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Transcript
Chapter 20: Electric Current What is current? Wire connecting two charged plates. Q Plates get discharged quickly. +++++++++++++ E ----------- Bulb lights temporarily Recall: Charging a capacitor requires separating charges. Q Rate of flow of charge is current. I q t charge flowed. C ampere( A) time interval. s Conventional current is opposite to flow of electrons. To maintain current one needs to keep separating charge. That’s what a battery does and it maintains V. Think, Answer; Talk to your neighbors, Answer Consider the following four situations. i) An ion (charge +Q) moves to the right. ii) A neutral hydrogen atom (proton, +e, and light electron, -e) moves right. iii) A beam of electrons in a TV shoots right iv) In an ionic solution, massive positive ions flow right, light negative electrons flow with equal speed to the left. In which of these is the net current ZERO? A: All of the above B: ii only C: iv only D: ii and iv only E: Some other combo/?? Ohm’s law: Resistance A battery in action A battery separates charges through chemical process and maintains positive and negative terminal. The potential difference (V) between the terminals of an ideal battery is called emf . Wbattery = q V q Resistivity Resistance, R, of a wire depends on three things: 1. area, A 2. length, L In ideal wire electrons can roam freely. There is no change in potential along the wire. But in real wire there are hurdles. So the electrons loose potential energy while moving along a wire. Unit of R: Current through pot. difference. V V I a resistance, resistance R A Metal wires, Cu, Ag, etc. have negligible resistance. +++++++++ -------- 3. material, L (larger area smaller resistance) (longer the wire more resistance) Tungsten has 3 times more than copper. A Unit = ? L A Resistivity is an electric property of material. It doesn’t depend on area and length. R R If resistivity is zero, the material is called superconductor. Problem: What is the resistance of a 4 m long and 2 mm diameter copper wire? Resistivity of Cu = 1.8 x 10-8 m. 1 Magnetic Levitation and high-speed train Water flow analogy of Current flow I R V What role do superconductors play in the levitation? Single resistor Circuit Vt I Vt R Vb Vb Ground pot = 0 Voltage drops linearly from upper to lower part of the resistor. V Vt Vb Vt Same as battery voltage. Power and Energy in Circuits The energy dissipation rate is called power U q V I V t t For an EMF source: For a resistor: P I V I 2 R V 2 R Pump Think, Answer; Talk to your neighbors, Answer A battery with voltage ("emf") of 5V is attached to resistor R. (We choose VA=0.) What is the voltage at points B, C, and D? Vb Once the charge reaches the low potential, battery Connecting wire. Same does work to increase its voltage everywhere. potential. Ohm’s law relates current, voltage and resistance as, V I For ideal battery, V emf R P What are the analogous components? P I What is the resistance of 100 W rated bulb at 120 V? 50 W? A: VB = VD =0, VC = 5 B: VB = 5, VC < 5, VD> 0 C: VB = 5, VC < 5, VD= 0 D: VB = VC = 5, VD=0 E: None of these. Alternating Current Source and Power An electric generator produces oscillating emf or current. 2 BA sin t where, 2 f T t or, t m sin t m i t R t Oscillating t m sin t current: i t R R or, i t I sin t p t i t t I sin 2 t i t I T t t V and I are in phase in R. 2 Average Power and RMS Values i t i t I sin t I Current in a light bulb t reverses direction 120 times in a second. p (t ) IV sin 2 t I 2 R sin 2 t always positive. p t So the power reaches a maximum 120 times a sec P but bulb glows steadily. Makes more sense to use t average power : I 1 1 V P I 2 R IV I rmsVrms where, I rms and, Vrms 2 2 2 2 2 I rms R Think, Answer; Talk to your neighbors, Answer We say household outlet is 120 V. In fact, the voltage of this AC source varies; the 120 V is A. The peak value of the voltage. B. The rms value of the voltage. C. The minimum value of the voltage. D.The average value of the voltage. Think, Answer; Talk to your neighbors, Answer Power transmission lines uses very high voltage. The reason behind using high voltage is because A. it is faster to transmit using high voltage. B. it is safer to transmit using high voltage. C. energy loss is less to transmit using high voltage. Problem: A hair dryer dissipates 1160 W when plugged into 120 V outlet. (a) What is the amplitude of voltage coming out of the outlet? (b) What is the resistance of the hair dryer heating coil? (c) What is the rms and peak current through the dryer? (d) What is the maximum power the dryer dissipates? Household electricity Why three pronged plug? Electric outlet are grounded parallel Circuits. 120 V rms 3 Electric fence Up to 5 mA - No harm 10 to 20 mA - May cause paralysis 100 to 300 mA - May cause ventricular fibrillation Energy used in a Household: Kilowatt hours Energy consumed by a device, E Pt J or watt seconds Commonly used unit is larger i.e., kilowatt hours = 1 unit. Problem: One evening you used a 240 W laptop, a 1380 W space heater, two 120 W light bulbs for five hours. How much will it cost if your utility company charges 11 cents per unit? Ground Fault Interrupter Circuit In kitchen and bathrooms where floor is damp, current may pass through your body. The built in sensing circuit compares currents in the hot wire and neutral wire. If the current in those two wires differs by as little as 5 mA, the GFI disconnects the circuit. 4