Electric Field Physics Department, New York City College of Technology

Key words Test charge Electric force Electric field Point charge Superposition principle Electric field lines

Test charge What is test Charge? A charge so small that the force it exerts does not significantly alter the distribution of other charges that are nearby.

Definition of electric field The electric field,, at any point in space is defined as the force exerted on a tiny positive test charge placed at that point divided by the magnitude of the test charge q:

Electric field The electric field is a vector whose direction is the direction of the force on a tiny positive test charge at that point, and whose magnitude is the force per unit charge. The SI units of : N/C

Electric field Disc 17, #10

Electric field of a point charge The electric field at a distance r from a single point charge Q has magnitude, therefore, E is independent of the test charge q, that is, E depends only on the charge Q which produces the field

Example #1 Find vector E for (a) and (b).

Example #1--Continued The direction is the electric field is toward the charge Q.

Calculate electric force If is given, then If q is positive, and point in the same direction. If q is negative, and point in opposite directions.

Superposition principle The electric field due to more than one charge is the vector sum of all the individual fields due to each charge:

Example #2 Calculate E at P.

Example #2--Continued The electric field points to the left.

Example #3

Example #3--Continued

The direction of E B is along the +x direction.

Electric field lines A pictorial description of the electric field

Electric field lines They indicate the direction of the electric field; the field points in the direction tangent to the field line at any point. The lines are drawn so that the magnitude of the electric field, E, is proportional to the number of lines crossing unit area perpendicular to the lines. The closer together the lines, the stronger the field. They start on positive charges and end on negative charges; and the number starting or ending is proportional to the magnitude of the charge.

Electric field lines

Electric fields and conductors The electric field inside a conductor is zero in the static situation. Any net charge on a conductor distributes itself on the surface. The electric field is always perpendicular to the surface outside of a conductor.