Electric Fields and Charging Methods

Electric Field Basics

A charged object exerts an electric force on surrounding charged objects, analogous to how mass exerts gravitational force.
An electric field measures the force felt per unit of charge experiencing the field.
A uniform electric field has a constant electric field strength throughout.
The electric field created by a point charge has a strength that varies with distance from the charge.

Electric Field strength (E) is measured in Newtons per Coulomb (N/C).
Electric field strength (E) is the ratio of the force experienced (F) by a test charge to the quantity of charge on the test charge (q).
- E = \frac{F}{q}
The electric field strength created by a source charge (Q) varies with location.
Combining Coulomb's Law with the equation above yields a second equation for electric field (E) using source charge (Q) and distance (d).
- E = k \frac{Q}{d^2}

Occurs when two objects are rubbed together, causing a transfer of electrons.
Electrons are more easily stripped from or added to atoms on the outside.
The triboelectric series determines which material gains electrons (becomes negatively charged) and which loses electrons (becomes positively charged).
Materials listed in a triboelectric series:
- Loses electrons: Rabbit fur, Hair, Glass, Mica, Wool
- Gains electrons: Silk, Paper, Wood, Amber, Rubber balloon, Polystyrene, Acrylic, Polyethylene, Teflon (PTFE)

Requires a charged object to start.
When a charged object touches a neutral object, it shares its charge.
A negatively charged object touching a neutral object results in two negatively charged objects.
A positively charged object touching a neutral object results in two positively charged objects.

Diagram i: A metal sphere with excess negative charge approaches a neutral electroscope.
Diagram ii: Upon contact, electrons move from the sphere to the electroscope and spread uniformly.
Diagram iii: The metal sphere has less excess negative charge and the electroscope now has a negative charge.
Diagram i: A neutral metal sphere rests on an insulating platform.
Diagram ii: A positively charged aluminum plate touches the metal sphere, drawing electrons off the sphere and onto the aluminum plate.
Diagram iii: The aluminum plate has less excess positive charge, and the metal sphere now has an excess positive charge.

Occurs when a charged object is connected to the earth, returning the object to neutral.
A positively charged object pulls electrons from the Earth to become neutral.
A negatively charged object pushes its excess electrons to the Earth to become neutral.

Occurs when charges within a neutral object are forced to separate or move to opposite sides.
Requires another charged object to assist in separating the charges.

A positively charged glass rod near a neutral conducting sphere causes charge distribution.

Requires a charged object and a ground.
The charged and neutral objects do not touch; the charged object gets close enough to create polarization in the neutral object.
To create a positive charge via induction, use a negatively charged object.
To create a negative charge via induction, use a positively charged object.

Diagram i: Two metal spheres are mounted on insulating stands.
Diagram ii: The presence of a negative charge induces electrons to move from sphere A to B, polarizing the system.
Diagram iii: Sphere B is separated from sphere A using the insulating stand, resulting in opposite charges on the two spheres.
Diagram iv: The excess charge distributes itself uniformly over the surface of the spheres.
Diagram i: Two metal spheres are mounted on insulating stands.
Diagram ii: The presence of a positive charge induces electrons to move from sphere B to A, polarizing the system.
Diagram iii: Sphere B is separated from sphere A using the insulating stand, resulting in opposite charges on the two spheres.
Diagram iv: The excess charge distributes itself uniformly over the surface of the spheres.