Electric Charges, Forces & Fields
All matter is made up of charged particles. There are two types of charge, + and -.
Opposite charges attract, like charges repel.
We usually describe the electric force in terms of the electric field. For now, this is a
convenient tool, but we’ll see that the electric field is real.
Electric fields cannot penetrate a conductor. All excess charge stays on the surface and
the field is zero inside.
Rub a plastic rod with a piece of wool.
Negative charges are immobile on the rod’s surface.
The other end is still neutral.
The positive charge on the wool is equal to the negative charge on the rod.
Charge is transferred to a piece of metal upon contact.
The charges repel each other.
Charge spreads over the surface of the metal very fast.
Charging by contact
An electrically neutral molecule + friction → positive molecule ion + negative
The bonds were broken by friction.
The positive half of the molecule lost an electron as the bond broke.
The negative half of the molecule gained an extra electron as the bond broke.
Fixed positive, mobile negative
The nucleus contains positive protons.
The electron cloud is negatively charged.
Forces from an external charges cause the negative charge and the positive
charge to be slightly offset. Hydrogen bonding
The slightly negative oxygen is attracted to the slightly positive hydrogen of a
Water molecules form hydrogen bonds with four neighbors.
Electrostatic forces make DNA do its thing.
The base pairs of the two strands can only join in certain combinations.
Three hydrogen bonds join guanine and cytosine.
Two hydrogen bonds join thymine and adenine.
The bases are comprised of hydrogen, nitrogen, carbon, and oxygen.
Units & Equations
The unit of charge is the coulomb (C).
The symbol for charge is q, or Q.
A typical charge on a rubbed object is about 1 nC.
Particle Mass (kg) Charge (C)
Proton 1.67 x 10 +e = 1.60 x 10
Electron 9.11 x 10 -e = -1.60 x 10
K ∣ 1q ∨ 2 2
F = r