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Admin: • Assignments: • Fifth assignment complete. • No assignment this week (need to get through some more material) • Labs and discussion sections run as usual this week (introduction to the oscilloscope) • No labs in the week after spring break. • Test feedback? I will be grading this week. • Was it a) too long or b) OK • Was it a) too difficult or b) OK Copyright © 2009 Pearson Education, Inc. 3i1-i2-2iSC=8 -i1+9i2-3iSC=0 -2i1-3i2+5iSC=0 http://www.math.odu.edu/~bogacki/cgi-bin/lat.cgi?c=sys Copyright © 2009 Pearson Education, Inc. Electric field Force on a charge Charge density Electric field due to an infinitely large conducting charged plate Copyright © 2009 Pearson Education, Inc. Two parallel plates. There is an electric field between two large parallel plates or sheets, which are very thin and are separated by a distance d which is small compared to their height and width. One plate carries a uniform surface charge density σ (=Q/A) and the other carries a uniform surface charge density -σ as shown (the plates extend upward and downward beyond the part shown). Where is this heading?... Capacitors (Chapter 24) Copyright © 2009 Pearson Education, Inc. 24-1 Capacitors A capacitor consists of any two conductors that are close but not touching. A capacitor has the ability to store electric charge. Copyright © 2009 Pearson Education, Inc. 24-1 Capacitors Parallel-plate capacitor connected to battery. (b) is a circuit diagram. Copyright © 2009 Pearson Education, Inc. • What is the current through the capacitor? • What is the voltage across the capacitor? 24-2 Determination of Capacitance For a parallel-plate capacitor as shown, the field between the plates is E = Q/ε0A. Integrating along a path between the plates gives the potential difference: Vba = Qd/ε0A. This gives the capacitance: Copyright © 2009 Pearson Education, Inc. E = Q/ε0A Copyright © 2009 Pearson Education, Inc. So Vba = Qd/ε0A. When a capacitor is connected to a battery, the charge on its plates is proportional to the voltage: The quantity C is called the capacitance Capacitance is the ability of an object to store an electrical charge. Unit of capacitance: the farad (F): Copyright © 2009 Pearson Education, Inc. 1 F = 1 C/V. 24-2 Determination of Capacitance Example 24-1: Capacitor calculations. (a)Calculate the capacitance of a parallel-plate capacitor whose plates are 20 cm × 3.0 cm and are separated by a 1.0-mm air gap. (b) What is the charge on each plate if a 12-V battery is connected across the two plates? (c) What is the electric field between the plates? (d) Estimate the area of the plates needed to achieve a capacitance of 1 F, given the same air gap d. Copyright © 2009 Pearson Education, Inc.