Electric charges. Basics of Electricity

Transcription

1 Electric charges Basics of Electricity

2 Electron has a negative charge Neutron has a no charge Proton has a positive charge But what is a charge?

3 Electric charge, like mass, is a fundamental property of matter. Inside atoms found in matter, attraction between positive and negative charges holds the atoms together.

4 Virtually all the matter around you has electric charge because atoms are made of electrons and protons (and neutrons). But... ordinary matter has zero net (total) charge, most matter acts as if there is no electric charge at all.

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6 Greater affinity for e - : stick to electrons more and tend to gain electrons becoming negative Less affinity for e - : don t hold electrons as tight and are more likely to lose electrons and become positive

7 Charging by Friction A wool cloth does not have much affinity for electrons. Becomes Positive PVC becomes negative

8 (Hold electrons tightly) Most likely to gain electrons and become negative (Hold electrons loosely) Most likely to lose electrons and become positive) PVC Rubber Cotton Paper Silk Fur Wool Nylon Hair Acetate Glass

9 Material through which electrons move freely Examples (gold, silver, copper, and aluminum) The general rule is that good thermo conductors are good electric conductors

10 Metals tend to share electrons in electron clouds electrons are free to move around making them better conductors.

11 Electrical Insulator Electrical Conductor

12 Material through which electrons do not freely move Examples: rubber, paper, plastic, air

13 Removing a static charge by producing a path to the ground Electrons move from a negatively charged objects to the ground until the object is neutral Electrons move from ground to neutralize positively charged objects The earth both accepts and gives electrons while remaining overall neutral Grounding wand for Van De Graaff generator

14 Section 2: Charging Objects Three Ways of putting a charge on an object 1. Friction 2. Induction 3. Conduction

15 1. Charging by Friction Charging by rubbing objects that have different affinities for electrons together

16 Induction (charging without contact) 1. Bring a charged object (rod) close to a neutral one (ball) without contact 2. The electrons in the ball will be repelled leaving a positive side The now positive sided ball with be attracted to the negative rod

17 Induction (charging without contact) Induction is only a temporary change without contact therefore electrons are not transferred The charge induced is opposite Take away the rod and a neutral charge will return

18 Conduction (charging with contact) Conduction is a more permanent change with contact; electrons are transferred and then isolated. Charge conducted is the same After conduction the ball and rod will repel each other

19 The unit of charge is the coulomb (C). The name was chosen in honor of Charles Augustin de Coulomb ( ), the French physicist who performed the first accurate measurements of the force between charges.

20 Electric forces are incredibly strong. A millimeter cube of carbon the size of a pencil point contains about 77 coulombs of positive and negative charge.

21 3 factors affecting the magnitude of the force between two charged objects: 1. Charge on the objects 2. Distance between objects 3. Material separating objects

22 When using this equation: A positive force (F) signifies repulsion Both charges (Qs) must be positive or both negative A negative force (F) signifies attraction One charge (Q 1 or Q 2 ) must be positive and the other negative

23 el Coulomb s Law is similar to Newton s Law of Gravity Similarities: They both are used to calculate a field force Both forces have an inverse square relationship to distance They are both related by a constant Differences: Force of gravity is always attractive Electrostatic force can be either attractive or repulsive Gravities constant is very small since gravity is a very weak force F g relates force created by a masses, F el relates force created by charges

24 el Both electric and gravitational forces are field forces because objects do not have to touch to be subjected to the force. Field forces are sometimes called forces at a distance. Magnetic forces work similarly.

25 Electrical Fields Electrical Field (E): an area of electrical influence around a charged object. Variable E Unit: newton per coulomb (N/C)

26 Arrows point away from the positive and toward the negative In the direction a positive charge would travel in the field Spacing of lines show field strength

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28 Two formula s: E: electrical field (N/C) F: force (N) Q: charge (C) k: constant (For air, k = 9.0 x 10 9 N m 2 /C 2 ) d: distance (m)

29 All charge lies on the surface of a conductor Electrical field inside a conductor is zero E = 0 inside conductor

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31 The metal outside the car gives the car a path to the ground

32 Here is more proof of shielding

33 Discharge occurs when the electric field around a conductor becomes so strong. The air is ionized helping the charge make a break for the ground.

34 Arc- a rapid discharge producing heat, light, and sound.

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36 Electrical forces Lightning is caused by a giant buildup of static charge. The cloud, air, and ground can act like a giant circuit. All the accumulated negative charges flow from the cloud to the ground, heating the air along the path (to as much as 20,000 C) so that it glows like a bright streak of light.

37 Charges are separated in storm clouds; + top and bottom Ground under the cloud becomes positively charged by induction V = millions of volts; causes arc discharge with tremendous energy.

38 Electric current is caused by moving electric charge. Electric current comes from the motion of electrons. current

39 A capacitor is a storage device for electric charge. Capacitors can be connected in series or parallel in circuits, just like resistors.

40 A capacitor can be charged by connecting it to a battery or any other source of current. A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

41 Continue on for intro to electricity and circuits.

42 Electric current is similar in some ways to a current of water. Like electric current, water current can carry energy and do work. A waterwheel turns when a current of water exerts a force on it.

43 An electric circuit is a complete path through which electric current travels. A good example of a circuit is the one found in an electric toaster.

44 Wires in electric circuits are similar in some ways to pipes and hoses that carry water.

45 When drawing a circuit diagram, symbols are used to represent each part of the circuit.

46 Electrical symbols are quicker and easier to draw than realistic pictures of the components.

47 A resistor is an electrical device that uses the energy carried by electric current in a specific way. Any electrical device that uses energy can be shown with a resistor symbol.

48 Inside a Light Bulb

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50 Current only flows when there is a complete and unbroken path, or a closed circuit. Flipping a switch to the off position creates an open circuit by making a break in the wire.

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53 Electric current is measured in units called amperes, or amps (A) for short. One amp is a flow of a certain quantity of electricity in one second. The amount of electric current entering a circuit always equals the amount exiting the circuit.

54 How can you tell there is water flowing in this picture?

55 You can see currents on the surface of the water showing in which direction the water is moving. The flow of electricity is called current and is measured in amperes or amps for short.

56 Electricity is the flow of electrons through a wire, like water flows in a river. Water flows in currents in a river, the flow of electricity is called current The flow of electricity is measured in amperes or amps for short

57 Voltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy. Voltage is measured in volts (V). A voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.

58 9 volt transistor battery on the tongue hurts. 1.5 volt flashlight battery 120 volt household outlet can be very painful and deadly!

59 A difference in voltage provides the energy that causes current to flow.

60 A useful meter is a multimeter, which can measure voltage or current, and sometimes resistance. To measure voltage, the meter s probes are touched to two places in a circuit or across a battery.

61 Measuring battery voltage Voltage can be considered a measure of electrical pressure Measuring tire pressure Measuring household water pressure

62 A battery uses stored chemical energy to create the voltage difference. Three 1.5-volt batteries can be stacked to make a total voltage of 4.5 volts in a flashlight.

63 A pump is like a battery because it brings water from a position of low energy to high energy.

64 Water meter measuring water flow If you want to measure current you must force the current to pass through the meter. Multimeters can measure two types of current: alternating current (AC) and direct current (DC).

65 Circuit breakers and fuses are two kinds of devices that protect circuits from too much current by making a break that stops the current.

66 Resistance is the measure of how strongly an object resists current flowing through it. The relationship between electric current and resistance can be compared with water flowing from the open end of a bottle.

67 The total amount of resistance in a circuit determines the amount of current in the circuit for a given voltage.

68 Electrical resistance is measured in units called ohms. This unit is abbreviated with the Greek letter omega (Ω).

69 The current in a circuit depends on voltage and resistance. Ohm s law relates current, voltage, and resistance with one formula.

70 If you know two of the three quantities, you can use Ohm s law to find the third.

71 A toaster oven has a resistance of 12 ohms and is plugged into a 120-volt outlet. How much current does it draw?

72 1. Looking for: current in amps 2. Given R = 12 ; V = 120 V 3. Relationships: I = V R 4. Solution I = 120 V = 10 A 12

73 Every electrical device is designed with a resistor that causes the right amount of current to flow when the device is connected to voltage. A

74 The resistance of many electrical devices varies with temperature and current. A light bulb s resistance increases when there is more current because the bulb gets hotter when more current passes through it.