Chapter 23 Electric Current

1. Chapter 23Electric Current Physics 1 (Garcia) SJSU

3. Physics 1 (Garcia) SJSU

4. recall that, when the ends of a conducting material are at different temperatures, heat energy flows from the higher temperature to the lower temperature • The flow ceases they reach the same temperature. • potential difference—charge flows from one end to the other as long as there is a difference in electrical potential. • Without a potential difference, no charge flows. • Connect a wire to a Van de Graaff generator and the other end to the ground • The flow will be brief, because they quickly reach a common potential with the ground.

5. . • Electric potential is measured in volts and acts like an “electrical pressure” that can produce a flow of charge, or current • Current (the rate of electrical flow) is measured in amperes (or, simply, amps, and abbreviated A) • The resistance - restrains this flow is measured in ohms (Ω).

6. One ampere is a rate of flow equal to 1 coulomb of charge per second. • (Recall that 1 coulomb, the standard unit of charge, is the electric charge of 6.25 billion billion electrons.) • In a wire that carries 5 amperes, 5 coulombs of charge pass any cross section in the wire each second. • In a wire that carries 10 amperes, twice as many electrons pass any cross section each second.

7. a current-carrying wire is not electrically charged. • Under ordinary conditions, negative conduction electrons swarm through the atomic lattice made up of positively charged atomic nuclei. • So there are as many electrons as protons in the wire. Whether a wire carries a current or not, the net charge of the wire is normally zero at every moment.

8. Flow • When the flow is in only one direction, we’ll call it direct current (dc); • when the flow is back and forth, we’ll call it alternating current (ac). • Electric current can deliver electric power, measured, just like mechanical power, in watts (W) or in thousands of watts, or kilowatts (kW).

9. Current Water flows from the reservoir of higher pressure to the reservoir of lower pressure; flow stops when the pressure difference ceases. Water continues to flow because a difference in pressure is maintained with the pump. Physics 1 (Garcia) SJSU

10. Electric Current Just as water current is flow of water molecules, electric current is the flow of electric charge. In circuits, electrons make up the flow of charge. ON OFF Physics 1 (Garcia) SJSU

11. Electrons make up the flow of charge. conduction electrons=charge carriers, one or more electrons from each metal atom are free to move throughout the atomic lattice. Protons do not move because they are bound inside the nuclei of atoms that are more or less locked in fixed positions. In conducting fluids—such as in a car battery—positive ions typically compose the flow of electric charge

12. Ammeter Ammeter Demo: Ammeter Ammeter measures electrical current. Current increases as the voltage increases. Due to charge conservation, same current into and out of light bulb. Light Bulb CURRENT Power Supply Ammeter readings always the same. (adjustable voltage) Galvanometer is an ammeter with both positive & negative Physics 1 (Garcia) SJSU

13. Voltage Sourcesprovide a difference in electrical voltage- an electrical pumpBatteries & Generators Charges flow only when they are “pushed” or “driven.” A sustained current requires a suitable pumping device to provide a difference in electric potential—a voltage. Aluminum Lemon Copper Simple Chemical Battery Simple Mechanical Generator Physics 1 (Garcia) SJSU

14. Batteries and electric generators do work to pull negative charges away from positive ones. • In chemical batteries, this work is usually, but not always, done by the chemical disintegration of zinc or lead in acid, and the energy stored in the chemical bonds is converted to electric potential energy

15. Chemical Battery Batteries separate positive and negative charges by using a chemical reaction. Chemical potential energy is converted into electrical energy.

16. Generators, such as alternators in automobiles, separate charge by electromagnetic induction • The work done by whatever means in separating the opposite charges is available at the terminals of the battery or generator. • These different values of energy per charge create a difference in potential (voltage). • voltage provides the “electrical pressure” to move electrons through a circuit joined to these terminals.

17. Rechargeable Battery Eventually the battery’s chemicals are consumed unless the reaction can be reversed by passing a current into the battery. Automobile battery is recharged while the gasoline engine is running since the engine powers a generator that produces a recharging current. Starting the car Engine running

18. We can distinguish between the ideas of charge flowing through a circuit and voltage placed, or impressed, across a circuit) by considering a long pipe filled with water. • Water will flow through the pipe if there is a difference in pressure across (or between) its ends. • Water flows from high-pressure to the low-pressure end. • Only the water flows, not the pressure.

19. Electric charge flows because of the differences in electrical pressure (voltage). • Charges flow through a circuit because of an applied voltage across the circuit. • You don’t say that voltage flows through a circuit. • Voltage doesn’t go anywhere, for it is the charges that move. • Voltage produces current (if there is a complete circuit).

20. Resistance • a) In a hydraulic circuit, a narrow pipe (green) offers resistance to water flow. • (b) In an electric circuit, a lamp or other device (shown by the zigzag symbol for resistance) offers resistance to electron flow.

21. Electrical Resistance Current depends not only on the voltage but also on the electrical resistance of the conductor. The wider the wire is, the less the resistance. A short wire offers less resistance than a long wire. More water flows through a thick hose than through a thin one connected to a city's water system (same water pressure). Jumper cables use thick wires so as to minimize the electrical resistance.

22. Electrical Resistance • Electrical resistance is measured in ohms. The Greek letter omega, Ω, is the symbol • Georg Simon Ohm, a German physicist -1826, discovered simple and important relationship among voltage, current, and resistance • Superconductors • The resistance of some materials reaches zero at very low temperatures.

23. Ohm’s Law Relation between current, voltage, and resistance is Ohm’s law, (Current) = Ampere is unit of current; symbol is A Volt is unit of voltage; symbol is V Ohm is unit of resistance; symbol is  The symbol of resistance in an electric circuit is . (Voltage) (Resistance)

24. How much current is drawn by a lamp that has a resistance of 60 Ω when a voltage of 12 V is impressed across it? 1/5 A. This is calculated from Ohm’s law: 12 V/60 Ω = 0.2 A

25. What is the resistance of an electric frying pan that draws 12 A when connected to a 120-V circuit? 10 Ω. Rearrange Ohm’s law to readResistance = voltage/current = 120 V/12 A = 10 Ω

26. Ohm’s law tells us that a potential difference of 1 volt established across a circuit that has a resistance of 1 ohm will produce a current of 1 ampere • If a voltage of 12 volts is impressed across the same circuit, the current will be 12 amperes. • The resistance of a typical lamp cord is much less than 1 ohm, while a typical lightbulb has a resistance of more than 100 ohms. • Remember that, for a given potential difference, less resistance means more current. • In the interior of computers and tv receivers, current is regulated by circuit elements called resistors, whose resistance may be a few ohms or millions of ohms.

27. Lab: Ohm’s Law Measuring voltage, current, and resistance in simple circuits to verify Ohm’s law. Ammeter Galvanometer Battery Resistor Physics 1 (Garcia) SJSU

28. Jacob’s Ladder Electrical resistance through air is greater for a larger gap. Biblical Jacob’s Ladder Physics Jacob’s Ladder Physics 1 (Garcia) SJSU

29. Ammeter Demo: Resistance of Water Pure water has very high resistance; impurities, such as salt, lower resistance. Light Bulb When salt dissolves the sodium and chlorine atoms are charged (ions). These mobile charges carry the current in the water. CURRENT Power Supply Salty Water Physics 1 (Garcia) SJSU

30. Ohm’s Law and Electric Shock • Current is a flow of charge, pressured into motion by voltage and hampered by resistance. • The damaging effects of shock are the result of current passing through the body which depends both on the voltage that is applied and on the electrical resistance of the human body.

31. The resistance of one’s body • ranges from about 100 ohms if it is soaked with salt water • to about 500,000 ohms if the skin is very dry. • We usually cannot feel the current produced by 12 volts, and 24 volts just barely tingles. If our skin is moist, 24 volts can be quite uncomfortable.

32. Electric Shock The damaging effects of shock are the result of current passing through the body. Effects of Electric Shock on Human Body From Ohm's law, current depends on the voltage and on electrical resistance. When dry, skin’s resistance around 100,000 . Resistance drops as low as 100  when wet and salty. Physics 1 (Garcia) SJSU

33. The bird can stand harmlessly on one wire of high potential, but it had better not reach over and grab a neighboring wire! Why not? Physics 1 (Garcia) SJSU

34. A grounded plug • Any charge that builds up on an appliance is therefore conducted to the ground, thereby preventing accidental shock.

35. Electric shock can overheat tissues in the body and disrupt normal nerve functions. • It can upset the rhythmic electrical patterns that maintain proper heartbeat, and it can upset the nerve center that controls breathing. • In rescuing shock victims, the first thing to do is to locate and turn off the power source. Then do CPR until help arrives. • For heart-attack victims, on the other hand, a properly administered electric shock can sometimes be beneficial in getting the heartbeat started again

36. Nervous System Nervous systems in animals use electrical currents to signal the contraction and relaxation of muscles. Frog leg jumps when electrical current passes through it.

37. Conduction in Human Heart The most important electrical signal in our body is the periodic signal that contracts and relaxes our heart muscle to pump blood. Without a constant flow of blood the brain can suffer permanent damage. SA AV

38. Conduction in Human Heart The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial (SA) node of the heart to be propagated to (and stimulate) the myocardium (muscle of the heart). When the myocardium is stimulated, it contracts, pumping blood in the body. As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the Atrioventricular (AV) node. The AV node functions as a critical delay in the conduction system. Without this delay, the atria and ventricles will contract at the same time, and blood won't flow effectively from the atria to the ventricles. SA AV Physics 1 (Garcia) SJSU

39. Check Yourself • What causes electric shock—current or voltage? • Electric shock occurs when current is produced in the body, which is caused by an impressed voltage. So the initial cause is the voltage, but it is the current that does the damage.

40. Direct & Alternating Current Direct current (DC) is current that flows in only one direction. Alternating current (AC) is current that flows back and forth with alternating direction. Physics 1 (Garcia) SJSU

41. DC/AC • direct current in a circuit • Electrons move from the repelling negative terminal toward the attracting positive terminal, always moving through the circuit in the same direction. • Even if the current occurs in unsteady pulses, so long as electrons move in one direction only, it is dc. • Most residential and commercial circuits are ac because electric energy in the form of ac can easily be stepped up to high voltage for long-distance transmission with small heat losses, then stepped down to convenient voltages where the energy is consumed

42. Why does the US & Europe have different standard voltage? • US- prior to 1900, 110 volts was the standard b/c bulbs glowed as bright as gas lamps, but would blow out bulbs if higher • By the time electricity became popular in Europe, engineers had figured out how to manufacture light bulbs that would not burn out so fast at higher voltages. • Power transmission is more efficient at higher voltages, so Europe adopted 220 volts as their standard. Physics 1 (Garcia) SJSU

43. DC vs. AC Easy to produce small DC currents using batteries, which also have low voltages. For major power lines, less ohmic heating if high voltage AC current is used instead of DC.

44. Converting ac to dc Diodes. As the symbol suggests, current flows in the direction of the arrow but not in the reverse direction • You can operate devices on ac instead of batteries with an ac–dc converter. • the converter uses a diode, a tiny electronic device that acts as a one-way valve to allow electron flow in one direction only • Since alternating current changes its direction each half cycle, current passes through a diode only half of each period. • The output is a rough dc, and it is off half the time. To maintain continuous current while smoothing the bumps, a capacitor is used

45. a capacitor acts as a storage reservoir for charge • Just as it takes time to raise the water level in a reservoir when water is added, it takes time to add or remove electrons from the plates of a capacitor • . A capacitor, therefore, produces a retarding effect on changes in current flow. • It opposes changes in voltage and smooths the pulsed output. Physics 1 (Garcia) SJSU

46. Speed and Source of Electrons in a Circuit • When we make a noncellular telephone call, the electrical signal carrying our voice travels through connecting wires at seemingly infinite speed. This signal is transmitted through the conductors at nearly the speed of light. •  It is not the electrons that move at this speed. • electrons inside metal at room temperature have an average speed of a few million kilometers per hour, they make up no current because they are moving in all possible directions. There is no net flow in any preferred direction.

47. When a battery or generator is connected, an electric field is established inside the conductor. • The electrons continue their random motions while simultaneously being nudged by this field. • It is the electric field that can travel through a circuit at nearly the speed of light. The conducting wire acts as a guide or “pipe” for electric field lines

48. Electrons in Conductors Electrons in a wire are in constant, rapid, but random motion. With direct current the electrons slowly drift down the wire. With alternating current the electrons slosh back and forth. Without Voltage With Voltage although an electric signal travels at nearly the speed of light in a wire, the electrons that move in response to this signal actually travel slower than a snail’s pace.

49. Power utilities do not sell electrons. They sell energy. You supply the electrons. • Energy is carried by the pulsating electric field and causes vibratory motion of the electrons that already exist in the lamp filament

50. common misconception 1 • Electrons that are free to move in a conductor are accelerated by the electric field impressed upon them, but not because they bump into one another. True, they do bump into one another and other atoms, but this slows them down and offers resistance to their motion.