Electric Fields and Forces

Electric Fields and Forces

Gravity is a field force. A small object in the gravitational field feels the gravitational force and is attracted to the more massive object as a result.

A field exists around all objects with mass. Its strength increases with more mass and closer to the object.

What would happen to an object in this gravitational field?

-feels a force toward Earth-accelerates toward Earth

Calculate the acceleration due to gravity.

Fg = mgg=Fg / m

How can we calculate the strength of this gravitational field?

Replace Earth with a negative charge.

This is called the source charge (Q).

Replace the object with a small positive charge, called the test charge (q).

+Always positive unless stated otherwise.

- E-field lines go toward a negative source charge.

What would happen to the test charge?

+- Feels a force toward source charge- Accelerates toward source charge

What if the source charge is positive?

+

- E-field lines go away from a positive source charge.

- Feels a force away from source charge- Accelerates away from source charge

Recall: g = Fg / m

E = Fe / q OR

Fe = qE

How can we calculate the strength of this electric field?

Electric field strength -Units N/C

test charge

Fg = Fg

mg = GMm r2

g = GM r2

Similar to…

E = kQ r2

k is Coulomb’s constantk = 9 x 109 Nm2/C2

Another way…

source chargeDistance between

source charge and either another charge or location

- Similar to Newton’s Universal Law of Gravitation

- F = GMm r2

- Calculates the electrical force between 2 charges.

Coulomb’s Law

E=F/q E = kQ r2

F = kQq r2

F = kQq r2

Coulomb’s Law

Use the absolute value of all charges (q or Q) in all of the above equations!

VERY IMPORTANT NOTE!!!!!!!