Electrostatics

Electric force is given by the formula above.

k=9.0*109(N*m2/C2)

C is the unit for charge, the Coulomb(Coo-Lome)

Like charges repel.

Opposite charges attract.

Example: A 1 µC charge and a -3 µC charge are separated by a distance of 5*10^-3m. Draw a force diagram and solve for the electric force between them.

F = 1080N

Ratios:-The electric force is proportional to each of the charges. -If you double one charge, you double the force.-If you double one charge and triple the other, the force gets multiplied by a factor of 6.

-The electric force is inversely proportional to the square of the distance between the charges.-If you triple the distance, the force gets divided by nine (three squared is nine).

Electric forces stack.

To calculate the electric force, solve for the force due to each individual charge then add the forces together.

EX. Calculate the force on the 1C charge.

F = 7.2*10^10N

Calculate the force on the 1C charge.

F = 2.7*10^10N

The force due to the positive charge is repulsive (to the left) and the force due to the negative charge is attractive (to the right), so we have to subtract them to find the total force.

Fnet=F1+F2F = (2.7*10^10N) – (7.2*10^10N)F = -4.5*10^10N (to the left)

Solve for the magnitude and direction of the net force on the 2C charge.

Solution: The 1.5C charge is pulling downward on the 2C charge with a force of

F = 1.08*10^11N

The 3C charge pushes to the left with a force of

F = 5.4*10^10N

This now becomes a forces problem. First, place the vectors front to back (the front of one touches the back of the other).

Next, use the Pythagorean theorem to get the total force.

F2= Fx2+Fy

2

F=1.207*1011 N

Next, use the sine, cosine, or tangent to solve for the angle.

tan(θ)= θ=

θ=63.4o

Electric Fields

= F/q

Electric fields tell the force per unit charge.

They point in the direction a positive charge would feel a force.

Electric fields point away from a positive charge and toward a negative charge.

Positive charges push the field lines.

Negative charges pull on field lines.

Greater charge means a stronger field.

Field at a Single Point

• Solve for the field due to each of the separate charges at a point.

• Add the arrows together.

• Notes: The direction of the field is in the direction a positive charge would feel a force

• When adding vector quantities (like forces or fields), go through one arrow and then through the other.

Electric PotentialGravitational Electric

Force Fg = mg

Energy Eg = mgh ∆x

Potential Vg = Eg/m V = Ee/q

V is potential. Gravitational potential is measured in Joules per kilogram while electric potential is measured in Joules per Coulomb (also called Volts).

Potential

Gravitational potential is gravitational potential energy per unit mass.

Electric potential is equal to electric energy per unit charge.

Vg = Eg/m = gh

V = Ee/q = E∆x