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Chapter Eight: Electricity and Magnetism • 8.1 Electricity • 8.2 Electrical Circuits and Electrical Power • 8.3 Properties of Magnets Investigation 8A Electricity • How does electricity work? 8.1 Electric and magnetic forces • All matter has electrical (and magnetic) properties because the atoms that make up matter are held together by electromagnetic forces. 8.1 Electric and magnetic forces • Magnetic forces can attract or repel because there are two kinds of magnetic poles, called north and south. • A north pole repels another north pole but attracts a south pole. 8.1 Electric and magnetic forces • Like magnetism, electric charge comes in two types. We call them positive and negative. • A positive charge attracts a negative charge and vice versa. • Two similar charges repel each other. 8.1 Electric charge • Electric charge, like mass, is also fundamental property of matter. • Inside atoms found in matter, attraction between positive and negative charges holds the atoms together. 8.1 Electric charge • Because ordinary matter has zero (total) charge, most matter acts as if there is no electric charge at all. • A tiny imbalance in either positive or negative charge is the cause of static electricity. 8.1 Electric current • Electric current is caused by moving electric charge. • Electric current comes from the motion of electric charges that are much smaller than atoms. • These charges are called electrons. e- 8.1 Electric current • Electric current is similar in some ways to a current of water. • Like electric current, water current can carry energy and do work. • A waterwheel turns when a current of water exerts a force on it. 8.1 Electric current • Electric current is what makes an electric motor turn or an electric stove heat up. • Electric current is measured in amperes honor of Andre-Marie Ampere. 8.1 Electric current • Electric current can carry great deal of energy. • An electric motor the size of a basketball can do as much work as five big horses or twenty strong people. 8.1 Voltage • When two containers of water are the same height, no water will flow. • No current flows when there is zero voltage difference, which is why a dead battery won’t light a bulb. 8.1 Voltage • Current flows from higher voltage to lower voltage. • It is a difference in voltage that makes electrical current flow just as a difference in height makes water current flow. 8.1 Batteries • Since electric current in wires is invisible, think of a “current” of marchers, each carrying a bucket of energy. • The battery refills the buckets with fresh energy and the bulb uses the energy to make light and heat. 8.1 Batteries • A pump is like a battery because it brings water from a position of low energy to high energy. 8.1 Batteries • A battery uses stored chemical energy to create the voltage difference. • Three 1.5-volt batteries can be stacked to make a total voltage of 4.5 volts in a flashlight. 8.2 Electric Circuits and Electrical Power • An electric circuit is a complete path through which electric current travels. • A good example of a circuit is the one found in an electric toaster. 8.2 Current in a circuit • Current only flows when there is a complete and unbroken path, or a closed circuit. • Flipping a switch to the “off” position creates an open circuit by making a break in the wire. 8.2 Current in a circuit • The amount of electric current flowing into any part of a circuit must be the same as the amount flowing back out. 8.2 Electrical power • Electrical power is measured in watts, just like mechanical power. • A 100-watt electric light bulb uses 100 joules of energy every second. 8.2 Electrical Power • To calculate power in an electric circuit you multiply the voltage and current together. What is the power in this circuit? 8.2 Resistance • Resistance (R) controls how much current flows for a given voltage. • When resistance is low, current flows easily. 8.2 Resistance This light bulb has a resistance of 1 ohm. • Electrical resistance is measured in ohms. • If a circuit has a resistance of 1 ohm, then a voltage of 1 volt causes a current of 1 amp to flow. 8.2 Ohm’s law • The current in a circuit depends on voltage and resistance. • Ohm’s law relates current, voltage, and resistance with one formula. 8.2 Ohm’s law • Voltage and current are directly related. • Doubling the voltage doubles the current. • Resistance and current are inversely related. • Doubling the resistance cuts the current in half. • If you know two of the three quantities, you can use Ohm’s law to find the third.