Explain the term „Electric Field
Strength“
Coulomb’s Law
Explain why the definition of “voltage” given in grade 9 and the term
“electric potential difference” are
actually the same
Explain the term “Electric Potential”
How can you calculate the electric
field strength inside a capacitor?
The picture shows the electrical
and the charges on a metallic
needle. What is typical about
this field?
What are the rules for drawing
electric field lines?
Are electric potential and electric
potential energy identical?
Force between two charges:
The force on acting upon a charge q in a field is
proportional to the charge.
r
r F
E = is independent of the charge because this is
q
the force per Coulomb.
F=
k Q1 Q2
r2
( k = 9.0 x 109 Nm²/C²)
(no need to memorize k !)
You need to do work if you transport a positive charge q from the negatively to the positively charged plate of a capacitor. It gains the
potential energy
P.E. = Work done = Force x distance
=Exqxd
Electric potential V is potential energy per
Coulomb, hence U = P.E. / q = E x d.
(This formula is only valid in a homogeneous
field.)
The unit of el. potential is 1 V.
O The field lines are perpendicular to the surface. If they weren’t charges would flow along
the surface
O Charges reside only on the surface because they repel each other hence they try to
get as far away as possible
q
F
The electric potential difference ∆U between two
points is also known as Voltage.
For instance the terminations of a power supply
have different electric potentials. As long as they are
not connected, the charges have only potential energy. After being connected the charges will transform this P.E. into other energy forms by doing
work.
V = Work / charge was the definition of voltage
given in grade 9.
V = ∆U is actually the same because ∆U = ∆P.E. / q
and ∆P.E. = Work done.
E=V/d
(Voltage divided by distance)
o The electric field is greatest on edges, tips
etc.
No !
Electric potential is
P.E. per Coulomb!
U = P.E. / q
O They point in the same direction as the force on a positive
charge
O The more field lines you draw,
the stronger the field
O They are perpendicular to the
surface of conductors