AP Physics 2 Electric Charge, Force, and Fields

Three basic rules for Electric Charge

Electric Charge is always conserved

Charge is quantized (any charge that’s > e will be a multiple of e)

Like charges repel and unlike charges attract

Charge Polarization

The Slight Seperation of positive and negative charge in a neutral object.

What happens if a charged object is brought near an insulator?

The electron charge shifts so that one side of the molecule will have a slight positive charge and the other a slight negative charge.

What is a conductor?

Materials like metals that have loosely bound electrons that can easily move from one atom to another.

What is an insulator?

Materials where electric charges don’t move freely

What is a semiconductor?

insulators in their pure state that can be made to be conductors by adding small amounts of other atoms to them.

Basically, a material that can have its electrical properties changed via doping

Three methods of charging

Friction

Conduction

Induction

Charging by Friction

An object gets charged when it’s rubbed against another object

Charging by Conduction

An object gets charged by making contact with an already charged object

Charging by Induction

A body gets charged by a charged object without any contact

How does charging by conduction work?

The electrons move from the highest electron-dense object to the lowest electron-dense object (from the most negative to least negative).

Quantifying Charge

# of excess electrons = \frac{q}{e}

Where q = charge

and e = the magnitude of a charge

Electrostatic Force

The force that charged particles exert on each other. It’s directly proportional to the strengths/size of the charges and inversely proportional to the distance² of the centers of the particles.

Why do we take the absolute value when we calculate the strength of an electrostatic force?

Because charges can be either positive or negative and we don’t want to confuse the product of the charges with the direction of the force.

When calculating electrostatic force, If the known charge is positive and the calculated force is < 0, what is the unknown 2nd charge?

Negative

When do we know when charges are attractive towards eachother?

When the charges are opposite in sign (+/-).

How do you calculate the acceleration of a proton being attracted to an electron?

a_{p}=\frac{F_{e}}{m_{p}}

Electric Field

The thing that exerts an electric force on all other charges within it. All charged objects have it.

What happens if an uncharged conductor is placed in an external electric field?

The free electrons inside the conductor will move until they reach equilibrium.

Once that’s reached, the excess charge will all be on the surface of the conductor with the electric field inside of the conductor equaling 0.