Download presentation

Presentation is loading. Please wait.

Published byArianna Pierce Modified over 4 years ago

1
1 Electricity & Magnetism I Dr Chris Booth

2
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

3
3 TOPIC 1 Electric Charge & Coulombs Law

4
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

5
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)

6
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)

7
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.

8
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.

9
9 Connect sphere with wire to ground Macroscopic view: positive charges escape to ground Microscopic view: electrons are attracted from ground, neutralising positive charge.

10
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!)

11
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.

12
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?

13
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

Similar presentations

Presentation is loading. Please wait....

OK

Electric Forces and Fields

Electric Forces and Fields

© 2018 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google