Electricity

ElectricityChapter 191Electric Circuits19.12ElectricityElectric current is created by the motion of electrons through materialsElectric circuits are complete pathways for current to flow Closed Circuit a complete loop of wire which allows current flowOpen Circuit an incomplete or open loop in which current cannot flow.Short Circuit an accidental, extra, easier pathway for current to flow

3Circuit Diagrams

4Circuit Components/SymbolsBatteryDiode

ResistorBulb

CapacitorGround

SwitchIntegrated Circuit

Transistor

5Current and Voltage19.26Electric Potential -- VoltageVoltage is electric potential energy, measured in volts. DIFFERENCES in the electric potential between two points is what makes electrons move and created current.Current is the amount of charge flowing per secondmeasured in Amperes. An Ampere is a very large amount of charge so we usually deal with milli-, micro- and nano-Amperes.7PotentialThe electric potential V, or just the potential, is the potential energy per unit charge

The units of potential are J/C1 J/C 1 V8Potential DifferenceThe potential difference between points A and B is defined as the change in the potential energy (final value minus initial value) of a charge q moved from A to B divided by the size of the charge V = VB VA = PE / q0The potential difference V between two points A and B is the work done per unit charge against the field in moving a unit positive charge q0 from A to B with no acceleration Potential difference is not the same as potential energy9Potential Difference, cont.Another way to relate the energy and the potential difference: DPE = q DVBoth electric potential energy and potential difference are scalar quantitiesUnits of potential differenceV = J/C

10Example 1A small sphere carrying a positive charge of 10.0 mC is moved against an E-field through a potential difference of +12.0 V. How much work was done by the applied force in raising the potential of the sphere?Given: q0 = 10.0 mC and V = +12.0 VFind: W11Example 1Solution: Problem Type - Electrostatics/potential difference; this example is about potential, and its definition in terms of work.The defining relationship between V, q0, and W is Eq. (16.3)(VB VA) = W(A B)/q

W= Vq0 = ( + 12.0V)(10.0X 10-6C) = 120 mJ12Energy and Charge MovementsA positive charge gains electrical potential energy when it is moved in a direction opposite the electric fieldA negative charge loses electrical potential energy when it moves in the direction opposite the electric fieldIf a charge is released in the electric field, it experiences a force and accelerates, gaining kinetic energyAs it gains kinetic energy, it loses an equal amount of electrical potential energy13Energy and Charge MovementsWhen the electric field is directed downward, point B is at a lower potential than point AA positive test charge that moves from A to B loses electric potential energyIt will gain the same amount of kinetic energy as it loses in potential energy

14Summary of Positive Charge Movements and EnergyWhen a positive charge is placed in an electric fieldIt moves in the direction of the fieldIt moves from a point of higher potential to a point of lower potentialIts electrical potential energy decreasesIts kinetic energy increases15Summary of Negative Charge Movements and EnergyWhen a negative charge is placed in an electric fieldIt moves opposite to the direction of the fieldIt moves from a point of lower potential to a point of higher potentialIts electrical potential energy increasesIts kinetic energy increasesWork has to be done on the charge for it to move from point A to point B

16Measuring VoltageA Multimeter measures voltage/current, depending upon its setting.

A reading of +1.5V means that the electric potential at the RED probes location is 1.5V HIGHER than that at the BLACK probes location17Meters in a Circuit Voltmeter A voltmeter is used to measure voltage (potential difference)Connects to the two ends of the bulb

18Creating VoltageBatteries create potential differences (voltages) through chemical reactions. There are MANY types:Primary Batteries NOT rechargeableGrocery-Store Batteries Zinc-Carbon zinc casing with a carbon rod embedded in an electrolyte pasteAlkaline CellsTransistor CellsLantern BatteriesMiniature Batteries --Silver-OxideMercuryLithium19Creating VoltageSecondary Batteries ARE rechargeableLead-Acid car batteriesNickel-CadmiumNickel-Metal-HydridePhotovoltaic Solar CellsFuel Cells Hydrogen combines with oxygen to create energy and water. Methanol can also be used20BatteriesTwo different solid conductors immersed in an active solution (electrolytes) function as a batteryChemical energy stored in interatomic bonds is converted into electrical-PE as the solution and the electrodes become involved in the chemical reactionA potential difference that can be used to supply energy and sustain a current in an external circuit is called an electromotive force emfThe emf is the voltage measured across the terminals of a source when no current is being drawn from or delivered to it21Cells in SeriesVoltage across the series-connected battery is the sum of the voltages across each constituent cell+++1.5 V cells1.5 V3.0 V4.5 V++Voltages AddVoltages SubtractA steady-state current can only exist in a closed circuit, and the same current flows in and out of the load. I is a direct current.For cells in series the current remains unchanged as it passes in and out of each elementV = V1 + V2 + V3I = I1 = I2 = I312322Cells in ParallelCells attached in parallel form a battery whose voltage is the same as the individual voltages but whose current capacity is the sum of the individual current outputs.LOAD1.5 V1.5 V1.5 V+++123V = V1 = V2 = V3I = I1 + I2 + I323Measuring CurrentUsing a multimeter on its current setting, current must be made to flow THROUGH the meter. Thus, using the multimeter as an Ammeter instead of a Voltmeter

24Meters in a Circuit Ammeter

An ammeter is used to measure currentIn line with the bulb, all the charge passing through the bulb also must pass through the meter25Electric CurrentWhenever electric charges of like signs move, an electric current is said to existThe current is the rate at which the charge flows through this surfaceLook at the charges flowing perpendicularly to a surface of area A

The SI unit of current is Ampere (A)1 A = 1 C/s

26Electric CurrentThe direction of the current is the direction positive charge would flowThis is known as conventional current directionIn a common conductor, such as copper, the current is due to the motion of the negatively charged electronsIt is common to refer to a moving charge as a mobile charge carrierA charge carrier can be positive or negative27Example 2A constant downward electron beam transports 3.20 mC of negative charge in 200 ms across the vacuum chamber of an electron microscope. Determine the beam current and the number of electrons traversing the chamber per second.given: q = 3.20 10-6 C and t = 200 10-3 sfind: I and the number of electrons per second 28Example 2Solution: Use the definition of current

The current is upward and equal to 1.60 x 10-5 C/s. The current transfers -1.60 x 10-5 C/s, and so the number of electrons transported per second, each with a charge of -1.60 x 10-5 C/s, is

16.0 mA

1.00 1014 electrons/s29Electrons in a CircuitThe drift speed is much smaller than the average speed between collisionsWhen a circuit is completed, the electric field travels with a speed close to the speed of lightAlthough the drift speed is on the order of 10-4 m/s the effect of the electric field is felt on the order of 108 m/s30Electrical Resistance and Ohms Law19.331ResistanceDefined as a measure of how easily (or NOT) current flows through a material.In a conductor, the voltage applied across the ends of the conductor is proportional to the current through the conductorThe constant of proportionality is the resistance of the conductorALL materials have SOME resistance, even wires and batteries, but this is so small we can ignore it.A multimeter can be set to measure resistance32ResistanceUnits of resistance are ohms (W)1 W = 1 V / AResistance in a circuit arises due to collisions between the electrons carrying the current with the fixed atoms inside the conductor**TOTAL resistance (R) in a circuit determines the current (I) given a certain voltage (V)**33Georg Simon Ohm1787 1854Formulated the concept of resistanceDiscovered the proportionality between current and voltages

34Ohms LawThe current in the circuit varies directly with voltage and inversely with resistance

This statement has become known as Ohms Law rewritten

Ohms Law is an empirical relationship that is valid only for certain materialsMaterials that obey Ohms Law are said to be ohmic

35Ohms LawAn ohmic deviceThe resistance is constant over a wide range of voltagesThe relationship between current and voltage is linearThe slope is related to the resistance

36Ohms LawNon-ohmic materials are those whose resistance changes with voltage or currentThe current-voltage relationship is nonlinearA diode is a common example of a non-ohmic device

37Example 3

What is the current through the depicted circuit?38Example 4A small ohmic light bulb is placed in series with two D-cells. The ammeter in series with the bulb reads the current in the circuit (0.50 A) without introducing any appreciable voltage drop across its own terminals. The voltmeter++

V = 3VI = A attached to the terminals of the bulb reads the voltage across it (3.0 V) without introducing any appreciable change in the current through the bulb. What is the resistance of the bulb? 39Example 4Given: At the bulb, V = 3.0 V and I = 0.50 AFind: RSolution: Use Ohm's Law. The total voltage across the bulb is the net voltage produced by the batteries, 1.5 V + 1.5 V.

6.0 W

40Example 5Suppose someone falling out of a tree grabs an overhead power line. The wire has a resistance of 60 microhms per meter and is carrying a dc current of 1000 amps. With hands a meter apart, what is the voltage across him? Will the unfortunate soul get much of a shock?Given: R/L = 60 mW/m and I = 1000 AFind: V across 1.0 m41Example 5Solution: Use Ohm's Law. The voltage drop across 1.0 m of the line isV = IR = (1000 A) (60 mW/m X 1.0 m)= 0.060 V = 60 mVYou know that you can handle the terminals of a 1.5-V dry cell without feeling any electricity, so 60 mV is much too small a voltage to push a detectable amount of current through a human body. 42Temperature Variation of ResistivityFor most metals, resistivity increases with increasing temperatureWith a higher temperature, the metals constituent atoms vibrate with increasing amplitudeThe electrons find it more difficult to pass through the atoms43Types of ResistorsFixed have well-defined an unchanging resistance values indicated by the color-codings on the outside

**5th band (if present) indicates 5 change in performance after 1000 hours of use**44Types of ResistorsVariable Potentiometers allows for smooth changes in the level of resistance in a circuit. Volume control on audio equipment is one application.

45InsulatorsInsulators are materials in which electric charges do not move freelyGlass and rubber are examples of insulatorsWhen insulators are charged by rubbing, only the rubbed area becomes chargedThere is no tendency for the charge to move into other regions of the material46ConductorsConductors are materials in which the electric charges move freely in response to an electric forceCopper, aluminum and silver are good conductorsWhen a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material47SemiconductorsThe characteristics of semiconductors are between those of insulators and conductorsSilicon and germanium are examples of semiconductors48SuperconductorsA class of materials and compounds whose resistances fall to virtually zero below a certain temperature, TCTC is called the critical temperatureThe graph is the same as a normal metal above TC, but suddenly drops to zero at TC

49SuperconductorsThe value of TC is sensitive to Chemical compositionPressureCrystalline structureOnce a current is set up in a superconductor, it persists without any applied voltageSince R = 050SuperconductorGood conductors do not necessarily exhibit superconductivityOne application is superconducting magnets

51