Physics II
Preparation for First Test Physics II
Coulomb’s law: It is one of the fundamental equations of electromagnetism. It gives you
the force on a charge due to the attraction or repulsion of another charge.
Fk
q 1q 2
, where k  9x10 9 Nm 2 /C 2
2
r
Remember that force is a vector, so direction is important. If two or more charges
produce forces you will need to add vectors, not magnitudes. Make sure that the
denominator, which is the distance between the charges, is squared.
You can also divide the force by the charge at that point and get a quantity called electric
field:
E
F
q
 k 21 , where k  9x10 9 Nm 2 /C 2
q2
r
This is also a vector, so be careful when adding several electric fields. Use the vector
rules that you learned in Physics I. Do not add magnitudes.
Electric Potential: The definition is work done on a test charge divided by the test
charge. So you can write:
V
q
Work
, and that means that the field produced by a point charge is V  k
r
qtest
This is a scalar quantity! That makes our lives easier because we don’t need to deal with
components, directions etc. When you want the potential due to several charges just add
them as numbers. Be careful with the signs though, a positive charge produces positive
potential and a negative charge negative potential.
Also, notice that the denominator is just “r” no squares here.
Definition of electric current: I=Q/time We used this definition to look at electrolysis.
Capacitors: Their capacitance is defined as C=Q/V.
Also:
Energy 
1
1
Q2
CV 2  QV 
2
2
2C
is the energy stored in a capacitor
DC circuits: We studied Ohm’s law V=RI, which is an equation that you should
remember the rest of your career when thinking about Physics II in the spring of 2016.
We also discussed:
V2
P=IV=
 RI 2 Power dissipated by an electric resistance
R