LECTURE 1 CHARGE AND COULOMB’S LAW Lecture 1
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¨ Reading chapter 21-1 to 21-3. ¤ Electrical charge n Positive and negative charge n Charge quantization and conservation ¤ Conductors and insulators ¤ Induction ¤ Coulomb’s law Demo: 1
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¨ Charged rods on turntable ¤ Charge can be positive or negative. ¤ Like charges repel each other. ¤ Opposite charges attract each other. Charge quantization
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¨ The unit of charge is the coulomb (C). ¨ Charge is quantized which means it always occurs in an integer multiple of e = 1.602×10-19 C. ¨ Each electron has a charge of negative e. ¨ Each proton has a charge of positive e. ¨ Point of interest: ¤ Quarks can have smaller charges than an electron but they do not occur as free particles. ¤ The charge of an up quark is +2/3 e. ¤ The charge of a down quark is -1/3 e. ¤ A proton consists of 2 up quarks and 1 down quark, total charge +e. ¤ A neutron consists of 1 up quark and 2 down quarks, total charge 0. Conservation of charge
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¨ The total charge is conserved.
¨ When we charge a rod, we move electrons from one place to another leaving a positively charged object (where we removed electrons) and an equal magnitude negatively charged object (where we added electrons).
¨ Point of interest: + ¤ Detecting anti-neutrinos:νe + p → n + e ¤ The proton (p) and positron (e+) have the same positive charge. ¤ The anti-neutrino ( ν e ) and neutron (n) are not charged. Example: 1
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¨ How many electrons must be transferred to a body to result in a charge of q = 125 nC? Conductors and insulators
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¨ In a conductor charged particles are free to move within the object. ¤ Metals are good electrical conductors because typically the outermost electron (conduction electron) from the atom disassociates from a particular atom and is free to move about the metal.
¨ In an insulator charged particles are not free to move within the object.
¨ Semiconductors have properties intermediate between conductors and insulators; their properties change with their chemical composition.
¨ Photoconductive materials become conductors when light shines on them.
¤ Photocopier and laser printer use photoconductive material. Quiz: 1
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¨ Electroscope ¤ A needle electroscope has a part of the conductor (the needle) free to rotate. ¤ Because like charge repels, the needle is repelled from the rest of the conductor.
¨ Conducting and Non-conducting “T” ¤ Demonstration of a conductor (aluminum) and an insulator (plastic) using an electroscope Quiz: 2
10 Induction
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¨ If we bring a charged object next to a metal, the electrons in the metal will either ¤ move towards a positively charged object. ¤ move away from a negatively charged object.
¨ Due to conservation of charge, this must leave a positive charge where the electrons have moved from. Charging by induction
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¨ Induction spheres ¤ Demonstration of charging a conductor by induction Coulomb’s law
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¨ The force by charged object 1 on charged object 2 is given by
9 2 2 where q1 and q2 are the charges on the two objects, k = 8.99×10 N m /C (Coulomb’s constant), r12 is the distance between the objects, and � ̂ is a unit vector pointing from object 1 to 2.
¨ The forces on the two charges are action-reaction forces.
¨ Newton’s law of gravity (� = � ) and Coulomb’s law of electric force look similar. Demo: 4
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¨ Metal rod on pivot ¤ Demonstration of attraction between a charged object of either sign and a conductor due to induction in the conductor Example: 2
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¨ Two small spheres, each with mass m = 5.0 g and charge q, are suspended from a point by threads of length L = 0.30 m. What is the charge on each sphere if the threads make an angle θ = 20º with respect to the vertical? Quiz: 3
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