PHYS 1444 – Section 003 Lecture #10AnnouncementsElectric CurrentExample 25 – 1Direction of the Electric CurrentOhm’s Law: Resistance and ResistorsOhm’s Law: ResistanceExample 25 – 3Ohm’s Law: ResistorsOhm’s Law: Resistor ValuesResistivityExample 25 – 4Example 25 – 5Temperature Dependence of ResistivityElectric PowerSlide 16Example 25 – 7Power in Household CircuitsExample 25 – 10Monday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu1PHYS 1444 – Section 003Lecture #10Monday, Oct. 3, 2005Dr. Jaehoon Yu•Electric Current•Ohm’s Law: Resistance and Resistor•Resistivity•Electric Power•Power in Household CircuitsToday’s homework is homework #6, due noon, next Tuesday!!Monday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu2Announcements•First term exam next Wednesday, Oct. 12–Time: 1 – 2:20 pm–Location: SH103–Coverage: CH. 21 – 25–Style: Mixture of multiple choice and numeric problems–There will be two more exams•Monday, Nov. 7, and Monday, Dec. 5–Two best of the three will be chosen for your final grading •Reading Assignment–CH25 – 6Monday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu3Electric Current•When a circuit is powered by a battery (or a source of emf) the charge can flow through the circuit.•Electric Current: Any flow of charge–Current can flow whenever there is potential difference between the ends of a conductor (or when the two ends have opposite charges)•The current can flow even through the empty space–Electric current in a wire can be defined as the net amount charge that passes through the wire’s full cross section at any point per unit time (just like the flow of water through a conduit…)–Average current is defined as:–The instantaneous current is:–What kind of a quantity is the current?I Q t=D DI dQ dt=Unit of the current?C/s1A=1C/sIn a single circuit, conservation of electric charge guarantees that the current at one point of the circuit is the same as any other points on the circuit.ScalarMonday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu4Example 25 – 1 Current is the flow of charge: A steady current of 2.5A flows in a wire for 4.0min. (a) How much charge passed by any point in the circuit? (b) How many electrons would this be? Current is total amount charge flow through a circuit in a given time. So from we obtain The total number of electrons passed through the circuit is Q I tD = DQ I tD = D =eN =2.5 4.0 60 600C� � =QeD=21196003.8 101.6 10CelectronsC-= ��Monday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu5Direction of the Electric Current•What do conductors have in abundance?–Free electrons•What happens if a continuous loop of conducting wire is connected to the terminals of the battery?–Electrons start flow of through the wire continuously as soon as both the terminals are connected to the wire. How?•The potential difference between the battery terminals sets up an electric field inside the wire and parallel to it•Free electrons in the conducting wire get attracted to the positive terminal•The electrons leaving negative terminal flow through the wire and arrive at the positive terminal–Electrons flow from negative to positive terminal–Due to historical convention, the direction of the current is opposite to the direction of flow of electrons Conventional CurrentMonday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu6Ohm’s Law: Resistance and Resistors•What do we need to produce electric current?–Potential difference•Georg S. Ohm experimentally established that the current is proportional to the potential difference ( )–If we connect a wire to a 12V battery, the current flowing through the wire is twice that of 6V, three times that of 4V and four times that of 3V battery.–What happens if we reverse the sign of the voltage?•It changes the direction of the current flow•Does not change the magnitude of the current–Just as in water flow case, if the height difference is large the flow rate is large If the potential difference is large, the current is large.I V�Monday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu7Ohm’s Law: Resistance•The exact amount of current flow in a wire depends on–The voltage–The resistance of the wire to the flow of electrons•Just like gunk in water pipe slows down water flow•Electrons are slowed down due to interactions with the atoms of the wire•The higher the resistance the less the current for the given potential difference V–So how would you define resistance?•So that current is inversely proportional to the resistance–Often it is rewritten as–What does this mean?•The metal conductor’s resistance R is a constant independent of V.–This linear relationship is not valid for some materials like diodes, vacuum tubes, transistors etc. These are called non-ohmic VRI=V IR=Ohm’s LawUnit?ohmsW1.0 1.0 /V AW=Monday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu8Example 25 – 3 Flashlight bulb resistance: A small flashlight bulb draws 300mA from its 1.5V battery. (a) What is the resistance of the bulb? (b) If the voltage drops to 1.2V, how would the current change?From Ohm’s law, we obtain If the resistance did not change, the current is R =I =Would the current increase or decrease, if the voltage reduces to 1.2V? VI=1.5300VmA=1.55.00.3VA= WVR=1.20.24 2405.0VA mA= =WMonday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu9Ohm’s Law: Resistors•All electric devices offer resistance to the flow of current.–Filaments of light bulbs or heaters are wires with high resistance to cause electrons to lose their energy in the wire–In general connecting wires have low resistance compared to other devices on the circuit•In circuits, resistors are used to control the amount of current–Resistors offer resistance of less than one ohm to millions of ohms–Main types are•“wire-wound” resistors which consists of a coil of fine wire•“composition” resistors which are usually made of semiconductor carbon•thin metal films•When drawn in the circuit, the symbol for a resistor is:•Wires are drawn simply as straight linesMonday, Oct. 3, 2005 PHYS 1444-003, Fall 2005Dr. Jaehoon Yu10Ohm’s Law: Resistor Values•Resistors have its resistance color-coded on its body•The color-coding follows the convention below: Color Number Multiplier ToleranceBlack 0 1=100Brown 1 101Red 2
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