Chapter 23 Electric Current (cont.)
· Direct current and alternating current · Electric power · Electric circuits 04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 1 Direct Current and Alternating Current
· Electric current may be dc or ac. · By dc, we mean direct current, which refers tot he flowing of charges in one direction.
Pulsating dc Time Time graphs of dc. ­ 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. 04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 2 Alternating Current
· 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). Time graphs of ac. 04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 3 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.
04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 4 Electric Power
· 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 The power and voltage ­ Units of voltage: volts on the light bulb read ­ Units of electric power: watts "100 W 120 V". How many amperes of · For example, if a lamp rated at 120 watts current will flow operates on a 120-volt line, it will draw a through the bulb? current of 1 ampere. How about a 60-watt lamp on a 120-volt line?
04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 5 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.
04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 6 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.
04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 7 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.
04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 8 · 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.
Dr. Jie Zou PHY 3050G Department of Physics 9 · · · 04/07/04 Homework 11 (04/07/04)
· Chapter 23, P. 456, Exercises: 24, 43, 44, 45. 04/07/04 Dr. Jie Zou PHY 3050G Department of Physics 10