Vern J. Ostdiek Donald J. Bord Chapter 7 Electricity (Section 4)

Vern J. OstdiekDonald J. Bord

Chapter 7Electricity(Section 4)

7.4 Electric Circuits and Ohm’s Law

• An electric current will flow in a lightbulb, a radio, or other such device only if an electric field is present to exert a force on the charges.

• A flashlight works because the batteries produce an electric field that forces electrons to flow through the lightbulb.


• An electric circuit is any such system consisting of a battery or other electrical power supply, some electrical device such as a lightbulb, and wires or other conductors to carry the current to and from the device.


• The power supply acts like a “charge pump”: • it forces charges to flow out of one terminal, go

through the rest of the circuit, and flow into the other terminal.

• Electrons typically move through a circuit quite slowly, about 1 millimeter per second.

• In this respect, an electric circuit is much like the cooling system in a car in which the water pump forces coolant to flow through the engine, radiator, and the hoses connecting them.


• The concepts of energy and work are used to quantify the effect of a power supply in a circuit. In a flashlight, for instance, the batteries cause electrons to flow through the bulb’s filament.

• Because a force acts on the electrons and causes them to move through a distance, work is done on the electrons by the batteries. • In other words, the batteries give the electrons

energy. • This energy is converted into internal energy and

light as the electrons go through the lightbulb.


• This leads to the concept of electric voltage. Voltage The work that a charged particle can do divided by the size of the charge.

• The energy per unit charge given to charged particles by a power supply.

• The SI unit of voltage is the volt (V), which is equal to 1 joule per coulomb.

• Voltage is measured with a device called a voltmeter.


• A 12-volt battery gives 12 joules of energy to each coulomb of electric charge that it moves through a circuit. • Each coulomb does 12 joules of work as it flows

through the circuit.



• If we return to the analogy of a battery as a charge pump, the voltage plays the role of pressure. • A high voltage causing charges to flow in a circuit is

similar to a high pressure causing a fluid to flow.


• Even when the circuit is disconnected from the power supply and there is no charge flow, the power supply still has a voltage. • In this case, the electric charges have potential

energy. • Voltage is also referred to as electric potential.


• The size of the current that flows through a conductor depends on its resistance and on the voltage causing the current. • Ohm’s law, named after its discoverer, Georg Simon

Ohm, expresses the exact relationship. • Ohm’s Law: The current in a conductor is equal to

the voltage applied to it divided by its resistance:

• The units of measure are consistent in the two equations:

• if I is in amperes and R is in ohms, then V will be in volts.


• By Ohm’s law, the higher the voltage for a given resistance, the larger the current.

• The larger the resistance for a given voltage, the smaller the current. • By applying different sized voltages to a given

conductor, one can produce different-sized currents.


• A graph of the voltage versus the current will be a straight line with a slope that is equal to the conductor’s resistance. • Reversing the polarity of the voltage (switching the

“–” and “+” terminals) will cause the current to flow in the opposite direction.

7.4 Electric Circuits and Ohm’s LawExample 7.1

• A lightbulb used in a 3-volt flashlight has a resistance equal to 6 ohms. • What is the current in the bulb when it is switched

on? • By Ohm’s law,


• A small electric heater has a resistance of 15 ohms when the current in it is 2 amperes.• What voltage is required to produce this current?


• Not all devices remain “ohmic”—that is, obey Ohm’s law—as the voltage applied to them changes. • Often, instead of remaining constant, the resistance

of a conductor changes when the voltage changes. • At higher voltages, a larger current flows through

the filament of a lightbulb, so its temperature is also higher. • The resistance of the hotter filament is

consequently greater.



• Some semiconductor devices, called diodes, are designed to have very low resistance when current flows through them in one direction but very high resistance when a voltage tries to produce a current in the other direction.

• Water with salt dissolved in it generally has lower resistance when higher voltages are applied to it: • doubling the voltage will more than double the

current. A graph of V versus I for ordinary tap water is less steep at higher voltages.


• Many electrical devices are controlled by changing a resistance.

• The volume control on a radio or a television simply varies the resistance in a circuit. • Turning up the volume reduces the resistance, so

more current flows in the circuit, resulting in louder sound.

• A dimmer control used to change the brightness of the lights in a room works the same way.

7.4 Electric Circuits and Ohm’s LawSeries and Parallel Circuits

• In many situations, several electrical devices are connected to the same electrical power supply. • A house may have a hundred different lights and

appliances all connected to one cable entering the house.

• An automobile has dozens of devices connected to its battery.

• There are two basic ways in which more than one device can be connected to a single electrical power supply—• by a series circuit and by a parallel circuit


• In a series circuit, there is only one path for the charges to follow, so the same current flows in each device.


• In such a circuit, the voltage is divided among the devices: • the voltage on the first device plus the voltage on

the second device, and so on, equals the voltage of the power supply.

• For example, if three lightbulbs with the same resistance are connected in series to a 12-volt battery, the voltage on each bulb is 4 volts. • If the bulbs had different resistances, each one’s

“share” of the voltage would be proportional to its resistance.


• Notice that the current in a series circuit is stopped if any of the devices breaks the circuit.


• A series circuit is not normally used with, say, a number of lightbulbs because if one of them burns out, the current stops and all of the bulbs go out. • A string of Christmas lights that flash at the same

time uses a series circuit so that all the bulbs go on and off together.


• In a parallel circuit, the current through the power supply is “shared” among the devices while each has the same voltage.


• The current flowing in the first device plus the current in the second device, and so on, equals the current output by the power supply. • There is more than one path for the charges to

follow—in this case, three. • If one of the devices burns out or is removed, the

others still function. • The lightbulbs in multiple-bulb light fixtures are in

parallel so that if one bulb burns out, the others remain lit.

• Often, the two types of circuits are combined: • one switch may be in series with several lightbulbs

that are in parallel.


• Three lightbulbs are connected in a parallel circuit with a 12-volt battery. The resistance of each bulb is 24 ohms.• What is the current produced by the battery?

• The voltage on each bulb is 12 volts. Therefore, the current in each bulb is

• The total current supplied by the battery equals the sum of the currents in the three bulbs.


• The concept of voltage is quite general and is not restricted to electrical power supplies and electric circuits.

• Whenever there is an electric field in a region of space, a voltage exists because the field has the potential to do work on electric charges. • The strength of an electric field can be expressed in

terms of the voltage change per unit distance along the electric field lines.


• For example, air conducts electricity when the electric field is strong enough to ionize atoms in the air.

• The minimum electric field strength required for this to happen is between 10,000 and 30,000 volts per centimeter, depending on the conditions. • This means that if there is a spark one-fourth of an

inch long between your finger and a doorknob, the voltage that causes the spark is at least 7,500 volts.


• As transistors and other components on integrated circuit chips (ICs) are made smaller, even the low voltages that are used to make them operate (typically around 1 volt) produce very strong electric fields.• Inside modern ICs, electric field strengths can

reach 400,000 V/cm. • Designers of ICs must keep this in mind because

electric fields only about 25 percent stronger than this can disrupt circuit processes.

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Vern J. Ostdiek Donald J. Bord Chapter 7 Electricity (Section 4)