Chapter 23 Electric Current (cont.)

Transcription

1 Chapter 23 Electric Current (cont.) Direct current and alternating current Electric power Electric circuits 1

2 Direct Current and Alternating Current Pulsating dc Time Time graphs of dc. Electric current may be dc or ac. By dc, we mean direct current, which refers tot he flowing of charges in one direction. Even if the current occurs in unsteady pulses, so long as electrons move in one direction only, it is dc. A battery produces direct current (dc) in a circuit because the terminals of the battery always have the same sign. 2

3 Alternating Current Time graphs of ac. By ac, we mean alternating current: electrons in the circuit are moved first in one direction and then in the opposite direction, alternating to and fro about relatively fixed positions. This is accomplished by alternating the polarity of voltage at the generator or other voltage source. 60-hertz current: nearly all commercial ac circuits in North America involve voltages and currents that alternate back and forth at a frequency of 60 cycles per second (60 -hertz). 3

4 Converting ac to dc Household current is ac. The current in a battery-operated device such as a pocket calculator is dc. You can operate these devices on ac instead of batteries with an ac-dc converter. The ac-dc converter uses a diode. Diode: a tiny electronic device that acts as a one-way valve to allow electron flow in only one direction. A diode is represented by the symbol. Current flows in the direction of the arrow but not in the reverse direction. a. When input to a diode is ac, b. Output is pulsating dc. 4

5 Electric Power The power and voltage on the light bulb read 100 W 120 V. How many amperes of current will flow through the bulb? The rate at which electric energy is converted into another form such as mechanical energy, heat, or light is called electric power. Electric power is equal to the product of current and voltage. Power = current voltage Units of current: amperes Units of voltage: volts Units of electric power: watts For example, if a lamp rated at 120 watts operates on a 120-volt line, it will draw a current of 1 ampere. How about a 60-watt lamp on a 120-volt line? 5

6 Electric Circuits Circuit: any path along which electrons can flow is a circuit. For a continuous flow of electrons, there must be a complete circuit with no gaps. A gap is usually provided by an electric switch that can be opened or closed to either cut off or allow energy flow. Devices are commonly connected in a circuit in one of two ways: Series or Parallel. 6

7 Series Circuits Series circuit: when connected in series, the devices in the circuit form a single pathway for electron flow between the terminals of the battery or a generator. Important characteristics of a series circuit Electric current has but a single pathway through the circuit. Thus the current passing through each electrical device is the same. The voltage, or potential difference, across each device is proportional to its resistance. The total voltage across the circuit is the sum of all voltage across individual devices. Ohm s law applies to each individual devices. Main disadvantage of a series circuit: If one device (e.g. a bulb) fails, current in the whole circuit ceases. 7

8 Parallel Circuits Parallel circuits: when connected in parallel, devices in the circuit form branches, each of which is a separate path for the flow of electrons. A break in any one path does not interrupt the flow of charge in the other paths. Each device operates independently of the other devices. Major characteristics of parallel circuits: Each device connects the same two points A and B of the circuit and the voltage is the same across each device. The total current in the circuit divides among the parallel branches. The amount of current in each branch is inversely proportional tot he resistance of the branch. The total current in the circuit equals the sum of the currents in its parallel branches. Ohm s law applies to each individual devices. 8

9 Some Examples Alternating current is normally produced by a A) battery. B) generator. C) both of these. D) neither of these. On some early automobiles both headlights went out when one bulb burned out. The headlights must have been connected in A) parallel. B) series. The electric power of a lamp that carries 2A at 120 V is A) 1/6 watts. B) 2 watts. C) 60 watts. D) 20 watts. E) 240 watts. The current through two identical light bulbs connected in series is 0.25 A. The voltage across both bulbs is 110 V. The resistance of a single light bulb is A) 22 ohms. B) 44 ohms. C) 220 ohms. D) 440 ohms. E) none of these. 9

10 Homework 11 (04/07/04) Chapter 23, P. 456, Exercises: 24, 43, 44,