ELECTROSTATICS -Fields

Michael Faraday developed the idea of a field as being a sphere of influence to explain how a force could affect an object at a distance without contact (such as gravity)

Fields can be scalar or vector Gravity, electric and magnetic fields are vector fields because they have magnitude and direction When an object is placed in the field, both objects experience a force. e.g. gravity

Test Charge (always positive) An electric field exists around a charged particle. When another charged particle enters this field an electric force is exerted on both particles Test Charge (always positive) Fe - + Fe - - The direction of the electric field is defined as the direction a positive particle would move if placed in the field. (Often called a test charge)

Electric field lines begin at positive charges and end on negative charges and are at right angles to the charged object. Field lines show the path that a positive test charge would follow if placed in the field. The closer the electric field lines, the stronger the electric field Field lines do not cross

MULTIPLE CHARGES: the net electric force determines the shape of the field lines Two like charges: Fnet The net electric field is the vector sum of each charge’s electric field. Fnet

Two opposite charges: Fnet + +

Example: The figure below shows the electric field lines for a system of two point charges. (a)  What are the relative magnitudes of the charges? (b)  What are the signs of the charges? Solution: (a)  There are 32 lines coming from the charge on the left, while there are 8 converging on that on the right.  Thus, the one on the left is 4 times larger than the one on the right. (b)  The one on the left is positive; the one on the right is negative.

ELECTRIC FLUX is an measure of how many field lines there are per unit area

The Shape of a Conductor: The charges on a conductor are spread out as far as possible in order to make the energy of system as low as possible On a solid conductor, the charges are on the surface of the conductor (draw pointy conductor)

On a hollow conductor, the charges are on the outer surface The electric field inside a hollow conductor is zero

The charges tend to be closer together at edges and corners, so the field lines are closer together The electric field can be so strong at corners it can force electrons off molecules in the air (pointed lightning rods cause lightning to spark from the clouds to the rod and then into the earth and not flow through houses)

ELECTRIC FIELD STRENGTH The definition: electric field strength: measured in (N/C) Electric force acting on the particle in the field (N) Charge of the particle placed in the field (C) Electric field strength (N/C) e.g. How much force is exerted on a 3.00 μC balloon placed in a cat’s electrical field where the field strength is 4.00 x 105 N/C? =1.2 N

A class of drugs which target HIV-I reverse transcriptase, called non-nucleoside inhibitors, all bind to a specific pocket on the enzyme. Shown is a close-up of the binding of the drug TIBO-R86183 inside this pocket. The electrostatic characteristics of reverse transcriptase in the vicinity of the non-nucleoside inhibitor binding site is also shown. The electrostatic potential is mapped onto a solvent-accessible surface, along with some representative electric field lines

Finding electric field strength at a certain distance from a charged object: Coulomb’s Constant Charge of the object producing the field (C) The magnitude of the electric field strength (N/C) Distance from the object (m)

Example Find the magnitude of the electric field at a point 0.450 m from a 5.00 x 10-6 C charge. 2.22 x 105 N/C

2. A –2. 00 x 10-6 C charge is placed in an electric field 2. A –2.00 x 10-6 C charge is placed in an electric field. The charge experiences a force of 5.30 x 10-2 N to the left. Determine the magnitude and direction of the electric field. (Hint: draw a diagram) 2.65 x 104 N/C

3. A –3. 50 x 10-6 C charge is 0. 440 m to the left from a 3 3. A –3.50 x 10-6 C charge is 0.440 m to the left from a 3.00 x 10-6 C charge. Determine the magnitude and direction of the electric field at point P halfway between the charges. P 0.440 m -3.50 x 10-6 C 3.00 x 10-6 C 1.21 X 106 N/C [left]