Measuring Electric Fields Defining the Electric Field Michael

Measuring Electric Fields

Defining the Electric Field • Michael Faraday suggested the idea of a force field to explain how the electric force can act at a distance. • An electric field is a property of the space around a charged object that exerts forces on other charged objects. • You cannot see an electric field, but there are ways to detect that an electric field is present.

Defining the Electric Field • You can envision the force from an electric field by modeling its effects on a small charged object—a test charge (q’)—at some location. • If there is an electrostatic force on the object, then there is an electric field at that point. • According to Coulomb’s law, the force is directly proportional to the strength of the test charge (q’), so the ratio of the force to the strength of the test charge is a constant.

Defining the Electric Field • The electric field at point A, the location of q’, is represented by the following equation. Electric Field Strength • The direction of an electric field is the direction of the force on a positive test charge. • The magnitude of the electric field is newtons per coulomb (N/C).

Defining the Electric Field N Use with Example Problem 2. Problem An electric field is measured using a positive test charge of 3. 0× 10− 6 C. The test charge experiences a force of 0. 24 N at an angle of 15° north of east. What are the magnitude and direction of the electric field at the location of the test charge? 15° q’ E

Defining the Electric Field • You can make a model of an electric field by using arrows to represent the field vectors at various locations. • The arrow’s length represents the field strength. • The direction of the arrow represents the field direction.

Defining the Electric Field • You can use a test charge to map the electric field resulting from any collection of test charges.

Defining the Electric Field If, and only if, the charge q is a point charge or a uniformly charged sphere, you can calculate its electric field from Coulomb’s law.

Defining the Electric Field Use with Example Problem 3. Problem Find the magnitude of the electric field at a point 0. 15 m to the right of a sphere of a charge of − 4. 0× 10− 6 C.

Modeling the Electric Field • You can represent electric fields with electric field lines. An electric field line indicates the direction of the force due to the electric field on a positive test charge. • The spacing between the lines indicates the electric field’s strength. The field is stronger where the lines are spaced more closely.

Modeling the Electric Field • Electric field lines are directed toward negative charges and away from positive charges. • Electric field lines are only a way of representing electric fields. They are not the electric field itself.

Modeling the Electric Field When more than one electric charge is present, the electric fields add together.