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1 Electricity & Magnetism I Dr Chris Booth

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2 Two lectures per week (for 6 weeks): Wednesday 11:10Hicks LT07 Fridays 11:10Hicks LT01 Syllabus (see sheet) 7 topics (plus revision session) 1 or 2 lectures per topic Organisation

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3 TOPIC 1 Electric Charge & Coulombs Law

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4 Electric charge is everywhere! Atoms:positive nucleus negative electrons Unit of charge: Coulomb (C) – very large! Charge on proton = e = 1.6 10 –19 C Charge on electron = –e (So > 6 10 18 electrons to make 1 C!) Introduction

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5 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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6 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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7 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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8 Insulating rodIsolated 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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9 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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10 Coulombs Law Force between 2 charges Q and q: Proportional to product of charges Qq Proportional to 1 / square of separation Qrq Const. of proportionality k = 8.99 10 9 N m 2 C –2 (Check the units!)

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11 Normally written 0 = 8.85 10 –12 C 2 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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12 Superposition The force due to one charge is unaffected by others. Just add vectors F tot = F 1 +F 2 +F 3 +... 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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13 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 = 1.67 10 –27 kg; mass of electron = 9.11 10 –31 kg Magnitude of electron & proton charges = 1.6 10 –19 C

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