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Electricity Dr Chris Booth
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Organisation Two lectures per week (for 6 weeks):
Mondays 10:00 Hicks Building LT07 Fridays 10:00 Diamond LT05 Syllabus (see booklet) 7 topics (plus revision session) 1 or 2 lectures per topic
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TOPIC 1 Electric Charge & Coulomb’s Law
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Introduction Electric charge is everywhere! Atoms: positive nucleus
negative electrons Unit of charge: Coulomb (C) – very large! Charge on proton = e = 1.610–19 C Charge on electron = –e (So > 61018 electrons to make 1 C!)
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Normally, matter is neutral – equal numbers of e & p
Separated charge electrostatics Moving charge magnetic effects (as well) Rub plastic (with fur) plastic gains negative charge Rub glass (with silk) glass gains positive charge In each case – transfer of electrons. (Positive charge = deficit of electrons)
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Conductors & Insulators
Conductors – e.g. metals Some atomic electrons free to move within body of conductor Insulators – electrons tightly bound to atom or molecule (Semiconductors – small number of free charges – depends on impurity level)
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Forces & Induction Like charges ←+ +→ ←– –→ repel
Unlike charges +→ ←– attract Conductor: Add extra electrons at one end. They repel and spread throughout material Induction: Use one charged body to charge another – see illustrations.
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Insulating rod Isolated conducting sphere
Macroscopic view: charge on rod repels positive charge on sphere, attracts negative. Microscopic view: some free (negative) electrons in sphere are attracted, leaving excess positive charge behind.
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Connect sphere with wire to ground
Macroscopic view: positive charges escape to ground Microscopic view: electrons are attracted from ground, neutralising positive charge.
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Coulomb’s Law Force between 2 charges Q and q:
Proportional to product of charges Qq Proportional to 1 / square of separation Q r q Const. of proportionality k = 8.99109 N m2 C–2 (Check the units!)
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Normally written 0 = 8.8510–12 C2 N–1 m–2 (Why 4? – See later!) Vector form: Force on q due to Q if points from Q to q.
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Superposition The force due to one charge is unaffected by others.
Just add vectors Ftot = F1+F2+F3+... Example Charges +Q, +Q and –Q are arranged at the vertices of an equilateral triangle as shown. What would be the force on a charge q at the centre of the triangle?
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Example 2 Charges +9Q and –Q are a distance apart. At what point would there be no net force on a test charge q? Example 3 What is the ratio of the gravitational to the electrostatic force between the electron and proton in a hydrogen atom? Mass of proton = 1.6710–27 kg; mass of electron = 9.1110–31 kg Magnitude of electron & proton charges = 1.610–19 C
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