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Introduction to Electricity

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Section 1 Electric Charge and Static Electricity

Section 2 Electric Current and Electrical Energy

Section 3 Electrical Calculations

Section 4 Electric Circuits

Chapter 17

Concept Mapping



Bellringer

Write a definition for electric charge in your own words in your science journal. When do you experience electric charges most, in winter or in summer? Explain your reasoning.

Chapter 17



Objectives

• Describe how charged objects interact by using the law of electric charges.

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

• Compare conductors with insulators.

• Give two examples of static electricity and electric discharge.

Chapter 17



Electric Charge

• Charges Exert Force Atoms are composed of particles with electric charge. The law of electric charges states that like charges repel and opposite charges attract.

Chapter 17


Section 1 Electric Charge and Static ElectricityChapter 17



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 17



Charge It!

• Friction Charging by friction happens when electrons are “wiped” from one object onto another.

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

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

Chapter 17



Charge It!, continued

Chapter 17



Charge It!, continued

Chapter 17

• 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

Chapter 17

Click below to watch the Visual Concept.

Visual Concept




Moving Charges

Chapter 17

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

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



Static Electricity

Chapter 17

• 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.



Static Electricity, continued

Chapter 17

• 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.



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 responses in your science journal.

Chapter 17



Objectives

• Describe electric current.

• Describe voltage and its relationship to electric current.

• Describe resistance and its relationship to electric current.

Chapter 17



Objectives, continued

• Explain how a cell generates electrical energy.

• Describe how thermocouples and photocells generate electrical energy.

Chapter 17



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 17



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 17



Electric Current, continued

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

Chapter 17



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 17



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 17



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 17



Resistance, continued

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

Chapter 17


Factors That Affect Resistance

Chapter 17

Click below to watch the Visual Concept.

Visual Concept




Generating 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 17


Section 2 Electric Current and Electrical EnergyChapter 17



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 17



Bellringer

How fast is a nanosecond? A nanosecond (ns) is one-billionth of a second. Electrical signals travel at 30 cm/ns. Calculate how far electrical signals travel in 1 second.

Record your answers in your science journal.

Chapter 17



Objectives

• Use Ohm’s law to calculate voltage, current, and resistance.

• Calculate electric power.

• Determine the electrical energy used by a device.

• Compare the power ratings of different electrical appliances.

Chapter 17



Connecting Current, Voltage, and Resistance

• Georg Ohm (1789 –1854) studied the resistances of materials. He measured the current that resulted from different voltages applied to a piece of metal wire.

• Ohm’s Law Ohm found that the ratio of voltage to current is a constant for each material. This ratio is the resistance of the material.

Chapter 17

R VI, or V IR



Connecting Current, Voltage, and Resistance, continued

Chapter 17


Section 3 Electrical CalculationsChapter 17



Electric Power

• What Is Electric Power? The rate at which electrical energy is changed into other forms of energy is electric power. Electric power is calculated using the following equation:

Chapter 17

• Watt: The Unit of Power The unit for power is the watt (W). Another common unit of power is the kilowatt (kW). One kilowatt is equal to 1,000 W.

power voltage current, or P V I



Electric Power, continued

• Power Ratings When you read the wattage label on a light bulb, you are reading the bulb’s power rating. The power rating describes the rate at which an electric device uses electrical energy.

Chapter 17



Measuring Electrical Energy

• The amount of electrical energy used in a home depends the power of the electrical devices in the house and the length of the time that those devices are on. The equation for electrical energy is as follows:

Chapter 17

electrical energy power time, or E P t


Section 3 Electrical CalculationsChapter 17



Measuring Electrical Energy, continued

• Measuring Household Energy Use Different amounts of electrical energy are used each day in a home. Electric companies usually calculate electrical energy by multiplying the power in kilowatts by the time in hours.

• How to Save Energy Every appliance uses energy. Replacing items that have high power ratings with items that have lower ratings is a way to save energy.

Chapter 17



Measuring Electrical Energy, continued

• It’s All About the Bulb All of the lighting in a home could make up as much as 25% of a home’s energy consumption. So, buying and using energy–efficient light bulbs can make a big difference in the amount of energy used by a household.

• Energy–Saving Programs Many governments and organizations around the world have started programs to help save energy and natural resources.

Chapter 17



Bellringer

What happens when you turn the lights on? What allows lights to be turned on and off? Write your answer or draw an explanatory picture for later reference in your science journal. What happens beyond the confines of the room when you flip the light switch?

Chapter 17



Objectives

• Name the three essential parts of a circuit.

• Compare series circuits with parallel circuits.

• Explain how fuses and circuit breakers protect your home against short circuits and circuit overloads.

Chapter 17


Parts of an Electric Circuit

• Forming a Loop An electric circuit is a complete, closed path through which electric charges flow.

• All circuits need three basic parts: an energy source, wires, and a load.

Chapter 17 Section 4 Electric Circuits


Section 4 Electric CircuitsChapter 17


Parts of an Electric Circuit, continued

• A Switch to Control a Circuit Sometimes, a circuit also contains a switch. A switch is used to open and close a circuit.



Types of Circuits

• Series Circuits A series circuit is a circuit in which all parts are connected in a single loop.

• Uses for Series Circuits Series circuits are useful in wiring burglar alarms. If any part of the circuit fails, there will be no current in the system and the alarm will sound.



Types of Circuits, continued

• Parallel Circuits A parallel circuit is a circuit in which loads are connected side by side.

• Uses for Parallel Circuits Almost all appliances are built with parallel circuits so that they will keep working if part of the system fails.



Household Circuit Safety

• Circuit Failure Broken wires or water can cause a short circuit. In a short circuit, charges do not go through one or more loads in the circuit.

• Fuses A fuse has a thin strip of metal. Fuses keep charges from flowing if the current is too high.



Household Circuit Safety, continued

• Circuit Breakers A circuit breaker is a switch that automatically opens if the current is too high. Charges stop flowing.

• Electrical Safety Tips Do not overload circuits by plugging in too many electrical devices. Do not use electrical devices near water.



Introduction to Electricity

Use the terms below to complete the Concept Mapping on the next slide.

Chapter 17

Concept Mapping

battery

electrical energy

thermocouple

light energy

thermal energy


Introduction to ElectricityChapter 17

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electric force

static electricity section

electric field

electric discharge

static electricity chapter

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conservation of charge