Electric Fields Topic 6.2

Review What are the two types of charges? How do neutral object interact with charged objects? What is the difference between insulators and conductors? Can there be electricity in insulators? There are two types of electric charge (positive and negative) Static charges can be produced by the action of friction on an insulator Conductors contain many free electrons inside them (electrons not associated with one particular atom)

Static Electricity Demos What causes the movements? Describe the movements in terms of “action at a distance” How do you know there’s a force if it’s not touching? What’s happening to the charges in each interaction? A concept of FIELDS is needed

Electric Force What are the variables that affect the electric force on an object? It was found that Describe a method in finding the constant that will make this proportion into an equation Charles-Augustin de Coulomb (1736 –1806) Coulomb found that the force between two point charges is proportional to the product of the two charges F α q1 x q2 and inversely proportional to the square of the distance (r) between the charges F α 1/r2

Coulomb’s Law

Coulomb’s constant The constant k is sometimes written as k = 1/4πεo where εo is called the permittivity of free space. What is the value & unit for k?

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? F = 20N q2 q1 r F = ? N F = 480 N 2q1 3q2 r/2

Electric Fields Is it possible to have a force with only one object? What causes you to know there’s a force on you (or any object) if it’s not touch you? How do we know our description of forces (and fields) are correct? Can we ‘see’ field lines? 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)

Electric Field E = F/Q (in N.C-1) 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. What is the unit of an electric field? E = F/Q (in N.C-1) E = F/Q (in N.C-1)

Think Pair Share Is the electric field a vector or scalar? Describe the concept of field lines in one sentence What would the field lines look like for a single charge?

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

Test Charge Defined to be a positive charge that is small enough so it does not have an effect on the field

Test Charge Defined to be a positive charge that is small enough so it does not have an effect on the field The arrows show the direction of force that would be felt by a positive charge in the field

Negative Charge? In what direction will the test charge feel the force from the negative charge?

Negative Charge? In what direction will the test charge feel the force from the negative charge? The arrows show the direction of force that would be felt by a positive charge in the field

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. Field here due to both charges? q1 q2

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. Field due to q1 Field here due to both charges? q1 q2

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. Field due to q1 Field due to q2 Field here due to both charges? q1 q2

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. Field due to q1 Field due to q2 Resultant field q1 q2

Field around two point charges Sketch what you think the field lines should look like:

Field around two point charges

Field around two point charges Sketch what you think the field lines should look like:

Field around two point charges

Field between charged parallel plates V d

Field between charged parallel plates V Uniform field E = V/d d “Edge effects”