BYU s EV1 Electric Dragster Powered by a bank of 160 ultracapacitors. http://newsnet.byu.edu/story.cfm/44205 http://www.youtube.com/watch?v=fi_g2nja3_s
BYU s EV1 Electric Dragster A bank of 160 ultra-capacitors at C = 2700 F and m = 500 g each wired in series and charged to V T = 400 V. C T 160 1 1 1 C C 160 2700 160 16.88 F Why in series??? V V T 160 U m 400 160 2.5 V 8438 J 0.5 kg 17 J/g 1 2 2 U CV 2 U T 0.5(2700)(2.5) 160U 1.35 MJ 8438 J U T Suppose the capacitors had been wired in series. 1 U 2 0.5(160*2700)(400) 2 T CVT 34.6 GJ 430 2 m Escaping Earth s gravity only requires 50 kj/g!!! T kj/g
BYU s EV1 Electric Dragster After a 15 second sprint, the capacitor voltage drops to half of its starting value (200 V). Find the engine resistance (R). V ( t) V e i t / RC R RC C 21.64 16.88 ln V i t / V ( t) 1.28 15 ln(2) 21.64 s If the car mass is M =1000 kg, find the final velocity of the car after a 15 second sprint. 1 2 3 K U ( 2 T CT VTi VTf ) ( UT ) i 0.75(1.35 MJ) 1MJ 2 4 K 1 2 2 Mv v 2K / M 2(1MJ) /(1000 kg) 100 miles/hour 44.7 m/s
The switch closes at t = 0. After a long time, the current in the 2 resistor is: (A) zero (B) 2 A (C) 3 A (D) 3.6 A The voltage across the capacitor will be: (A) zero (B) 4 V (C) 6 V (D) 1.8 V If the switch is then opened again, the time constant of decay will be: (A) 1 s (B) 2 s (C) 3 s (D) 4 s In this case, the initial discharge current will be: (A) zero (B) 1.3 A (C) 2 A (D) 3 A
What time will this be? A. just after the switch is closed B. A long time after the switch is closed. What time will this be? A. just after the switch is closed B. A long time after the switch is closed.
If R2 is 0.5 Ω, what is the sign of the V in part A? A. + B. - C. Zero Quick Quiz: pp If R2=0.5Ω what direction is the current through the branch with the cap in part a? A. to the right, B. to the left C. Zero.
1Ω 1Ω 4Ω 2Ω How does the circuit discharge in part b, if the left side of the ca capacitor is +? Thought question. What is R effective? A. 0 to 1Ω, B. 1 to 2 Ω C. 2 to 4Ω D. more Part b. Assume R2 =1Ω, which side will have more current through it. A. Top or B. bottom?
Hint: This is a good time to draw diagrams into your Chap. Summaries. BTW Current is NEVER used up. Today: Pay attention to signs of quantities and directions of current flow. A. B. C. Circuits with RC elements. The 2 nd (voltage) Kirchhoff s Laws will be critical.
One source (a transformer) delivers power to many nearby homes wired in parallel. 120 V electrical outlets usually have three wires: (1) a live or hot supply (black). (2) a neutral return (white). Household Wiring (3) a safety ground (green or bare copper) http://www.wikipedia.org
Household Wiring The live wire is at 120 V relative to earth ground. The neutral wire is earth grounded at the transformer. The safety ground wire is earth grounded below house. Breaker box Appliances Ground
Not connected directly to neutral return wire. Directly earth grounded below the house. Attached to conducting parts of electrical device that someone might touch. Never carries current by design, only by accident! Safety Ground Wire
Electrocution (ma scale) http://ehs.okstate.edu/modules/electric/index.htm Damage depends on: (1) magnitude of current, (2) length of time, (3) body part touching live wire, (4) current path through body.
Open vs. Short Circuit An open circuit is when a connection is broken, so that no current can flow. Annoying but not generally dangerous. I = 0 A short circuit occurs when the supply and return leads make contact without passing through a resistive load. A very large current produces heat, sparks and possibly fire. I Fuses and circuit breakers limit the maximum short-circuit current.
Exam 3: 22 Mc and one HWS problem. What direction do Dipoles Point?
Things to do. Add diagrams to your CS. Dipoles. RC time decay and increase of voltages, charges and currents etc.
Be able to do Resistivity to resistance and vice versa problem. Crossed capacitor Multiple resistors or capacitors. See the summary on line.
Current Measurement (Ammeter) Measures current. Placed in series with circuit elements. Ideally, it should have no resistance, so as not to alter the current. Typically only works over a limited range.
Voltage Measurement (Voltmeter) Measures potential diff. Placed in parallel with a circuit element. Ideally, it should have infinite resistance, so as not to draw any current. Typically only works over a limited range.
Main component in analog meters for measuring I and V. Current flows through a coil to become an electromagnet, which experiences a springbalanced torque in the field of two permanent magnets. Torque and deflection are proportional to the current. Galvanometer
Non-zero R g Galvanometer as (ammeter) Galvanometer as (voltmeter) Non-infinite R g I g I p V s V g I R p I V R p R g R s R g I I p I g R R g p V V s V g R R s g