Bell Work

Unit 3: Field ForcesAction at a Distance

Using a test charge…

• Can be used to test a single charge.• Test charges can be any sign, but due to conception, we typically

assume them as positive.• Charge is so small (in comparison to primary charge) that they are

considered negligible in terms of the Electrostatic Force of interaction.

Then, re-define the variables in terms of the ‘test charge’…

The Electric Field

Can this same relationship be used for other field forces?Gravity?

Bell Work

1. Calculate the Electric Field of a charged object exerting an electrostatic force of 12N on a proton (our test charge), 2cm away.

2. Calculate the Electric Field of a -12C charge, 2 cm away from a test charge.

Bell Work

• Two positive charges, .1 m apart, are static. Directly between them, and .1 m in the +y direction sits a positive test charge. Each charge creates an electric field of 5 N/C at that point.

1. Using vector mathematics, resolve the two vectors to determine the total Electric Field at the point charge.

Bell Work

You have a positively charged ring with a radius of 2 cm.

1. Draw the resultant vector of the Electric Field at a point 5 cm away, if the ring is positioned such that it could slide over the point.2. Draw the resultant vector of the Electric Field at the center of a positively charged square conductor.

Getting a visual interpretation of a field:

Looking back…

Rules for drawing field lines

• 1. Lines start at positive charges and end in negative charges • 2. The number of field lines α the strength of the electric field• 3. Field lines can NEVER cross

Rule 1

Lines start at positive charges and end in negative charges

Rule 2

The number of field lines α the strength of the electric field

Rule 3

Field lines can NEVER cross

Bell Work

Draw the electric field for the following configuration of charges:

Practice…

How can we prevent lightning from damaging stuff – like buildings, animals, cars etc..

• Use the iPad to research lightning and lightning strike prevention. I want to know everything there is about lightning and how to protect ourselves and the things in our environment from it!

Special Fields: Conductors

Rules: 1. The electric field is 0 N/C inside a conductor. All charge in on the surface.2. The electric field is perpendicular to the outside surface at any point.3. On irregularly shaped conductors, charge tends to accumulate where the surface is sharper (the radius of curvature is smallest).

An example of this was in your reading…

Lightning !

What is lightning? (Video: 10:05)• http://www.youtube.com/watch?v=JJubgrI0T0g What causes lightning – weather perspective (12:00) • http://

www.youtube.com/watch?v=yxpR75apjbY What happens if you’re struck by lightning (4:00)• http://

www.youtube.com/watch?v=L1HhRAUqFqM • Google – Lightning strike victimsLightning Rods (part 1: 6:30)• https://

www.youtube.com/watch?v=zhu5pIrPw7U

Lightning Rods