physics exam 2 - ch20

field model

how forces from charges transmit through empty spaces from one charge to another

1. source charge alters spaces around it

2. the alteration of space = electric field

3. the field exerts a force on other charges

alteration is everywhere in space

electric field

alteration of space around a charge

units = N/C

gravitational field

alteration of space around a mass

magnetic field

alteration of space around a magnet

E (N/C)

electric field strength (magnitude of the electric field)

E = Force on x,y,z ÷ q

q = charge

if charge q experiences an electric force

then an electric field exists to cause the force

electric field vector

exists at every point in space

positive probe charge

vector points the same direction as the force

negative probe charge

vector points in the opposite direction of the force

electric field only depends on

source charges

• not dependent on magnitude of q

force on probe charge

F on q = (kqq/r², away or towards q)

electric field on a source charge

E = (kq/r², away or towards q)

away from q

if q > 0 because + charges push forces away from them

towards q

if q < 0 because - charges push forces towards them 2

electric field diagram

drawings of electric field vectors at a number of points around a charge

• a positive charge would have all vectors going outwards from charge

• a negative charge would have all vectors going towards the charge

field exists at all points but diagram only shows a few points

electric field vector

arrow = strength and direction of electric field at the point

length of vector is only significant relative to the lengths of other vectors

vectors do not stretch from 1 point to another

electric field vectors

only represent the force at the point

• so if vectors appear to cross they don’t actually cross

• only not possible if two directions of 1 point

electric field is stronger

when the electric field vectors are closer together

electric field of multiple charges

vector sum of the electric field from each of the charges

electric field at any point of an electric dipole

is by vector addition of the fields of the two charges

parallel plate capacitor

two electrodes

• spaced closely

• equal and opposite charges

horizontal components = cancel out

vertical components = add to give the electric field vector pointing from positive to negative plate

electrode

a conducting plate

uniform electric field

every point in the electric field has the same strength and direction

• like in a parallel-plate capacitor

E_o permittivity constant (epsilon naught)

8.85 × 10^-12

electric field of a parallel plate capacitor depends on

Q/A = charge to area ratio charge density

• closer the charges, larger the fields

• shape is not relevant

• spacing doesn’t affect

electric field lines

imaginary lines drawn through a region of space to visual the electric field

electric field vectors are

tangent to electric field lines

field lines closer together

are closer to the charge and where the field strength is the greatest

electric field in a parallel capacitor

travel from the positive to negative plate

• constant field = evenly spaced field lines

electric field lines

cannot cross

start at a positive charge and end on a negative charge

stronger field strength

higher acceleration at that point

heart

• cell membrane insulator

• surface of tissue on one side is polarized and other side isn’t

• large electric dipole that can generate electric field throughout the body

conductor in electrostatic equilibirum

electric field is 0

• no charges are moving

• electrostatic equilibrium

excess charges in a conductor

lie at its surface because any interior charges would create an electrical field

• forces are perpendicular to the charged surface

• if tangent then a force would exert and then the forces would have to move

repulsive forces of charges in a conductor

cause them to move as far apart without leaving the conductor

• perpendicular only not tangent

screening

using a conducting box to exclude electric fields in a region

• helps create electric fields of 0

• metal walls and wires can be used for screening

not uniform distribution of charge in a conductor

sharper point → excess charge accumulates = stronger electric field

• ex: lightning rod can accumulate charge

Force on q =

q * E

q = force on charge

E = electric field

DNA gel electrophoresis

1. fragments have a negative charge

2. the electrodes create an electric field on the gel

3. the fragments then travel from the - to the + charge

4. fragments of different sizes travel at different rates