electricity chapter 14. section 2 electric charge and static electricity objectives explain the role...

Electricity

•Chapter 14

Section 2 Electric Charge and Static Electricity

Objectives

• Explain the role of charged particles in atoms and in charged objects.

• Describe how electric force depends on charge and distance.

• Describe three ways in which an object can become charged.

• Compare conductors with insulators.

• Give two examples of static electricity and electric discharge.

Chapter 14


Electric Charge

• Charges And Forces Atoms are composed of particles with electric charge.

• The law of electric charges states that like charges repel and opposite charges attract.

• When you rub you hair on a balloon, why will it stick to the wall?

•Imbalance of charges•Hair becomes negative (more electrons)•Balloon becomes positive (fewer electrons)

Chapter 14

Section 2 Electric Charge and Static ElectricityChapter 14

The Structure of an Atom

Size of the Charges

• The unit of measure for electrical charge is the Coulomb

• Electrons and Protons have the same charge, but opposite– Electrons have a charge of -1.6x10-19C– Protons have a charge of +1.6x10-19C


Electric Charge, continued

• The Force Between Protons and Electrons Because protons and electrons have opposite charges, they are attracted to each other.

• The Electric Force and the Electric Field The force between charged objects is an electric force. An electric field is the region around a charged object in which an electric force is exerted on another charged object.

Chapter 14


The Law of Electric Charges

Chapter 14


Charge It! • Friction Charging by friction happens when

electrons are “wiped” from one object onto another.•Like a balloon through your hair

• Conduction Charging by conduction happens when electrons move from one object to another by direct contact.

•Like touching the handle of a door after shuffling your feed

• Induction Charging by induction happens when charges in an uncharged metal object are rearranged without direct contact with a charged object.

Chapter 14


Charge It!, continued

Chapter 14

• Conservation of Charge When you charge something by any method, no charges are created or destroyed. The numbers of electrons and protons stay the same.

• Detecting Charge You can use a device called an electroscope to see if something is charged.

Charging by Contact



Moving Charges

Chapter 14

• Conductors An electrical conductor is a material in which charges can move easily.

•Silver, gold, copper are best•Most other metals

• Insulators An electrical insulator is a material in which charges cannot move easily

•Air, cloth, paper, plastic, wood.


Static Electricity

Chapter 14

• Static electricity is the electric charge at rest on an object.

• Electric Discharge The loss of static electricity as charges move off an object is called electric discharge. One of the most dramatic examples of electric discharge is lightning. The next slide shows how lightning is formed.


How Lightning Forms

Chapter 14


Static Electricity, continued

Chapter 14

• Lightning Dangers It is particularly dangerous to be at the beach or on a golf course during a lightning storm. Even standing under a tree during a storm is dangerous.

• Lightning Rods A lightning rod is a pointed rod connected to the ground by a wire. Objects that are joined to Earth by a conductor, such as a wire, are grounded.

Electric Force

• The attraction of a positively charged particle to a negatively charged particle

• Electric force is proportional to charge and distance– Force is equal to the product of the charges

(Fe=CaCb)– Force is inversely proportional to the SQUARE of

the distance• Fe=1/(da-db)2

• If you double the distance, the force is one-fourth• If you half the distance, the force is 4 times

Electric Force

• Electric Force acts in a ‘Field’– Electric Field is the area in which the

oppositely charged items attract each other

Section 3 Electric Current and Electrical Energy

Bellringer

What is the difference between something that is direct and something that is alternating? How does the difference relate to electric current?

Record your response in your science journal.

Chapter 14


Objectives

• Describe electric current.

• Describe voltage and its relationship to electric current.

• Describe resistance and its relationship to electric current.

• Explain how a cell generates electrical energy.

• Describe how thermocouples and photocells generate electrical energy.

Chapter 14


Electric Current

• Electric current is the rate at which charges pass through a given point. Electric current is expressed in units called amperes, or amps.

• Making Charges Move When you flip a switch, an electric field is set up in the wire at the speed of light. The electric field causes the free electrons in the wire to move.

Chapter 14


Electric Current, continued

• Commanding Electrons to Move This electric field is created so quickly that all electrons start moving through the wire at the same instant. Think of the electric field as a command to the electrons to charge ahead.

Chapter 14


Electric Current, continued

• Alternating Current and Direct Current There are two kinds of electric current—direct current (DC) and alternating current (AC).

Chapter 14


Voltage

• Voltage is the potential difference between two points in a circuit. Voltage is expressed in volts (V).

• Voltage and Energy Voltage is a measure of how much work is needed to move a charge between two points.You can think of voltage as the amount of energy released as a charge moves between two points in the path of a current.

Chapter 14


Voltage, continued

• Voltage and Electric Current As long as there is a voltage between two points on a wire, charges will flow in the wire. The size of the current depends on the voltage.

• Varying Nature of Voltage Different devices need different levels of voltage.

Chapter 14


Resistance

• Resistance is the opposition to the flow of electric charge.

• Resistance and Material Good conductors, such as copper, have low resistance. Poor conductors, such as iron, have higher resistance.

• Resistance, Thickness, and Length Thick, short wires have less resistance than thin, long wires.

Chapter 14


• Resistance and Temperature In general, the resistance of metals increases as temperature rises.

Chapter 14

Resistance, continued

Chapter 14

Factors That Affect Resistance



Generating Electrical Energy

• Cells change chemical or radiant energy into electrical energy.

• Parts of a Cell A cell contains a mixture of chemicals called an electrolyte. Every cell also has a pair of electrodes made from conducting materials.

• Kinds of Cells Two kinds of cells are wet cells and dry cells. The electrolytes in dry cells are solid or pastelike.

Chapter 14


How a Cell Produces an Electric Current

Chapter 14


Generating Electrical Energy, continued

• Thermocouples Thermal energy can be converted into electrical energy by a thermocouple.

• Photocells A photocell converts light energy into electrical energy.

Chapter 14

Magnetism and Electricity

Use the terms below to complete the concept map on the next slide.

Concept Mapping

electric current

battery

charges

thermocouple

photocell

cell

Chapter 14

Magnetism and ElectricityChapter 14

FCAT

For the following questions, write your answers on a separate sheet of paper.

Chapter 14 Standardized Test Preparation

1. Charged particles in solar wind bombard Earth constantly. The image below shows how the paths of these charged particles are bent as they approach Earth. What causes the charged particles in the solar wind to change course as they approach Earth?

A. Earth’s gravity

B. Earth’s radiation

C. Earth’s electric field

D. Earth’s magnetic field


2. Several forces act on the objects in the image below. One force pushes the objects apart. Another force pulls the object together. What force pulls the two objects toward each other?

F. gravity

G. convection

H. electric force

I. magnetic force


3. After sliding down a plastic slide, Juan notices the hairs on his arm are standing on end. Which of the following forces pushes the hairs up?

A. atomic force

B. electric force

C. magnetic force

D. gravitational force


4. George wants to generate electrical energy in his house from a renewable resource. What device from the following choices could he use as a model?

F. a nuclear power plant

G. a coal fired power plant

H. a diesel electric generator

I. a solar roadside emergency phone


5. The sketch below shows a diagram of Angelica’s experiment. Angelica placed the “black box” in a hot attic during the day and observed the light bulb shining brightly. During the night, the light bulb went out.

Part A Identify the type of energy going into the light bulb that was generated by this system.

Part B Explain why the light bulb went out at night.


5. Part A Identify the type of energy going into the light bulb that was generated by this system.

Part A: Full-credit answers should include the following points: light bulbs are powered by electrical energy; the black box in the attic must generate electrical energy; the black box generates electrical energy from the thermal energy in the hot attic.

Part B Explain why the light bulb went out at night.

Part B: Full-credit answers should include the following points: at night, the sun does not warm up the attic; there is not enough thermal energy in the attic at night for the black box to generate the electrical energy required to illuminate the light bulb.


Section 1 Magnets and MagnetismChapter 14


The Structure of an Atom


The Law of Electric Charges

Chapter 14


How Lightning Forms

Chapter 14

Section 3 Electric Current and Electrical EnergyChapter 14


How a Cell Produces an Electric Current

Chapter 14

Section 3 Electric Current and Electrical EnergyChapter 14

electricity chapter 14. section 2 electric charge and static electricity objectives explain the role...

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