1 Electric Field

2 Chapter Objectives know the definition of, and basic uses for, the electric field. be able to sketch electric field lines. know how electrostatic shielding works.

3 Van der Graff A Van De Graff Machine is a mechanical-electric device that produces high voltage at safe levels of electric current. The parts include -A hollow metal ball -vertical pipe with rubber conveyor belt inside -hollow metal box with and electric motor inside Van De Graaff machines in the classroom are smaller than the machines in the laboratory. How does VDG work? -the roller(s) become charges through contact with the belt. One roller sucks charge from the metal comb and onto the belt. The other roller pushes electric charge from the belt onto the comb. As the belt is moving charge is sucked in at one end and puts charge out at the other end.

4 Van Der Graaff

5 Electrophorus

6 ELECTRIC FIELD Michael Faraday – developed the concept of “electric field”. TEST CHARGE: is used to map out the field resulting from any collection of charges; it’s very small in charge and size not being able to move the source from it’s location, or affect the electric field being measured. A charge creates an electric field around it in all directions. If a 2 nd charge is placed at some point in the field, the second charge interacts with the field at that point. The ELECTRIC FIELD is a vector quantity that relates the FORCE exerted on a test charge to the SIZE of the TEST CHARGE. The RATIO of FORCE to TEST CHARGE is independent of the size of the test charge. The length of the arrow shows the strength of the field. The vector quantity of the electric field DOES NOT DEPEND ON THE TEST CHARGE, but depends only on the SOURCE CHARGE, and the LOCATION of that specific point. DIRECTION of the electric field is the direction of the force on a positive test charge.

7 Picturing Electric Field Electric Field Lines are the lines to which the ELECTRIC FIELD VECTORS are TANGENT at any point. The STRENGTH of the electric field is indicated by the SPACING between the LINES. The field is STRONGER where the LINES are CLOSE together. It is WEAKER where the lines are SPACED FARTHER APART. Electric fields exist in three dimensions. The electric field vectors created by a POSITIVE SOURCE point/extend RADIALLY OUTWARD. The electric field vectors created by a NEGATIVE SOURCE point/extend RADIALLY INWARD. When there are two or more charges, the net field is the VECTOR SUM of the fields resulting from THE INDIVIDUAL CHARGES. For more than one charge, the lines become curved and they always LEAVE the POSITIVE charge and ENTER A NEGATIVE charge.

8 Dipole Here is the dipole once again, this time with the electric field lines drawn instead of the field vectors. If you drag the red test charge to any point on a field line, you will observe that the force on the charge is a vector tangent to the field line. This is because the force is parallel to the field vector at the location of the particle, and the field vector is in turn tangent to the field line.

9 Links Electric field 14 animations: Prof. Selman Hershfield – University of Florida

10 Problems 1.Two charges, Q1 and Q2, are situated on a horizontal line at a distance “a”. The charge on Q1 is (-5q) and it is at the origin. The charge on Q2 is (+2q) and it is a distance to the right of Q1. Find the point where the net electric field is zero. 2. (Problem 68) The electric field at the surface of the Earth is about 100 N/C and points toward the center of the Earth. What is the magnitude of the charge on the Earth? 3. (Problem 69) Calculate the size and direction of the electric field required if an electron of mass 9.1 x kg and electric charge -1.6 x C placed if it is to experience a force that will exactly cancel its weight at the Earth’s surface.

11 Problems 4. A positively charged oil drop whose mass is kg remains stationary under the influence of the earth’s gravitational field and a uniform electric field of 6.1 x N/C. find the magnitude of the charge on the oil drop. 5. A proton of mass 1/6 x kg and electric charge1/6 x C acquires a speed of 12 X m/s when it is accelerated by an electric field, E. find its magnitude. 6. A small positively charged object falls from rest in a uniform downward electric field, so that the force on the object is given by F=qE, where E is a constant. Write an equation giving its speed, v, after it has fallen a time t, in terms of its mass, m, and charge, q. Gravity may be disregarded.

12 Problems 7.A test charge of 1C is placed halfway between a charge of +5C and a charge of +3C that are 20 m apart. Find the magnitude and direction of the force on the test charge. 8. A proton and an electron are separated by a distance equal to the radius of a hydrogen atom (5 x m). If the charge of the electron is – 1.6 x 10 – 19 C, find the attractive force between them.