electric current

electric conductors and insulators

Some materials resist the flow of electric current more or less than other materials do. Materials that have low resistance to electric current are called electric conductors. Many metalsincluding copper, aluminum, and steelare good conductors of electricity. Water that has even a tiny amount of impurities in it is an electric conductor as well. Materials that have high resistance to electric current are called electric insulators. Wood, rubber, and plastic are examples of electric insulators. Dry air is also an electric insulator. You probably know that electric wires are made of metal and coated with rubber or plastic (see Figure 23.14). Now you know why. Metals are good electric conductors, so they offer little resistance and allow most of the current to pass through. Rubber and plastic are good insulators, so they offer a lot of resistance and allow little current to pass through. When more than one material is available for electric current to flow through, the current always travels through the material with the least resistance. Thats why all the current passes through a metal wire and none flows through its rubber or plastic coating.

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properties that affect resistance

For a given material, three properties of the material determine how resistant it is to electric current: length, width, and temperature. Consider an electric wire like one of the wires in Figure 23.14. A longer wire has more resistance. Current must travel farther, so there are more chances for it to collide with particles of wire. A wider wire has less resistance. A given amount of current has more room to flow through a wider wire. A cooler wire has less resistance than a warmer wire. Cooler particles have less kinetic energy, so they move more slowly. Current is less likely to collide with slowly moving particles. Materials called superconductors have virtually no resistance when they are cooled to extremely low temperatures.

electric current and materials

Electric current cannot travel through empty space. It needs a material through which to travel. However, when current travels through a material, the flowing electrons collide with particles of the material, and this creates resistance.

resistance

Resistance is opposition to the flow of electric charges that occurs when electric current travels through matter. The SI unit of resistance is the ohm (named for the scientist Georg Ohm, whom you can read about below). Resistance is caused by electrons in a current bumping into electrons and ions in the matter through which the current is flowing. Resistance is similar to the friction that resists the movement of one surface as it slides over another. Resistance reduces the amount of current that can travel through the material because some of the electrical energy is converted to other forms of energy. For example, when electric current flows through the tungsten wire inside an incandescent light bulb, the tungsten resists the flow of electric charge, and some of the electrical energy is converted to light and thermal energy.

using ohms law to calculate current

You can use the equation for current (above) to calculate the amount of current flowing through a material when voltage and resistance are known. Consider an electric wire that is connected to a 12-volt battery. If the wire has a resistance of 3 ohms, how much current is flowing through the wire? Current = 12 volts = 4 amps 3 ohms You Try It! Problem: A 120-volt voltage source is connected to a wire with 20 ohms of resistance. How much current flows through the wire?

ohms law

Voltage, or a difference in electric potential energy, is needed for electric current to flow. As you might have guessed, greater voltage results in more current. Resistance, on the other hand, opposes the flow of electric current, so greater resistance results in less current. These relationships between current, voltage, and resistance were first demonstrated by a German scientist named Georg Ohm in the early 1800s, so they are referred to as Ohms law. Ohms law can be represented by the following equation. Current (amps) = Voltage (volts) Resistance (ohms)

understanding ohms law

You may have a better understanding of Ohms law if you compare current flowing through a wire from a battery to water flowing through a garden hose from a tap. Increasing voltage is like opening the tap wider. When the tap is opened wider, more water flows through the hose. This is like an increase in current. Stepping on the hose makes it harder for the water to pass through. This is like increasing resistance, which causes less current to flow through a material. Still not sure about the relationship among voltage, current, and resistance? Watch the video at this URL: MEDIA Click image to the left or use the URL below. URL:

sources of voltage

Batteries like the one in Figure 23.11 are one of several possible sources of voltage needed to produce electric current. Sources of voltage include generators, chemical cells, and solar cells. Generators change the kinetic energy of a spinning turbine to electrical energy in a process called electromag- netic induction. You can read about generators and how they work in the chapter "Electromagnetism." Chemical and solar cells are devices that change chemical or light energy to electrical energy. You can read about both types of cells and how they work below.

explaining electric current

Why do charges flow in an electric current? The answer has to do with electric potential energy. Potential energy is stored energy that an object has due to its position or shape. An electric charge has potential energy because of its position in an electric field. For example, when two negative charges are close together, they have potential energy because they repel each other and have the potential to push apart. If the charges move apart, their potential energy decreases. Electric charges always move spontaneously from a position where they have higher potential energy to a position where their potential energy is lower. This is similar to water falling over a dam from an area of higher to lower potential energy due to gravity. In general, for an electric charge to move from one position to another, there must be a difference in electric potential energy between the two positions. The difference in electric potential energy is called potential difference, or voltage. Voltage is measured in an SI unit called the volt (V). For example, the terminals of the car battery in Figure 23.11 have a potential difference of 12 volts. This difference in voltage results in a spontaneous flow of charges, or electric current.

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introduction to electric current

Electric current is a continuous flow of electric charges. Current is measured as the amount of charge that flows past a given point in a certain amount of time. The SI unit for electric current is the ampere (A), or amp. Electric current may flow in just one direction, or it may keep reversing direction. When current flows in just one direction, it is called direct current (DC). The current that flows through a battery-powered flashlight is direct current. When current keeps reversing direction, it is called alternating current (AC). The current that runs through the wires in your home is alternating current. Graphs of both types of current are shown in Figure 23.10. You can watch an animation of both types at this URL: MEDIA Click image to the left or use the URL below. URL:

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solar cells

Solar cells convert the energy in sunlight to electrical energy. They contain a material such as silicon that absorbs light energy and gives off electrons. The electrons flow and create electric current. Figure 23.13 and the animation at the URL below show how a solar cell uses light energy to produce electric current and power a light bulb. Many calculators and other devices are also powered by solar cells.

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chemical cells

Chemical cells are found in batteries. They produce voltage by means of chemical reactions. A chemical cell has two electrodes, which are strips made of different materials, such as zinc and carbon (see Figure 23.12). The electrodes are suspended in an electrolyte. An electrolyte is a substance containing free ions that can carry electric current. The electrolyte may be either a paste, in which case the cell is called a dry cell, or a liquid, in which case the cell is called a wet cell. Flashlight batteries contain dry cells. Car batteries contain wet cells. Animations at the URL below show how batteries work. Both dry and wet cells work the same basic way. The electrodes react chemically with the electrolyte, causing one electrode to give up electrons and the other electrode to accept electrons. In the case of zinc and carbon electrodes, the zinc electrode attracts electrons and becomes negatively charged, while the carbon electrode gives up electrons and becomes positively charged. Electrons flow through the electrolyte from the negative to positive electrode. If wires are used to connect the two electrodes at their terminal ends, electric current will flow through the wires and can be used to power a light bulb or other electric device.

instructional diagrams

No diagram descriptions associated with this lesson

questions

The SI unit for electric current is the

-->  a. ampere.

b. volt.

c. ohm.

d. watt.

What does voltage mean?

a. electrical resistance

-->  b. potential difference

c. flow of electric charges

d. none of the above

Which statement about the electrodes in a battery is false?

a. Electrodes react chemically with the electrolyte.

b. Electrodes either give up or accept electrons.

c. Electrodes are either positive or negative.

-->  d. Electrodes are either a paste or a liqui

Copper electrical wires have a plastic coating because

a. plastic is a very good electric insulator.

b. plastic offers a lot of resistance to electric current.

c. current flows through the material with the least resistance.

-->  d. all of the above

Assume that a wire has 1.5 ohms of resistance. If the wire is connected to two 1.5-volt batteries, how much current will flow through the wire?

a. 3.0 amps

b. 2.3 amps

-->  c. 2.0 amps

d. 1.0 amps

Current flowing through a battery-powered flashlight is alternating current.

a. true

-->  b. false

An electric charge has potential energy because of its position.

-->  a. true

b. false

Electric charges always move from lower to higher potential energy.

a. true

-->  b. false

Electric current is a continuous flow of electric charges.

-->  a. true

b. false

Car batteries contain wet cells.

-->  a. true

b. false

Greater resistance always results in more current.

a. true

-->  b. false

Both dry cells and wet cells work the same basic way.

-->  a. true

b. false

Solar cells contain a material that absorbs electrons and gives off light.

a. true

-->  b. false

An electric generator changes kinetic energy to electrical energy.

-->  a. true

b. false

A wider wire has more resistance than a narrower wire.

a. true

-->  b. false

A battery with six electrodes contains six chemical cells.

a. true

-->  b. false

Electric current can travel through matter or across space.

a. true

-->  b. false

Current always travels through the material with the greatest resistance.

a. true

-->  b. false

Greater voltage results in more current.

-->  a. true

b. false

Ohms law states the relationships among current, voltage, and resistance.

-->  a. true

b. false

material that has low resistance to the flow of electric current

a. alternating current

b. electric insulator

c. electric current

d. direct current

-->  e. electric conductor

f. voltage

g. resistance

electric current that flows in only one direction

a. alternating current

b. electric insulator

c. electric current

-->  d. direct current

e. electric conductor

f. voltage

g. resistance

opposition to the flow of electric current

a. alternating current

b. electric insulator

c. electric current

d. direct current

e. electric conductor

f. voltage

-->  g. resistance

material that has high resistance to the flow of electric current

a. alternating current

-->  b. electric insulator

c. electric current

d. direct current

e. electric conductor

f. voltage

g. resistance

any continuous flow of electric charges due to a difference in voltage

a. alternating current

b. electric insulator

-->  c. electric current

d. direct current

e. electric conductor

f. voltage

g. resistance

electric current that keeps reversing the direction in which the current flows

-->  a. alternating current

b. electric insulator

c. electric current

d. direct current

e. electric conductor

f. voltage

g. resistance

difference in electric potential energy between two positions

a. alternating current

b. electric insulator

c. electric current

d. direct current

e. electric conductor

-->  f. voltage

g. resistance

For an electric charge to move from one position to another, there must be a difference between the two positions in

a. electrical conductivity.

-->  b. electric potential energy.

c. kinetic energy of particles.

d. resistance to electric charges.

Sources of voltage include

a. generators.

b. solar cells.

c. chemical cells.

-->  d. all of the above

Batteries produce electrical energy by

a. moving electrodes.

-->  b. chemical reactions.

c. thermal transfer.

d. nuclear fusion.

Which of the following materials is an electric conductor?

-->  a. copper

b. rubber

c. plastic

d. wood

An example of an electric insulator is

a. steel.

b. water.

-->  c. dry air.

d. aluminum.

Properties that affect the resistance of a material include its

a. width.

b. length.

c. temperature.

-->  d. all of the above

If a wire with a resistance of 4 ohms is connected to a 12-volt battery, how much current is flowing through the wire?

a. 48 amps

b. 16 amps

c. 4 amps

-->  d. 3 amps

diagram questions

No diagram questions associated with this lesson