Transcript Electric forces and electric fields

Electric forces and electric
fields
•
•
•
•
•
•
•
•
•
1Proprieties of electric charges
(Give an ex. of electrically charged)
Electric charge can be + or –
Like charges repel one another; and unlike
charges attract one another
Electric charge is always conserved
The object become charged because – charge
is transffered from one object to another
An object may have charge of ±e, ±2e, ±3e
e = 1.60219x10-19C
SI unit: C (Coulomb)
• 2 Insulators and conductors
• In conductors, electric charges move
freely in response to an electric force. All
other materials are called insulators (give
an ex. of each)
• Semiconductors are between conductors
and insulators.
• An object connected to a conducting wire
buried in the Earth is said to be grounded.
• Induction – charging of a conductor
• Charging an object by induction requires
no contact with the object inducing the
charge.
• 3. Coulomb’s Law
• An electric force has the following
properties:
• It is directing along a line joining the two
particles and is inversely proportional to
the square of the separation distance r,
between them
• It is proportional to the product of the
magnitudes of the charges, |q1|and |q2|, of
the 2 particles
• It is attractive if the charges are of the
opposite sign, and repulsive if the charges
have the same sign
• The magnitude of the electric force:
• F=ke (|q1||q2|/r2)
• ke – Coulomb constant
ke = 8.9875x109N m2/C2
• 4. Electric Field
• The electric field E produced by a charge
Q at the location of a small “test” charge qo
is defined as the electric force F exerted
by Q and qo divided by the charge qo .
• E=F/qo E=ke (|q|/r2)
• Si unit : N/C
•
•
•
•
•
•
Pb. Strategies:
1. Draw a diagram of the charges
2. Identify the charge of interest
3. Convert all units in SI
4. Apply Coulomb’s Law
5. Sum all the x- components of the
resulting electric force
• 6. Sum all the y-components of the
resulting electric force
• 7. Use Pythagorean theorem to find the
magnitude and the direction of the force
• 5. Electric field lines
• 1. The electric field E is tangent to the electric
field lines at each point
• 2. The number of lines per unit area through a
surface perpendicular to the lines is proportional
to the strength of the electric field in a given
region
• Rules for drawing electric field lines:
• -The lines for a group of point charges must
begin on + charge and end on – charge
• - The number of lines drawn leaving a + charge
or ending a – charge is proportional to the
magnitude of the charge
• - No two field lines can cross each other
• 6. Conductors in electrostatic equilibrium
• When no net motion of chartge pccurs within a
conductor, the conductor is in electrostatic
equilibrum
• 1. the electric field is zero inside of the material
• 2. any excess charge on an isolated conductor
resides entirely on its surface
• 3. the electric field just outside a charge
conductor is perpendicular to the conductor’s
surface
• 4. On an irregularly shaped conductor , the
charge accumulates at sharp points, where the
radius of curvature of the surface is smallest
• 9. electric flux and Gauss’s Law
• The electric flux ( the number of the field
lines) is proportional to the product of the
electric field and surface of the area
• ΦE =EA
• ΦE =EA cosθ
• For a close surface, the flux line passing
into the interior of the volume are negative,
and those passing out of the interior of the
volume are positive
•
•
•
•
E= ke q|/r2
A= 4πr2
ΦE =EA=4π ke q
Permittivity of free space:
εo=1/4π ke =8.85x10-12C2/N m2
• Gauss’s Law:
• ΦE =q/ εo
• The electric flux through any closed
surface is equal to the net charge inside
the surface divided by the permittivity