Module 6.2 - Electric Fields

Question 1

State is coulomb’s law

Answer 1

Like charges repel and opposite charges attract, with a force proportional to the product of the charges and inversely proportional to the square of the distance between them

Question 2

What is the coulomb’s law equation?

Answer 2

F = k(q1q2/r^2)

Question 3

Still don’t understand what permittivity is

Answer 3

should probably figure that out at some point

Question 4

What is electric field strength?

Answer 4

Electric field strength is the force acting on a charged particle inside an electric field per unit charge (the force a charge of +1C would experience if placed inside the field)

Question 5

Is electric field strength scalar or vector?

Answer 5

E is a vector, it points in the direction an imaginary positive charge would move if in the field

Question 6

Which way do electric field lines point?

Answer 6

Electric field lines point from positive to negative

Question 7

What is the relationship between electric field strength and distance?

Answer 7

Inverse proportion

E (=x 1/r^2

Question 8

For a plate charged at 400V above a grounded plate, at what distance is the potential 100V?

Answer 8

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ +400V
\+300
\+200
 - - - - - - - - - - - - - - - - - - - - - - - 
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0V

Question 9

What is the equation for permitivity

Answer 9

e = e(relative) * e(0)

Question 10

Why does capacitance increase with increasing permittivity?

Answer 10

With increased permittivity, it becomes easier to generate an electric field between the plates of the capacitor, creating a stronger attractive force allowing for more charge to be stored (I think permittivity is really confusing)

Question 11

What are some similarities between electric and gravitational fields?
(4 total)

Answer 11

• grav field is force per unit mass, and electric field (E) is force per unit positive charge
• Newton’s law for force of gravity is an inverse square law as well as coulomb’s law for electric force
• The field lines for a spherical mass and negative spherically symmetric charge look the same
• Gravitational potential and electric potential are both potential energy per unit mass/positive charge and both are zero at infinity

Question 12

What are some differences between gravitational and electric fields?
(3 total)

Answer 12

• Gravitational forces always attract whereas electric forces can be attractive or repulsive
• Objects can be shielded from electric fields but not gravitational fields
• The size of an electric force depends on the medium between two charges, but for gravitational force this makes no difference

Question 13

define electric potential?

Answer 13

Electric potential is the work done bringing a unit positive charge from a point infinitely far away to that point in the electric field

Question 14

when is electric potential positive and when is it negative?

Answer 14

V is positive when Q is positive (a repulsive force)

V is negative when Q is negative (an attractive force)

Question 15

How are electric potential and electric potential energy related?

Answer 15

Electric potential is the electric potential energy that a unit positive charge (+1C) would have at a certain point

Question 16

Where does the formula for the capacitance of an isolated point charge come from?

Answer 16

1) Substitute V = Q/C into the electric potential equation

2) Rearrange for C