Tuesday, Feb. 22, 2011 1PHYS 1444-02 Dr. Andrew BrandtPHYS 1444 – Section 02Lecture #9• Chapter 24• Chapter 25Tuesday Feb 22, 2011Dr. Andrew BrandtHW4 Ch 24 due Sat Feb. 26thMar 3 will be ½ period reviewHW5 Ch 25 will be due Fri. Mar. 4March 4: deadline to receive credit for late HW so solutions can be posted***Test 1 will be Tues. Mar. 8 on ch 21-25***• Dielectrics• Electric Current • Resistance• The Battery• Ohm’s Law: Resistors• Resistivity• Electric PowerTuesday, Feb. 22, 2011 2• What happens to the electric field within a dielectric?• Without a dielectric, the field is– What are V0and d?• V0: Potential difference between the two plates• d: separation between the two plates• For the constant voltage case, the electric field remains the same• For the constant charge: the voltage drops to V=V0/K, thus the field in the dielectric is– Reduced. Effect of a Dielectric Material on Field 00VEd00DVEVEEd dK K0DEEKPHYS 1444-02 Dr. Andrew BrandtTuesday, Feb. 22, 2011 3PHYS 1444-02 Dr. Andrew BrandtMolecular Description of Dielectric• So what makes dielectrics behave the way they do?• We need to examine this on a microscopic scale.• Let’s consider a parallel plate capacitor that is charged up +Q(=C0V0) and –Q with air in between.– Assume that charge cannot flow in or out• Now insert a dielectric– Dielectrics can be polar may have a permanent dipole moment. – Due to the electric field molecules may be aligned.Tuesday, Feb. 22, 2011 4PHYS 1444-02 Dr. Andrew BrandtMolecular Description of Dielectric• OK. Then what happens?• Then effectively, there will be some negative charges close to the surface of the positive plate and positive charges close to the negative plate– Some electric field does not pass through the whole dielectric but stops at the negative charge– So the field inside dielectric is smaller than the air• Since electric field is smaller, the force is smaller– The work need to move a test charge inside the dielectric is smaller– Thus the potential difference across the dielectric is smaller than across the airTuesday, Feb. 22, 2011 5PHYS 1444-02 Dr. Andrew BrandtExample 24 – 8Dielectric Removal: A parallel-plate capacitor, filled with a dielectric with K=3.4, is connected to a 100-V battery. After the capacitor is fully charged, the battery is disconnected. The plates have area A=4.0m2, and are separated by d=4.0mm. (a) Find the capacitance, the charge on the capacitor, the electric field strength, and the energy stored in the capacitor. (b) The dielectric is carefully removed, without changing the plate separation nor does any charge leave the capacitor. Find the new value of capacitance, electric field strength, voltage between the plates and the energy stored in the capacitor.(a) CQAd0KAd212 2 2 834.03.4 8.85 10 3.0 10 304.0 10mC N m F nFmEUCV863.0 10 100 3.0 10 3.0F V C CVd431002.5 104.0 10VVmm212CV28413.0 10 100 1.5 102F V J6PHYS 1444-02 Dr. Andrew BrandtSince the charge is the same ( ) before and after the removal of the dielectric, we obtainExample 24 – 8 cont’d(b) 0C0QQSince the dielectric has been removed, the effect of dielectric constant must be removed as well. 0E0U0VWhere did the extra energy come from?.The law of energy conservation is violated perhaps??External force has done work of 3.6x10-4J on the system to remove dielectric!!Wrong! Wrong! Wrong!CK212 2 2 934.08.85 10 8.8 10 8.84.0 10mC N m F nFm0QCK Q CKV3.4 100 340VV0Vd433408.5 10 844.0 10VV m kV mm20012CV212CKVK212KCVKU443.4 1.5 10 5.1 10JJTuesday, Feb. 22, 2011Tuesday, Feb. 22, 2011 7PHYS 1444-02 Dr. Andrew BrandtElectric Current and Resistance• So far we have been studying electrostatics:– The charges have been at rest• Now we will learn electrodynamics– Charges in motion• What is the electric current?– A flow of electric charge– Examples of things that use electric current?• In an electrostatic situation, there is no electric field inside a conductor but when there is current, there is a field inside a conductor– Electric field is needed to keep charges movingTuesday, Feb. 22, 2011 8PHYS 1444-02 Dr. Andrew BrandtThe Electric Battery• What is a battery?– A device that produces electrical energy from the stored chemical energy and produces electricity.• Electric battery was invented by Volta in 1790s in Italy– It was made of disks of zinc and silver based on his research that certain combinations of materials produce a greater electromotive force (emf), or potential, than others• Simplest batteries contain two plates made of dissimilar metals, electrodes– Electrodes are immersed in a solution, electrolyte– This unit is called a cell and many of these form a battery• Zinc and Iron in the figure are called terminalsTuesday, Feb. 22, 2011 9PHYS 1444-02 Dr. Andrew BrandtHow does a battery work?• One of the electrodes in the figure is zinc and the other carbon• The acid electrolyte reacts with the zinc electrode and dissolves it.• Each zinc atom leaves two electrons on the electrode and enters into the solution as a positive ion the zinc electrode acquires negative charge and the electrolyte becomes positively charged• The carbon electrode picks up the positive charge• Since the two terminals are oppositely charged, there is a potential difference between themTuesday, Feb. 22, 2011 10PHYS 1444-02 Dr. Andrew BrandtHow does a battery work?• When the terminals are not connected, a certain amount of zinc is dissolved into the solution establishing an equilibriumcondition.• How is a particular equilibrium potential maintained?– If the terminals are not connected:• the zinc electrode becomes negatively charged up to the equilibrium pint• zinc ions then recombine with the electrons in the zinc electrode• Why does battery go dead? – When the terminals are connected, the negative charges will flow away from the zinc electrode– More zinc atoms dissolve into the electrolyte to produce more charge– One or more electrodes get used up stopping the flow of chargeTuesday, Feb. 22, 2011 11PHYS 1444-02 Dr. Andrew BrandtElectric Current• When a circuit is powered by a battery (or a source of emf), charge can flow through the circuit.• Electric Current: Any flow of charge– Current can flow whenever there is potential difference between the ends of a conductor (or when the two ends have opposite charges)– Electric current in a
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