Presentation on theme: "Static charges will move if potential difference and conducting path exists between two points Electric field due to potential difference creates force."— Presentation transcript:
Static charges will move if potential difference and conducting path exists between two points Electric field due to potential difference creates force on charges
In solids, moving charges are electrons In liquids and gases, both positive and negative ions can move Electrolyte: substance whose aqueous solution conducts electric current Positive charge moving one direction is equivalent to negative charge moving in opposite direction
Rate of flow of electric charge through a cross section of a conductor Unit is ampere (A or amp); 1A = 1C/s Ampere is a fundamental unit of SI system Electrons flow from negative to positive Conventional current is flow of positive charges from positive to negative (thank you Ben Franklin)
Electrons are pushed by electric field established in conductor Electrons possess thermal velocity ~ 10 6 m/s, causes random collisions with atoms Speed due to electric field much less ~ 10 -4 m/s, called drift speed Collisions create resistance to flow of charge
Due to collisions of conduction electrons with atoms Unit is ohm ( ); 1 ohm = 1V/1A Circuit elements designed to provide measured amounts of resistance called resistors
Resistance of uniform conductor directly proportional to its length, inversely proportional to its cross sectional area Resistance increases when temperature increases for most metals Resistance depends on nature of the material: the resistivity (rho) has units of ohm cm; R = l/A
Low resistivity materials called conductors; most metals High resistivity materials called insulators; nonmetals In between are semiconductors: Si, Ge, B, Se; can act as conductors or insulators under certain circumstances
Most conductors have a wide temperature range where resistance is constant—called ohmic because they obey Ohm’s Law (I=V/R) Resistance increases at high temperature— light bulbs are non-ohmic Resistance of many semiconductors varies directly with temp.—digital thermometers
Discovered by Onnes (1908) while investigating low temp conductivity Resistance drops suddenly to zero at critical temperature Critical temp for most materials is a few kelvins, but newer composite materials found with superconductivity at higher temperatures
Practical uses include MRI machines, levitating, high speed trains, research
Current causes injury, not voltage Currents can be high if skin conductivity is high -- wet or salty Must be a potential difference for current to flow -- connection to high voltage not dangerous unless path to ground exists Grounded (3 wire) and polarized plugs help prevent shocks
DC: direct current, flow of charge in one direction only – batteries AC: alternating current, electrons vibrate back and forth; don’t actually flow through circuit In USA, current alternates between +120 V to -120V at 60 Hz AC can be transmitted for long distances with little loss due to heat
Often need DC for electronic devices (stereos, computers, TV, etc.) Diode acts as one way valve turning AC into pulsed DC 2 or more diodes together can provide smoother DC Capacitors also used to smooth out DC signal
Charged capacitor can discharge and move charges but only until potential on plates is equal For continuous current, need sustained potential difference and closed conducting path or circuit Work must be done on charges to maintain potential difference; called emf; Unit: volt; symbol: script E
If 1.5 C of charge passes a point in 5 s, what is the current? What is the symbol for current in an equation? What is the symbol for the resistance unit? What electronic component can act as a one- way valve to convert AC to DC?
Which type of current is produced by a battery? What 4 factors determine the resistance of a wire? Which wire will have the least resistance?
Circuit current is determined by emf (voltage) of source and resistance in circuit. where V is source voltage, I is source current and R T is total resistance in circuit Internal resistance of battery must be included in total resistance V = IR gives voltage drop across any resistance element in circuit
Emf source does work on charges Charges then do work on circuit components: resistors, bulbs, motors, etc. One coulomb of charge moved through potential difference of one volt equals one joule of work done, energy increase also 1J
W = qV = VIt (since q = It) For one electron moved through 1 volt, unit of work/energy is electron volt (eV) 1 eV = 1.60 x 10 -19 J
Work done on resistance by current appears as heat; can be desirable (oven, iron, heater) or not (motor, light, computer) Since resistance always present in normal circuits, some energy lost due to heat Joule’s Law: Q = I 2 Rt Use to calculate heat produced by resistance and current over a time period
Since power is work or energy transferred/time, P = VI For a resistive element, P = I 2 R power dissipated in a resistance If current is not known, P = V 2 /R For total power in circuit, use V of battery or other source and R T (total resistance) of circuit for R
Energy sold in kilowatt-hours, a unit of energy (power x time) 1 kW-hr means device used 1000 watts of power for one hour To minimize power loss in transmission lines, high voltages and fairly low currents used
If the resistance connected to a battery is reduced, what will happen to the current? If a 6V battery produces 2A of current, what is the resistance? If 10A of current passes through a resistance of 6 , what is the rate that electrical energy is converted to heat?
If a motor draws 10A of current from a 12V battery, what is the resulting power? Name three units used to express energy. When current passes through resistance, what is produced?
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