1. CHAPTER 17: ELECTRICITY 1

2. 17.1 ELECTRIC CHARGE AND FORCE CHAPTER 17: ELECTRICITY

3. ELECTRIC CHARGE Electricity: force created by a difference in charges (+, -) due to gained or lost electrons. Protons (+) are trapped in the nucleus, so moving electrons (-) are responsible for electricity Electric Charge: an electrical property of matter that creates electric and magnetic forces and interactions Like energy, electric charge is never created or destroyed The SI unit of electric charge is the coulomb, (C) 3

4. TRANSFER OF ELECTRIC CHARGE When charges between two points differ because electrons build up, the electric charge can be transferred This can happen through induction, conduction, or friction 4

5. TRANSFER OF ELECTRIC CHARGE Induction: presence of (-) charge forces electrons to move away (no contact involved) Conduction: when a charged object touches a neutral object and the charges transfer. Often called charging by contact 5

6. TRANSFER OF ELECTRIC CHARGE Friction: when objects are rubbed together and one material gains electrons, becoming negatively charged, and the other loses electrons, becoming positively charged Static electricity happens when the charges build up, but don’t move 6

7. TRANSFER OF ELECTRIC CHARGE Although charge can jump between points, it often travels through a conductor Electrical Conductor: a material in which charges can move freely, allows charges to flow The best conductors are metals because their electrons are not tightly bound to the nucleus Electrical Insulator: a material in which charges cannot move freely, does not allow charges to flow The electrons are tightly bound in insulators, most plastics are insulators 7

8. ELECTRIC FORCE Electric charges exert a force Electric Force: the attractive or repulsive interaction between any two charged objects Like charges repel, opposites attract This force decreases as the distance between charges increases 8

9. ELECTRIC FIELD Electric Field: the space around a charged object in which another charged object experiences an electric force (this is like gravitational forces!) Any charge that is placed in an electric field will be pushed or pulled by the field For example, if you have a + charge, and place a + charge near it, they will repel 9

10. 17.2 CURRENT CHAPTER 17: ELECTRICITY

11. LET’S REVIEW! Potential Energy: energy stored in an object due to its position Kinetic Energy: the energy an object possesses due to its motion 11

12. CURRENT Electric Current: rate at which charges pass through a given point (flow of electrons from point A to B) Related more to the amount than the speed The SI unit of current is the ampere (A) 1 amp = 6.25 x 10 18 electrons per sec. (6,250,000,000,000,000,000!) When charges flow, they carry energy that can be used to do work 12

13. ELECTRICAL POTENTIAL ENERGY Electrical Potential Energy: the ability to move an electric charge from one point to another Charges naturally move from an area with high electrical PE to an area with low electric PE Electrical PE is measured in volts (V) 13

14. ELECTRICAL POWER & ENERGY When electrons flow, they transfer energy and transform electrical energy into other forms of energy (sound, light, heat) Electrical Power: The rate at which electrical energy is converted into other forms of energy The SI unit for power is the watt (W) Electric power is calculated by multiplying the total current, I, by the voltage, V, in a circuit 14

15. VOLTAGE Voltage: The difference in charges between two points, also called potential difference Voltage sets charges in motion Think of a battery - there is voltage across the terminals One terminal is positive and the other is negative Electrons will always flow from (-) to (+) when they are connected 15

16. DIRECT CURRENT (DC) Flow of a charge always in one direction The amount of current can change, but it will always flow in the same direction For example – a battery In metals, moving electrons make up the current 16

17. DIRECT CURRENT (DC) Lightning is a form of DC caused by static electricity in clouds. Static is formed when air molecules move past each other (like clothes in a dryer), and negative charges group at the bottom of the cloud and transfer electrons to the ground, which has taken on a positive charge. 17

18. DIRECT CURRENT (DC) Battery: device which produces a continual direct current (DC), through a chemical reaction. When the (+) and (-) terminals are connected through a device, they produce a flow of electrons from negative to positive, which powers the device. 18

19. ALTERNATING CURRENT (AC) Flow of charge back and forth, changing its direction many times in one second. AC power is used all over the world because it’s cheaper and easier to make devices with AC power All of the appliances and lights in your house probably run off of AC power 19

20. CONVENTIONAL CURRENT Electrons move from areas of negative charge to positive Conventional current moves in the other direction – positive to negative The direction of current is opposite to the direction that the electrons move 20

21. ELECTRICAL RESISTANCE Resistance: a material’s opposition to the flow of electrons Resistance is caused by internal friction, which slows the movement of charges through a conducting material All materials have some resistance Conductors are low, insulators are high Resistor: certain type of conductor used to control current 21

22. ELECTRICAL RESISTANCE Resistance is measured in Ohms (Ω) 1 Ω = 1 V/A Ohm’s Law: resistance in a circuit equals voltage difference, divided by current Ohm’s Law provides a way to measure the resistance of objects and materials 22

23. ELECTRICAL RESISTANCE Semiconductors: materials that have electrical properties between those of insulators and conductor, and conduct under certain conditions. Remember your periodic table! Superconductors: some metals and compounds that have zero resistance when their temperature falls below the critical temperature Critical temps range based on the superconductor – can be as low as –272 ºC (–458 ºF) and as high as –123 ºC (–189 ºF) 23

24. ELECTRICAL RESISTANCE Once current is established in a superconductor, it will continue even if voltage is removed, making them very useful Superconducting magnets are so strong they are used to levitate trains (MagLev Trains) which reduces the friction between a normal train and the track For any conductor, an increase in temperature will increase resistance 24 BILL NYE

25. 17.3 CIRCUITS CHAPTER 17: ELECTRICITY 25

26. CIRCUITS Electric Circuit: pathway along which an electric current can travel, en route to a device. Closed Circuit: Complete electrical path in which current circulates Open Circuit: An incomplete electrical path with no charge flow and therefore no current Switches interrupt the flow of charges in a circuit. You can use a switch to open and close a circuit. 26

27. SERIES CIRCUITS Series Circuit: only one path from the source through all of the loads and back to the source Current in each device is the same, but resistances and the voltage across each device may differ If one element along the path in a series circuit is removed, the circuit will not work A string of old Christmas lights is an example of a series circuit 27

28. PARALLEL CIRCUIT Parallel Circuit: two or more paths for current to flow through Voltage is the same across each device, and the sum of all currents equals the total current A break in any one path does not interrupt the flow of electric charges Newer Christmas lights and household circuits are an example 28

29. CIRCUIT BREAKERS When electrical wires carry more than a safe level of current, the circuit is overloaded. Overloaded circuits can cause fires Short Circuit: When current stops short of the intended device Circuit Breaker: device which prevents too much electricity from flowing through a circuit, can be reset usually with the flip of a switch. 29

30. FUSES Fuse: an electrical device that contains a metal strip that melts when current in the circuit becomes too great Fuses melt to prevent circuit overloads. 30

31. SCHEMATIC DIAGRAMS Schematic Diagram: graphical representation of a circuit Uses lines to represent wires Uses symbols to represent components Schematic diagrams use standard symbols and can be used to describe all electrical devices 31

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