Chapter 16 Electricity. Section 1: Electric Charge and Force Section 2: Current Section 3: Circuits

Transcription

1 Chapter 16 Electricity Section 1: Electric Charge and Force Section 2: Current Section 3: Circuits

2 Key Terms Electric Charge and Force Electric Charge Electrical Conductor Electrical Insulator Electric Force Electric Field

3 Electric Charge and Force Electric Charge An electric charge is an electrical property of matter that creates electric and magnetic forces and interactions Like charges repel, and opposite charges attract The two charges are positive and negative Positive + Negative = Attraction Positive + Positive = Repulsion

4 Electric Charge and Force An object s electric charge depends on the imbalance of its protons and electrons All matter is made up of protons, neutrons, and electrons. When there is an imbalance in the number of protons and electrons the object becomes charged. The SI unit of electric charge is coulomb Proton = + 1.6x10-19 C Electron = - 1.6x10-19 C

5 Electric Charge and Force Conductors allow charges to flow; insulators do not Conductor a material in which charges can move freely and that can carry an electric current (Copper) Insulator a material that does not transfer current easily (Glass)

6 Electric Charge and Force Objects can be charged by the transfer of electrons Protons and neutrons are relatively fixed in the nucleus of an atom. The outermost electrons can be easily transferred from on atom to another. The direction of transfer depends on the material

7 Electric Charge and Force One material gains electrons becoming negatively charged and the other material loses electrons become positively charged The transfer of electrons by rubbing two objects together is called charging by friction Objects can also be charged without friction. This is called charging by contact.

8 Electric Charge and Force Charges move within uncharged objects A negatively charged rod brought near a metal doorknob induces a positive charge on the side of the doorknob closest to the rod and a negative charge on the side farthest from the rod. A charged object can attract and insulator by inducing a slight change in position of the electrons. This is called polarization

9 Electric Charge and Force Electric Force An electric force is the force of attraction or repulsion between objects due to charge Electric forces are responsible for events we cannot see. They hold atoms together as well as proteins which our bodies need

10 Electric Charge and Force Electric force depends on charge and distance The electric force between two objects is proportional to the product of the charges on the objects. The electric force is also inversely proportional to the square of the distance between two objects

11 Electric Charge and Force Electric force acts through a field Electric field is a region in space around a charged object that causes a stationary charged object to experience an electric field. Electric field lines can be used to show an electric field.

12 Electric Charge and Force

13 Electric Charge and Force

14 Current Key Terms Electrical Potential Energy Potential Difference Cell Current Resistance

15 Current Voltage and Current Electrical potential energy is the ability to move an electric charge from one point to another Electrical potential energy between two negative charges decrease as the distance between them increases

16 Current Potential difference is measured in volts Potential difference between any two points, the work that must be done against electric forces to move a unit charge from one point to the other The SI unit for potential difference is Volts (V) 1 Volt = 1Joule per Coulomb (1 J/C)

17 Cell is a device that is a source of electric current because of a potential difference, or voltage, between the terminals Current There is a voltage across the terminals of a battery The voltage across two terminals of a battery can range from 1.5 V (small battery) to 12 V (car battery)

18 Current Table 1 on pg 539 gives examples of different types of cells. Electrochemical cells contain an electrolyte, a solution that conducts electricity, and two electrodes, each a different conducting material. Can be either a wet cell or dry cell Wet cell liquid electrolyte

19 Current A voltage sets charges in motion When charges are accelerated by an electric field to move to a position of lower potential energy, an electric current is produced. A current is the rate that electric charges move through a conductor.

20 Current Ampere = 1 C of charge moving past a point in 1 second Batteries are direct current sources because the charge always moves from one terminal to the other in the same direction.

21 Current Conventional current is defined as movement of positive charge A negative charge moving in one direction has the same effect as a positive charge moving in the opposite direction.

22 Electrical Resistance Current Resistance is opposition posed by a material or a device to the flow of current Resistance can be calculated from current and voltage A conductor s resistance indicates how much the motion of charges within it is resisted

23 Current It is found by dividing the voltage across the conductors by the current. The SI unit is ohm Ω, which is equal to volts per ampere. Resistance Equation resistance = voltage/current or R =V/I

24 Current Resistor is a special type of conductor used to control current. Equations R = V/I = Ω V = IR = volts I = V/R = amp

25 Current Conductors have low resistance A good conductor allows electrons to flow easily under the influence of an electric field. Metals (copper) Some materials become superconductors below a certain temperature

26 Current Certain metals and compounds have zero resistance when their temperature falls below a certain temperature called the critical temperature. Superconductors have been used in electrical devices such as filters, powerful magnets, and Maglev high-speed express trains.

27 Current Semiconductors are intermediate to conductors and insulators Materials that semiconductors that are in their pure state, are insulators. As impurities are added the semiconductor is loses some of its insulating ability. Silicon and germanium are examples of semiconductors.

28 Current Insulators have high resistance Insulating materials are used to prevent electric current from leaking. Example: Plastic coating on wires Sometimes it is important to provide a pathway for current to leave a charged object. This is

29 Circuits Key Terms Electric Circuit Schematic Diagram Series Parallel Electrical Energy Fuse Circuit Breaker

30 Circuits What are Circuits? An electric circuit is a path through which charges can be conducted Closed Circuit - electrons are able to flow a complete path Open Circuit Electrons are unable to flow a

31 Schematic diagram is a graphical representation of a circuit that uses lines to represent wires and different symbols to represent components Circuits Switches interrupt the flow of charges in a circuit Schematic diagrams are used to represent circuits

32 Circuits

33 Circuits Table 2 pg 548 has more examples of Schematic Diagram Symbols Series and Parallel Circuits Series circuits have a single path for current Figure 17A pg 549

34 Circuits If one element along the path in a series circuit is removed, the circuit will not work The voltage across each device in a series circuit can be different Parallel circuits have multiple paths for current The voltage across each device is the same in a parallel circuit

35 Circuits When devices are connected in parallel, charges have more than one path to follow. The circuit can be complete even if one light bulb burns out. The current will take the path with least resistance.

36 Circuits Electric Power and Electrical Energy Electrical energy is the energy that is associated with charged particles because of their positions. Electric power is the rate at which electrical energy is used in a circuit

37 Circuits Electric power is the product of the total current (I) in the voltage (V) across a circuit Electric Power Equation power = current x voltage P = IV

38 Circuits If you combine the electric power equation with the equation V = IR, the power lost, or dissipated, by a resistor can be calculated P = I 2 R = V 2 /R Electric companies measure energy consumed in kilowatt-hours

39 Circuits Fuses and Circuit Breakers If to many objects are plugged into an outlet, the overall resistance will be lower. This means that the electrical wires carry more than a safe level of current When this happens the circuit is said to be overloaded which can lead to a fire.

40 Circuits When two wires touch, it creates an alternative pathway for the electrical current. This is a short circuit. Grounding appliances can reduce the risk electrical shocks from short circuits Fuses melt to prevent circuit overloads

41 Circuits A fuse is an electrical device that contains a metal strip that melts when current in the circuit becomes too great. A circuit overload will cause a fuse to blow out Circuit breakers open circuits with high current A circuit breaker is a switch that opens a circuit automatically when the circuit exceeds a

42 Circuits Circuit breakers uses a magnet or bimetallic strip that respond to current overload by opening the circuit Unlike fuses circuit breakers can be reset by turning the switch back on.