الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Lectures in General Physics
5.1 Definition of electric potential difference
We define the potential difference between two points A and B as the work
done by an external agent in moving a test charge q o from A to B i.e.
V B -V A = W AB / q o (5.1)
The unit of the potential difference is (Joule/Coulomb) which is known as
Volt (V)
Notice
Since the work may be (a) positive i.e V B > V A
(b) negative i.e V B < V A
(c) zero i.e V B = V A
You should remember that the work equals
r r
W = F . l = F cosθ
l
ex
ex
• If 0 < θ < 90 ⇒ cos θ is +ve and therefore the W is +ve
• If 90 < θ < 180 ⇒ cos θ is -ve and therefore W is -ve
• If θ = 90 between F ex and l ⇒ therefore W is zero
The potential difference is independent on the path between A and B. Since
the work (W AB ) done to move a test charge q o from A to B is independent on
the path, otherwise the work is not a scalar quantity. (see example 5.2)
Dr. Hazem Falah Sakeek