Electric Current and Resistance Moving Charges n Static

Electric Current and Resistance

Moving Charges n Static charges will move if potential difference and conducting path exists between two points n Electric field due to potential difference creates force on charges

Moving Charges n In solids, moving charges are electrons n In liquids and gases, both positive and negative ions can move n Electrolyte: substance whose aqueous solution conducts electric current n Positive charge moving one direction is equivalent to negative charge moving in opposite direction

Electric Current n Rate of flow of electric charge through a cross section of a conductor n Unit is ampere (A or amp); 1 A = 1 C/s n Ampere is a fundamental unit of SI system n Electrons flow from negative to positive n Conventional current is flow of positive charges from positive to negative (thank you Ben Franklin)

Electron Drift Speed n Electrons are pushed by electric field established in conductor n Electrons possess thermal velocity ~ 106 m/s, causes random collisions with atoms n Speed due to electric field much less ~ 10 -4 m/s, called drift speed n Collisions create resistance to flow of charge

Resistance n Due to collisions of conduction electrons with atoms n Unit is ohm (W); 1 ohm = 1 V/1 A n Circuit elements designed to provide measured amounts of resistance called resistors

Resistance Laws n Resistance of uniform conductor directly proportional to its length, inversely proportional to its cross sectional area n Resistance increases when temperature increases for most metals n Resistance depends on nature of the material: the resistivity (rho) has units of ohm cm; R = r l/A

Range of Resistivities n Low resistivity materials called conductors; most metals n High resistivity materials called insulators; nonmetals n In between are semiconductors: Si, Ge, B, Se; can act as conductors or insulators under certain circumstances

Resistance and Temperature n Most conductors have a wide temperature range where resistance is constant—called ohmic because they obey Ohm’s Law (I=V/R) n Resistance increases at high temperature— light bulbs are non-ohmic n Resistance of many semiconductors varies directly with temp. —digital thermometers

Superconductivity n Discovered by Onnes (1908) while investigating low temp conductivity n Resistance drops suddenly to zero at critical temperature n Critical temp for most materials is a few kelvins, but newer composite materials found with superconductivity at higher temperatures

Superconductivity n Practical uses include MRI machines, levitating, high speed trains, research

Electric Shock n Current causes injury, not voltage n Currents can be high if skin conductivity is high -- wet or salty n Must be a potential difference for current to flow -- connection to high voltage not dangerous unless path to ground exists n Grounded (3 wire) and polarized plugs help prevent shocks

Direct and Alternating Current n DC: direct current, flow of charge in one direction only – batteries n AC: alternating current, electrons vibrate back and forth; don’t actually flow through circuit n In USA, current alternates between +120 V to 120 V at 60 Hz n AC can be transmitted for long distances with little loss due to heat

Converting AC to DC n Often need DC for electronic devices (stereos, computers, TV, etc. ) n Diode acts as one way valve turning AC into pulsed DC n 2 or more diodes together can provide smoother DC n Capacitors also used to smooth out DC signal

EMF n Charged capacitor can discharge and move charges but only until potential on plates is equal n For continuous current, need sustained potential difference and closed conducting path or circuit n Work must be done on charges to maintain potential difference; called emf; Unit: volt; symbol: script E

Quick Quiz n If 1. 5 C of charge passes a point in 5 s, what is the current? n What is the symbol for current in an equation? n What is the symbol for the resistance unit? n What electronic component can act as a oneway valve to convert AC to DC?

Quick Quiz n Which type of current is produced by a battery? n What 4 factors determine the resistance of a wire? n Which wire will have the least resistance?

Ohm’s Law n Circuit current is determined by emf (voltage) of source and resistance in circuit. n where V is source voltage, I is source current and RT is total resistance in circuit n Internal resistance of battery must be included in total resistance n V = IR gives voltage drop across any resistance element in circuit

Energy of Electric Current n Emf source does work on charges n Charges then do work on circuit components: resistors, bulbs, motors, etc. n One coulomb of charge moved through potential difference of one volt equals one joule of work done, energy increase also 1 J

Energy of Electric Current n W = q. V = VIt (since q = It) n For one electron moved through 1 volt, unit of work/energy is electron volt (e. V) n 1 e. V = 1. 60 x 10 -19 J

Energy and Resistance n Work done on resistance by current appears as heat; can be desirable (oven, iron, heater) or not (motor, light, computer) n Since resistance always present in normal circuits, some energy lost due to heat n Joule’s Law: Q = I 2 Rt n Use to calculate heat produced by resistance and current over a time period

Power in Electric Circuits n Since power is work or energy transferred/time, P = VI n For a resistive element, P = I 2 R power dissipated in a resistance n If current is not known, P = V 2/R n For total power in circuit, use V of battery or other source and RT (total resistance) of circuit for R

Power Companies n Energy sold in kilowatt-hours, a unit of energy (power x time) n 1 k. W-hr means device used 1000 watts of power for one hour n To minimize power loss in transmission lines, high voltages and fairly low currents used

345 - 750 k. V 345, 000 V 138 k. V 24 k. V 25, 000 V 5000 V 240 V 12 k. V

Quick Quiz n If the resistance connected to a battery is reduced, what will happen to the current? n If a 6 V battery produces 2 A of current, what is the resistance? n If 10 A of current passes through a resistance of 6 W, what is the rate that electrical energy is converted to heat?

Quick Quiz n If a motor draws 10 A of current from a 12 V battery, what is the resulting power? n Name three units used to express energy. n When current passes through resistance, what is produced?