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

2. 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.

3. 7.4 Electric Circuits and Ohm’s Law 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.

4. 7.4 Electric Circuits and Ohm’s Law 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.

5. 7.4 Electric Circuits and Ohm’s Law 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.

6. 7.4 Electric Circuits and Ohm’s Law 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.

7. 7.4 Electric Circuits and Ohm’s Law 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.

8. 7.4 Electric Circuits and Ohm’s Law

9. 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.

10. 7.4 Electric Circuits and Ohm’s Law 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.

11. 7.4 Electric Circuits and Ohm’s Law 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.

12. 7.4 Electric Circuits and Ohm’s Law 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.

13. 7.4 Electric Circuits and Ohm’s Law 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.

14. 7.4 Electric Circuits and Ohm’s Law Example 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,

15. 7.4 Electric Circuits and Ohm’s Law Example 7.2 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?

16. 7.4 Electric Circuits and Ohm’s Law 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.

17. 7.4 Electric Circuits and Ohm’s Law

18. 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.

19. 7.4 Electric Circuits and Ohm’s Law 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.

20. 7.4 Electric Circuits and Ohm’s Law Series 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

21. 7.4 Electric Circuits and Ohm’s Law In a series circuit, there is only one path for the charges to follow, so the same current flows in each device.

22. 7.4 Electric Circuits and Ohm’s Law 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.

23. 7.4 Electric Circuits and Ohm’s Law Notice that the current in a series circuit is stopped if any of the devices breaks the circuit.

24. 7.4 Electric Circuits and Ohm’s Law 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.

25. 7.4 Electric Circuits and Ohm’s Law In a parallel circuit, the current through the power supply is “shared” among the devices while each has the same voltage.

26. 7.4 Electric Circuits and Ohm’s Law 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.

27. 7.4 Electric Circuits and Ohm’s Law Example 7.3 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.

28. 7.4 Electric Circuits and Ohm’s Law 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.

29. 7.4 Electric Circuits and Ohm’s Law 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.

30. 7.4 Electric Circuits and Ohm’s Law 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.