ELECTRIC CIRCUITS Chapter Twenty Electric Circuits 20 1

ELECTRIC CIRCUITS

Chapter Twenty: Electric Circuits Ø 20. 1 Charge Ø 20. 2 Electric Circuits Ø 20. 3 Current and Voltage Ø 20. 4 Resistance and Ohm’s Law

Chapter 20. 1 Learning Goals ØDefine static electricity and discuss its causes. ØExplain what it means when an object is electrically charged. ØDiscuss the relationship between like and unlike charges.

Investigation 20 C Electric Charge ØKey Question: What is static electricity?

20. 1 Electric charge ØElectric charge, like mass, is also fundamental property of matter. ØInside atoms found in matter, attraction between positive and negative charges holds the atoms together.

20. 1 Charge ØVirtually all the matter around you has electric charge because atoms are made of electrons and protons (and neutrons). ØBecause ordinary matter has zero net (total) charge, most matter acts as if there is no electric charge at all.

20. 1 Electric and magnetic forces ØWhether two charges attract or repel depends on whether they have the same or opposite sign. ØA positive charge attracts a negative charge and vice versa. ØTwo similar charges repel each other.

20. 1 Electrical forces ØThe unit of charge is the coulomb (C). The name was chosen in honor of ØCharles Augustin de Coulomb (17361806), the French physicist who performed the first accurate measurements of the force between charges.

20. 1 Electrical forces ØElectric forces are incredibly strong. ØA millimeter cube of carbon the size of a pencil point contains about 77 coulombs of positive and negative charge.

20. 1 Electrical forces Ø Lightning is caused by a giant buildup of static charge. Ø The cloud, air, and ground can act like a giant circuit. Ø 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.

Chapter Twenty: Electric Circuits Ø 20. 1 Charge Ø 20. 2 Electric Circuits Ø 20. 3 Current and Voltage Ø 20. 4 Resistance and Ohm’s Law

Chapter 20. 2 Learning Goals ØDefine electricity. ØDescribe the components of an electric circuit. ØExplain the difference between a closed circuit and an open circuit.

20. 2 Electric current ØElectric current is caused by moving electric charge. ØElectric current comes from the motion of electrons. current

20. 2 Electric current ØElectric current is similar in some ways to a current of water. ØLike electric current, water current can carry energy and do work. ØA waterwheel turns when a current of water exerts a force on it.

20. 2 Electric Circuits ØAn electric circuit is a complete path through which electric current travels. ØA good example of a circuit is the one found in an electric toaster.

20. 2 Electric Circuits ØWires in electric circuits are similar in some ways to pipes and hoses that carry water.

20. 2 Electric Circuits ØWhen drawing a circuit diagram, symbols are used to represent each part of the circuit.

20. 2 Electric Circuits ØElectrical symbols are quicker and easier to draw than realistic pictures of the components.

20. 2 Resistors ØA resistor is an electrical device that uses the energy carried by electric current in a specific way. ØAny electrical device that uses energy can be shown with a resistor symbol.

20. 2 Current in a circuit ØCurrent only flows when there is a complete and unbroken path, or a closed circuit. ØFlipping a switch to the “off” position creates an open circuit by making a break in the wire.

Chapter Twenty: Electric Circuits Ø 20. 1 Charge Ø 20. 2 Electric Circuits Ø 20. 3 Current and Voltage Ø 20. 4 Resistance and Ohm’s Law

Chapter 20. 3 Learning Goals ØExplain how current flows in an electric circuit. ØDefine voltage and describe how it is measured. ØDiscuss the function of a battery in an electric circuit.

Investigation 20 A ØKey Question: Electricity How do you measure voltage and current in electric circuits?

20. 3 Current and voltage ØElectric current is measured in units called amperes, or amps (A) for short. ØOne amp is a flow of a certain quantity of electricity in one second. ØThe amount of electric current entering a circuit always equals the amount exiting the circuit.

20. 3 Voltage ØVoltage is a measure of electric potential energy, just like height is a measure of gravitational potential energy. ØVoltage is measured in volts (V). ØA voltage difference of 1 volt means 1 amp of current does 1 joule of work in 1 second.

20. 3 Voltage ØA difference in voltage provides the energy that causes current to flow.

20. 3 Voltage ØA useful meter is a multimeter, which Øcan measure voltage or current, and sometimes resistance. ØTo measure voltage, the meter’s probes are touched to two places in a circuit or across a battery.

20. 3 Batteries ØA battery uses stored chemical energy to create the voltage difference. ØThree 1. 5 -volt batteries can be stacked to make a total voltage of 4. 5 volts in a flashlight.

20. 3 Batteries ØA pump is like a battery because it brings water from a position of low energy to high energy.

20. 3 Measuring current Ø If you want to measure current you must force the current to pass through the meter. ØMultimeters can measure two types of current: alternating current (AC) and direct current (DC).

20. 3 Measuring current Ø Circuit breakers and fuses are two kinds of devices that protect circuits from too much current by making a break that stops the current.

Chapter Twenty: Electric Circuits Ø 20. 1 Charge Ø 20. 2 Electric Circuits Ø 20. 3 Current and Voltage Ø 20. 4 Resistance and Ohm’s Law

Chapter 20. 4 Learning Goals ØUse Ohm’s law to relate current, voltage and resistance. ØApply Ohm’s law to solve problems. ØClassify materials as conductors, insulators, and semiconductors.

Investigation 20 B Resistance and Ohm’s Law ØKey Question: What is the relationship between current and voltage in a circuit?

20. 4 Resistance ØResistance is the measure of how strongly an object resists current flowing through it. ØThe relationship between electric current and resistance can be compared with water flowing from the open end of a bottle.

20. 4 Resistance ØThe total amount of resistance in a circuit determines the amount of current in the circuit for a given voltage.

20. 4 Resistance ØElectrical resistance is measured in units called ohms. ØThis unit is abbreviated with the Greek letter omega (Ω).

20. 4 Ohm’s Law ØThe current in a circuit depends on voltage and resistance. ØOhm’s law relates current, voltage, and resistance with one formula. ØIf you know two of the three quantities, you can use Ohm’s law to find the third.

Solving Problems ØA toaster oven has a resistance of 12 ohms and is plugged into a 120 -volt outlet. ØHow much current does it draw?

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

20. 4 Resistance of common objects ØEvery electrical device is designed with a resistor that causes the right amount of current to flow when the device is connected to voltage.

20. 4 Resistance of common objects ØThe resistance of many electrical devices varies with temperature and current. ØA light bulb’s resistance increases when there is more current because the bulb gets hotter when more current passes through it.

You are Wired! ØYour nervous system uses specialized cells called neurons to transfer electrical signals from one part of your body to another. A neuron has three basic parts: the cell body; a long, thin portion called the axon; and fingerlike projections called dendrites.