Electric Force

SWBAT describe the electric force conceptually and solve for the force mathematically.

Luke, use the force!

I can’t, I don’t have

enough electrons…

Rub your feet on the carpet you dummy!!!

Charges

• A proton has a charge of 1.60 X 10 -19 C • A electron has a charge of -1.60 X 10 -19 C • C = Coulomb

Electric Force

• The electric force depends on two things:– Charge: greater the charge on the objects, greater force– Distance between charges: closer greater force, further

apart weaker force

• Follows Newton’s 3rd Law: 2 charges of any size will experience the same force as the other, but in the opposite direction.

Coulomb’s Law• Charles Coulomb was the first scientist to express this

force mathematically, in the 1780’s.

• Fe= (kq1q2)/r2

• Notice similarity to the force of gravity

• ‘r’ is the distance between the charges• ‘q’ is the 2 charges• ‘k’ is the proportionality constant • kC= 8.99 X 109 N*m2/C2

Aren’t I shocking!

Practice

• Note: It is usually easier to use the absolute values of the forces while solving, then decide afterwards whether it is an attractive or repulsive force.

• Note: An Electron and Proton both have the same charge of 1.60 × 10-19 C. Electron is negative – Proton is positive

• Practice 1: A positive charge of 6.0 x 10-6 C is 0.03 m from a second positive charge of 3.0 x 10-6 C. Calculate the force between them and indicate if it is attractive or repulsive.

Practice

• Practice 2: What force exists between a positive charge of 1.5 x 10-5 C and a negative charge of -6.0 x 10-6 C, which are 5.0 cm apart.(Note the large forces involved between electric charges given in millionths of a coulomb also called a microcoulomb).

• Practice 3: A negative charge of 6.0 x 10-6 C exerts an attractive force of 64.8 N on a second charge 0.05 m away. What is the sign and magnitude of the second charge?

Practice• The hydrogen atom has the simplest structure of all atoms. Its

nucleus is a proton (mp = 1.67 x 10-27 kg), outside of which there is a single electron (me = 9.11 x 10-31 kg) at an average separation distance of 5.3 x 10-11 m. – Solve for the electrical and gravitational forces (G = 6.67 x

10-11) between the proton and the electron in a hydrogen atom The electrical forces that subatomic particles exert on one another are so much stronger than their mutual gravitational forces, that gravitation can be completely ignored.

– The electrical attraction of the proton for the electron causes the centripetal force needed to hold the electron in orbit. Find the electron’s speed.

How to SolveGiven/ Constants: • r= 5.3 x 10-11 m• q=1.60 X 10 -19 C (an electron is negative/ a

proton is positive)• mp = 1.67 x 10-27 kg, me = 9.11 x 10-31 kg

• kC= 8.99 X 109 N*m2/C2

• G = 6.67 x 10-11 N*m2/kg2

• Plug into Fe and Fg

Net Electric Force

• Electric force is often caused by more than 2 charged particles.

• The net electric force is the vector sum of all the individual electric forces acting on an object.

• This called the principle of superposition

Charges in Line

• Three point charges are placed on the x-axis as shown. Find the net electrostatic force on q1.

Charges in a Plane

• A configuration of three charges is shown. What is the net electrostatic force on q3?