Download Review for Chapter 7 - the law of electric charges:

Review for Chapter 7
- the law of electric charges:
- opposite electric charges attract each other.
- similar electric charges repel each other
- charged objects attract some neutral objects
- charging objects.
i. by friction. the electrostatic series determines what objects will hold electrons and which
will give electrons when rubbed together
ii. by contact. an object with some charge touches another previously neutral object. the
touched object gains a charge of the same sign (positive/negative) as the object with which
it was touched
iii. by induction. an object with some charge is moved close to an uncharged object. the
electrons are repelled or attracted by the charged object and move toward one side of the
originally uncharged object
- electric forces
Q = Ne , where e = 1.6 ¥10-19 , Q is charge and N is the number of electrons
Fµ
†
†
1
and F µ q1q2 , where F is the force of the electric field, r is the distance between two
r2
†
†
†
specific charged objects, and q1 & q2 are the charges of the two objects
†
†
†
- Coulomb’s law: the force “from a distance” between two charges is proportional to the product of
the charges of the two
and inversely proportional to the square of the distance between
† particles
†
them
F=k
q1q2
2
, where k = 9.00 ¥10 9 N ⋅ Cm 2
2
r
Coulomb’s law only applies when the charges are very small and the two objects are small
†
†
compared to the distance between them.
- field of force. a field of force exists in a region of space when an appropriate object placed at any
point in the field experiences a force
- electric field lines.
-
+
†
†
by convention, electric field lines start at positive charges and end on negative charges.
field lines never cross.
- the electric field at any point is defined as the electric force per unit positive charge and is a
vector quantity:
r
r FE
e=
, and the units are N / C
q
if the positively charged object is small enough to†be considered a point charge, then the electric
†
field at any point a distance r from that point is directed radially outward with a magnitude of:
r
r FE
qq
q
e =
= k 1 22 = k 12
q
qr
r
†
for a conductor in static equilibrium, the electric field is zero inside the conductor, because the
†
charge is all on the surface, and the electric field extends perpendicular to the surface of the
charged object
- static equilibrium: a term meaning “at rest”, referring to acceleration, not charge - the “static”
part is static as opposed to dynamic equilibrium, which means moving, but not accelerating.
- electric potential energy.
EE = k
q1q2
r
- if q1 and q2 are opposites, E E will be negative, and the charges will be attracted to each other.
†
- if q1 and q2 are of the same sign, the charges will repel and E E will be positive
†
† energy is stored by moving charged objects closer together
-†electrical potential
†
-†potential energy per unit positive charge is the electric
† potential, V . V represents the amount of
work necessary to move a unit positive test charge from rest at infinity to rest at any specific point
in the electric field of a charged object.
† †
V=
V=
†
EE
q
kq1 q 2
r
q
q
V = k 1 , with units
r
†
J
C
=V
- 1V is the electric potential at a point in an electric field when 1J of work is required to move
†
†
1C of charge from infinity to that point.
†
†
- electric potential energy ≠ electric potential ( E E ≠ V ) †
- E E = qV . potential difference is the change in electrical potential from having a charge move
from one point to another:
†
†
DE E = q ⋅ DV
r DV
- e =
. the magnitude of the strength of an electric field in N / C is the potential difference
r
†
divided by the distance between the objects
†
†
- motion of uncharged particles. a particle placed in an electric field will experience a force moving it
r
in a specific direction if it is free to move. its acceleration is described by: a =
†
r
FE
m
- as an object is repelled by another charged object, the electric energy is turned to kinetic
energy, and all the energy is conserved: -DE E = DE K
-differences between Coulomb’s Law and Newton’s Law of Universal Gravitation.
†
Coulomb’s Law
Newton’s Law
an inverse square law
an inverse square law
proportional to products of quantities
characteristic of particular force
proportional to products of quantities
characteristic of particular force
acts along a line passing through the centres of acts along a line passing through the centres of
the objects
the objects
can attract or repel
can only attract!
2
very large constant k = 9.00 ¥10 9 N ⋅ Cm 2
†
2
very small constant G = 6.67 ¥10-11 N ⋅ kgm 2
†