Chapter Eight: Electricity and Magnetism 8.1 Electricity 8.2 Electrical Circuits and Electrical Power 8.3 Properties of Magnets

Investigation 8A How does electricity work? Electricity

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 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. This light bulb has a resistance of 1 ohm.

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.