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Exam #1

Exam #1. Attention: Midterm test #1 will be given in Lecture class on Wednesday October 5 starting at noon. One hour long For students requiring extra time: Please contact the Learning Assistance Services Fill the form, let me sign it and arrange the time for your test

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Exam #1

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  1. Exam #1 • Attention: Midterm test #1 will be given in Lecture class on Wednesday October 5 starting at noon. • One hour long • For students requiring extra time: • Please contact the Learning Assistance Services • Fill the form, let me sign it and arrange the time for your test • If you have a conflict with a religious holiday – alternative time – Monday 10/3 – 3pm • Sign up after class Lecture VI

  2. Work to move a charge How much work has to be done by an external force to move a charge q=+1.5 mC from point a to point b? Work-energy principle + 30cm + 20cm 15cm 25cm + - Q1=10mC Q2=-20mC Lecture VI

  3. Determine E from V • Think ski slopes • If V depends on one coordinate x • E is directed along x from high V to low • If V depends on x,y,z Lecture VI

  4. E near metal sphere • Find the largest charge Q that a conductive sphere radius r=1cm can hold. • Air breakdown E=3x106V/m Near surface: E=V/rLarger spheres can hold higher voltage Lecture VI

  5. Capacitance Physics 122 Lecture VI

  6. Concepts • Primary concepts: • Capacitor and capacitance • Storing electric energy and charge • Dielectrics Lecture VI

  7. Laws • Charge on a capacitance • Capacitance formula • Capacitance with dielectric • Energy in a capacitance Lecture VI

  8. Capacitance • Two parallel plates are called a capacitor. When capacitor is connected to a battery plates will charge up. Note: net charge =0 • C – coefficient, called capacitance, property of the capacitor. • Capacitance is measured in Farad (F=C/V) Lecture VI

  9. Electric field in a capacitor E=const • E= V/d • points from high potential to low • When V is fixed (same battery), E depends only on the d. • Potential • High next to + plate • Low – next to - plate Lecture VI

  10. Capacitance • Capacitance depends on the geometry of a capacitor • e0 = 8.85. 10-12 C2/N m2- • permittivity of free space • A – area of plates (m2) • Same sign charges want to “spread out” – to hold more charge need large area • d – distance between plates (m) • Opposite sign charges “hold” each other, attraction is stronger for shorter d A d Lecture VI

  11. Dielectrics • Put non-conductive material (dielectric) between plates • Can hold more charge  capacitance increases • K(>1) – dielectric constant Lecture VI

  12. Charging up a capacitor • Find the work needed to charge a capacitor C to voltage V • Take small charge dq and move it across the capacitor, which is at voltage V at this moment • dW=Vdq Lecture VI

  13. Energy storage • Work to charge a capacitor= potential energy stored in the capacitor • To use the right formula, watch what is kept constant • V=const – if C connected to a battery • Q=const - if C disconnected Lecture VI

  14. Inserting dielectric • Capacitor is connected to a battery, supplying voltage V. How will the energy stored in the capacitor change if we insert a dielectric (K=2)? • CKC=2C – capacitance increases V stays const – same battery Q changes Lecture VI

  15. Inserting dielectric • Capacitor is charged to charge Q and disconnected from a battery. How will the energy stored in the capacitor change if we insert a dielectric (K=2)? • CKC=2C – capacitance increases V can change Q stays const – charge conservation Lecture VI

  16. Inserting dielectric Disconnected battery – energy decreases Dielectric will be “sucked in” Connected battery – energy increases Dielectric will be “pushed out” Lecture VI

  17. Test problem • Between two very large oppositely charged parallel plates at which of the three locations A, B and C electric potential is the greatest? • A A • B B • C C • D Equal at all three locations. Lecture VI

  18. Capacitor connections No DC current through capacitor, Just store charge. • Three capacitors connected to a battery in Parallel: • Same voltage at capacitors C1, C2, C3 • V1=V2=V3=V • Different charges: • Q1=C1V • Q2=C2V • Q3=C3V • Total charge stored on three capacitors: • Q=Q1+Q2+Q3=V(C1+C2+C3) • Ceq=C1+C2+C3 Lecture VI

  19. Capacitor connections • Three capacitors connected in series • Same charge on each capacitor • (QA=0  -Q1+Q2=0) • Q1=Q2=Q3 • Different voltages V=V1+V2+V3 Lecture VI

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