PHYS 102 Lecture 1: chapter 16 electric force electric field
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Chapter 16 Electric Potential
16.1 Electric Potential Energy and Potential Difference
● There are two types of electric charges in nature;Positive charge (proton)
and negative charge (electron).
● An object is positively charged if it has lost electrons.
● An object is negatively charged if it has gained electrons.
Conductors are materials in which electric charge (electrons) can move quite
freely. Metals such as copper and aluminum are conductors.
Insulators are materials in which electric charge (electrons) are not free to move.
Materials such as glass, rubber, and plastic are insulators.Semi-conductors have
electrical properties between metals and insulators.
3. Coulomb’s Law state that the electrostatic force between two charged
particles separated by a distance r is given by:
F= 1 q1q2 4πεo r2
1 =k=9×10−9 Nm2/C24πε o
εo is the permitivity of free space and k is the electrostatic constant, q1 and
q2are the charges of the two particles and r is the distance between the two
Prepared by Dr. A. Mekki
• If the two charges have same signs, there is repulsion between them.
If the charge q in inside the uniformly charged shell then the force on the charge
q is ZERO.
● The elementary charge is that of the electron e = 1.6 x 10-19 C. Any
charge on a body is an integer multiple of the electron charge,i.e., Q = n
e, n = ±1, ± 2,...
● Electric charge is always conserved. It can be transferred from one body
to another but cannot be lost.
Electric Potential Energy
• Define the change in electric potential energy (electric potential energy
difference) when a
point charge q moves from some point to another point, as the negative of the
work done by
the electric force to move the charge from point to point
• Eg. Consider electric field between two equally but oppositely charged parallel
separation is small compared to their width and height, so the field E will be
most of the region
• Consider a tiny positive point charge q placed at point A very near the positive
• This range is so small that it doesn’t affect E
• If charge q is released, the electric force will do work on the charge and
accelerate it toward
the negative plate
• Work W done by the electric E to move the charge a distance d is:
W = Fd = qEd
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