Electric Force and Electric field 1. There are two types of electric charge (positive and negative)

Electric Force and Electric field 2. Static charges can be produced by the action of friction on an insulator

Electric force and electric field 3. Conductors contain many free electrons inside them (electrons not associated with one particular atom)

Electric Force and Electric field 4. Charge is conserved. The total charge of an isolated system cannot change. I’m indestructible! So am I!

Electric Force and Electric field The force between two charges was investigated by Charles Augustin Coulomb in 1785

Electric Force and Electric field Coulomb found that the force between two point charges is proportional to the product of the two charges F α q 1 x q 2 and inversely proportional to the square of the distance (r) between the charges F α 1/r 2

Coulomb’s law It follows that F α q 1 q 2 r 2 or F = kq 1 q 2 r 2

Coulomb’s law F = kq 1 q 2 r 2 The constant k is sometimes written as k = 1/4πε o where ε o is called the permittivity of free space.

Calculations using Coulomb’s law The force between two charges is 20.0 N. If one charge is doubled, the other charge tripled, and the distance between them is halved, what is the resultant force between them? q1q1 q2q2 r r/2 2q 1 3q 2 F = 20N F = ? N

Calculations using Coulomb’s law F = kq 1 q 2 /r 2 = 20.0N x = k2q 1 3q 2 /(r/2) 2 = 6kq 1 q 2 /(r 2 /4) = 24kq 1 q 2 /r 2 x = 24F = 24 x 20.0 = 480 N q1q1 q2q2 r r/2 2q 1 3q 2 F = 20.0N x = 480 N

Electric field An area or region where a charge feels a force is called an electric field. The electric field strength at any point in space is defined as the force per unit charge (on a small positive test charge) at that point. E = F/q (in N.C -1 )

Force on a charge This means the force on a charge q is given by F = Eq If the charge is a proton or electron F = Ee where e = 1.6 x C

Electric field around a point charge If we have two charges q 1 and q 2 distance r apart F = kq 1 q 2 /r 2 Looking at the force on q 1 due to q 2, F = Eq 1 F = kq 1 q 2 /r 2 = Eq 1 E (field due to q 2 ) = kq 2 /r 2 q1q1 q2q2 NOT in data book

Electric field Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition. q1q1 q2q2 Field here due to both charges?

Electric field Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition. q1q1 q2q2 Field here due to both charges? Field due to q 1

Electric field Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition. q1q1 q2q2 Field here due to both charges? Field due to q 1 Field due to q 2

Electric field Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition. q1q1 q2q2 Resultant field Field due to q 1 Field due to q 2

Electric field patterns An electric field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines.

Electric field patterns An electric field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines. The arrows show the direction of force that would be felt by a positive charge in the field

Electric field patterns An electric field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines. The arrows show the direction of force that would be felt by a positive charge in the field

Electric field patterns An electric field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines. The closer the lines are together, the stronger the force felt. This is an example of a radial field

Field around a charged metal sphere E = 0 inside the sphere

Field around two point charges

Field between charged parallel plates Uniform field E = V/d V d “Edge effects” NOT in data book

Remember! The force F on a charge q in a field E is F = Eq

Parallel plates E = V/d and E = F/q So V/d = F/q Useful!!!!

Electric field hockey!