Chapter 22 Current Electricity

1. Chapter 22Current Electricity An electric circuit is formed when a conductive path is created to allow free electrons to continuously move. The continuous movement of free electrons through the conductors of a circuit is called an electric current I. The flow of positive charge is called conventional current. The rate of flow of electric charge, or electric current, I, is measured in coulombs per second. A flow of one coulomb per second is called an ampere, A. 1A = 1C/s The force motivating electrons to "flow" in a circuit is called voltage. Voltage is a specific measure of potential energy that is between two points. When we speak of voltage in a circuit, we are referring to the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point If net charge q passes by a point a in a conducting wire in time t, the current Ia at that point is: Ia = q/t

2. Power measures the rate at which energy is transferred. If a generator transfers one joule of kinetic energy to electric energy each second, it is transferring energy at the rate on one joule/s, or one watt. The power or energy delivered to the motor per second, is represented by the equation: Power P=IV

3. Example Problem A 6-V battery delivers a 0.50-A current to an electric motor that is connected across its terminals. What power is consumed by the motor? If the motor runs for 5.0 minutes, how much electric energy is delivered?

4. The property that determines how much current will flow is called the resistance, R. It is measured by placing a potential difference across two points on a conductor and measuring the current, it is defined as: I is the electric current in amperes, V is the potential difference in volts, and R is the resistance of the conductor measured in ohms (?). 1 ? is the resistance that permits a current of 1 A to flow when a potential difference of 1 V is applied across the resistance. Resistance and Ohm�s Law

5. The amount of current in a circuit depends on the amount of voltage available to motivate the electrons, and also on the amount of resistance in the circuit to oppose electron flow.

6. V/I is constant for a given conductor, therefore the device that has this constant resistance obey Ohm�s Law. Most metallic conductors obey Ohm�s law, but many devices do not obey Ohm�s Law, for example a transistor radio or pocket calculator contain transistors and diodes, that do not obey Ohm�s law. Wires used to connect electric devices have small resistance, (e.g 0.03 ? or 0.004 ? used in house wiring).

7. Resistors are devices designed to have a specific resistance. They are made of long, thin wires; graphite; or semiconductors. Are used to control the current in circuits or parts of circuits. Superconductors are materials that have zero resistance. It can conduct electricity without loss of energy.

8. A lamp dimmer switch allows continuous change in light intensity. It uses a variable resistor called a rheostat, or potentiometer. It consists of coil of resistance wire and sliding contact point. Resistance of the circuit changes by moving the contact to different areas of the wire. The light output of a lamp can be adjusted from bright with little wire in the circuit to dim with a lot of wire in the circuit. This type of device controls the speed of electric fans and electric mixers.

9. Rheostat

10. If enough current is present through your body, your breathing or heart can stop. Also the energy transferred can burn you. Wet skin have less resistance (1500 ?) than dry skin (105 ?), and the currents can rise to dangerous levels. (1.2 mA versus 80 mA)

11. Example problem Current through a Resistor A 30.0 V battery is connected to a 10.0 ? resistor. What is the current in the circuit?

12. Symbols of electric circuit

13. Diagramming Circuits

15. 22.2 Using Electric Energy An electric lamp changes electric energy into light. A motor converts electric energy to mechanical energy. Some energy is lost in incandescent bulbs and motors as they get hot. A space heater, a hot plate, and hair dryer are designed to convert almost all the electric energy into thermal energy. Power (P = E/t) is the rate at which energy is converted from one form to another Electric power dissipated (thermal energy) in a resistor is proportional to the square of the current that passes though it and to the resistance. P= IV = I (IR) = I2R The total energy that will be converted to thermal energy is E = Pt = I2Rt. To reduce this loss of energy, the current, I, or the resistance, R, must be reduced. Cables of high conductivity and large diameter (heavy and expensive) are used, as well as the current in the transmission lines is kept low.

16. Since P = IV, the current can also be reduced by increasing the voltage. Some long-distance lines use voltages of 500 000 volts. The resulting lower current reduces the I2R loss in the lines by keeping the I2 factor low. The output voltage from the generating plant can be reduced upon arrival at electric substations to 2400 V and again to 240 V or 120 V before use at home.

17. Example Problem Thermal Energy Produced by an Electric Current. A heater has a resistance of 10.0 O. It operates on 120.0 V. A. What is the current through the resistance? B. What thermal energy is supplied by the heater in 10.0 s?

18. The Kilowatt-Hour Electric companies are often called power companies, they really provide energy.

19. A kilowatt-hour from a power company costs about 12 cents. One kilowatt-hour supplied by dry cell batteries would cost hundreds of dollars! The electric energy (J�s/s = J) used by any device = rate of energy consumption in joules per second (watts)� number of seconds it is operated. 1J = watt-second (small energy unit) Kilowatt-hour = 1000 watts delivered continuously for 3600 seconds (1 hour) 1kWh= 1000 J/s�3600 s = 3.6 � 106 J

20. Example Problem The cost of Operating an Electric Device A television set draws 2.0 A when operated on 120 V. How much power does the set use? If the set is operated for an average of 7.0 h/day, what energy in kWh does it consume per month (30 days)? At 11� per kWh, what is the cost of operating the set per month?

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