Energy= P(in Watts) x t(in hours)

65 Power and Energy Example A 1000 W microwave oven operates for 6 minutes. What is the amount of energy used? Let s calculate the Electrical Energy used all three ways!!!

66 The KWh When calculating the power consumed in our homes, Hydro Quebec refers to the KWh. 1 KW = 1000W 1KWh = the amount of energy consumed in 1 hour. 1000W X 3600s = J And so, 1KWh = J

67 Cost of Using Electricity We use ALWAYS kwh to calculate the cost of using something. Example: If your house used 800 KWh of electricity in one month how much would it cost if Hydro Quebec charges 0.08 $/KWh.

68 Electrical Circuits Network in which electrical charges can flow continuously in a closed loop. Necessary components: A power supply (battery) Creates the potential difference Resistors or elements (light bulbs) use up electrical energy Wires that carry the charge between the battery and the resistors and back again current intensity is measured here.

69 Series circuit Elements are connected end to end. Characteristics: If one component is defective, the entire circuit stops working The energy used by the resistors adds up with each new resistor in the circuit. Rtotal = R1 + R2 + R3

70 Parallel Circuits This type of circuit contains at least 1 branch. Characteristics: If an element of the circuit is defective, the elements in the other branches can still function. The effect of each resistor is shared among the pathways. It doesn t add up Current intensity is shared among the different resistors.