* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Module 4, Lecture 4: Electric Current and Resistance Electric
Survey
Document related concepts
Surge protector wikipedia , lookup
Nanogenerator wikipedia , lookup
Galvanometer wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Giant magnetoresistance wikipedia , lookup
Power MOSFET wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Lumped element model wikipedia , lookup
Current source wikipedia , lookup
Rectiverter wikipedia , lookup
Superconductivity wikipedia , lookup
Thermal runaway wikipedia , lookup
Current mirror wikipedia , lookup
Transcript
Electric Current Direction Module 4, Lecture 4: Electric Current and Resistance Current – any motion of ___________ from one region to another. By convention, current is defined as flowing from + to – ____ - standard symbol used for current. dQ I dt Current Current density out Current units: _________ Current density units: ______ in Electrons actually flow in the ______ direction I J A 2 We will use the conventional current flow (which was established well before the discovery of electrons). DC - ______________ current – always in the same direction AC - ______________ current – continuously changes directions EF 152 Lecture 4-4 1 Lecture 4-4 Resistivity and temperature Resistance For many materials, the current is proportional to the potential (V) V IR A material in which V = is said to follow “_______ Law” V Ohm A : _____________ R EF 152 L : ___________ A L: _____________ Resistivity at Room Temperature Material Resistivity increases with increasing temperature (·m) 1.47 x 10-8 1.72 x 10-8 2.75 x 10-8 20 x 10-8 Conductors Graphite Silicon 3.5 x 2300 10-5 : Insulators Wood Glass Teflon Lecture 4-4 Semiconductors R R0 1 T Semiconductors temperature coefficient of resistance A copper wire increases temperature by 20°C. What is the increase in resistance? 108 – 1011 1010 – 1014 > 1013 A: _____________ EF 152 Resistivity decreases with increasing temperature Conductors Silver Copper Aluminum Steel 3 EF 152 Lecture 4-4 At temperatures below the resistivity is zero Superconductor Material α (1/°C) Conductors Silver Copper Aluminum Steel 3.8 3.9 3.9 5.0 x x x x 10-3 10-3 10-3 10-3 Semiconductors Germanium Silicon -50 x 10-3 -70 x 10-3 4 Energy and power in circuits Examples: P Vab I An 18 gauge lamp cord copper wire has a diameter of 1.02 mm and a cross sectional area of 8.17x10-7m2. Determine the resistance of a 15 ft (4.57 m) length of wire. 1.72 10 Power delivered to a resistor Units of Watts (W) 1 W = Ω · m AC vs. DC AC power is easier to _____ A typical circuit breaker will be tripped if more than 20 amps pass through it. For a 120 V circuit, determine the minimum resistance to keep the circuit breaker from tripping. 3000 W generator; 2Ω wire Volt Amp Loss 100 1000 EF 152 Lecture 4-4 5 EF 152 Lecture 4-4 Electromotive Force Example: A ___________ circuit is required for a steady current flow. Determine the power dissipated in the 4 Ohm resistor. Electromotive force (emf, ) causes current flow from lower to higher potential. emf is not really a force – it is an energy/charge quantity like potential. 6 Sources of emf: ________________________ Determine the current flow in the circuit. Two light bulbs operate at 120 V, but one has a power rating of 25 W while the other has a power rating of 100 W. Which one has the greater resistance? EF 152 Lecture 4-4 7 EF 152 Lecture 4-4 8