general physics ii fall 2007 - university of northern...

1

General Physics IISpring 2008

Electric Fields and Forces

2

Electric Field Lines• To visualize the electric field due to a collection of charges,

one can draw an arrow representing its vector value at each point in space. This procedure gets to be very cumbersome. A more elegant method is to draw electric field lines.

• An electric field line is drawn such that the electric field at any point along the line is tangent to the line.

• The direction of the field is indicated by an arrowhead on the line.

• The magnitude of the field is proportional to the density of thelines. Hence, there are twice the number of lines per unit area close to a charge as there are close to another charge half its magnitude.

• Field lines emanate from a positive charge and terminate on a negative charge.

• Electric field lines never cross.

3

Electric Field Lines

4

Electric Field Lines

Electric Dipole

5

Electric Field Lines

6

A set of electric field lines is directed as below. At which of the noted points is the magnitude of the field the greatest?

Checking Understanding

7

Electric Field Inside a Conductor• In the material of a conductor, the

charges move freely in response to an applied electric field.

• When the charges stop moving, there can be no electric field inside the material. (Otherwise, the charges would experience forces and move.)

• Thus, under electrostatic conditions, the electric field inside the material of a conductor is zero.

• Since the electric field is zero within the material of a conductor under electrostatic conditions, all excess charge must reside on the surface of the conductor.

E

---

+++

0inE =

E

---

+++polE

8

Electric Field at the Surface of a Conductor

• Under electrostatic conditions, electric field lines must meet the surface of a conductor at right angles to the surface. Thus, an electric field line must be perpendicular to the tangent to the surface at the point of intersection.

• Reasoning: If the electric field had a component parallel to the surface, there would be a force on the charges at the surface causing them to move. Under electrostatic conditions, there is no motion of charge, so such a parallel component cannot exist.

9

Uncharged Conductor in an External Electric Field

10

Workbook: Chapter 20, Question 35

11

Forces and Torques on Charges in Electric Fields

12

Electric Forces and Torques• An electric dipole consists of two charges of the same

magnitude but opposite in sign separated by a distance.

O

H H+

-Water molecule

13

A dipole is held motionless in a uniform electric field. For thesituation below, when the dipole is released, which of the following describes the subsequent motion?

A. The dipole moves to the right.

B. The dipole moves to the left.

C. The dipole rotates clockwise.

D. The dipole rotates counterclockwise.

E. The dipole remains motionless.

Checking Understanding

14

A dipole is held motionless in a uniform electric field. For thesituation below, when the dipole is released, which of the following describes the subsequent motion?

A. The dipole moves to the right.

B. The dipole moves to the left.

C. The dipole rotates clockwise.

D. The dipole rotates counterclockwise.

E. The dipole remains motionless.

Checking Understanding