Download AP Physics B Lesson XXX Electric Field Electric Field E (electric field

AP Physics B
Lesson XXX
Electric Field
I.
Electric Field
a.
E (electric field)
b.
Acts like the gravitational field (g)
c.
Describes Force/unit charge at a point in space
d.
E = F/q
i. Newtons/Coulomb
e.
It is a vector quantity
i. Magnitude and direction
1.
Measured from an arbitrary point (+) charge.
f.
The electric field points away from positive charges and towards negative charges.
g.
Electric Field for point charges
i. Ept charge = kcQ/r2
h.
Multiple charges
i. Enet = E1 + E2…. Etc.
ii. Example – Calculate the magnitude and direction of the electric field at point P which is
15cm to the right of a point charge Q = -3.0x10-6C.
iii. Answer – Epq = 1.2x106N/C to the left
iv. Example II - Two point charges, QA = -25μC and QB = +50 μC are separated by 10cm.
a) What is the magnitude and direction of the electric field at point P, located 2.0cm from
QA and 8.0cm from QB? b) If a proton at rest is placed at point P, what will be its
acceleration initially (mag and direction).
v. Answer – 6.3x108N/C to the left, 6.0x1016m/s to the left
vi. Example III – A positive charge at A causes an electric field at P that is directed along
the positive Y axis and has a value of 3.9x10 5N/C. The negative charge at B causes an
electric field at point P that is acting straight toward B and has a magnitude of
1.8x105N/C. What is the net electric field at point P due to both charges?
Epa
Epb
P
0.40m
B
A
0.30m
vii. Answer – 2.7x105N/C at an angle of 66°.
II. Static Electric Field Lines
a.
Offer a picture of how electric fields are established and how they may influence point charges.
b.
They do not really exist. They just offer a visual aid to help us understand how a point charge can
be influenced if placed in a field.
i. Rules for drawing electric fields.
1.
The lines must begin on positive charges or at infinity and must terminate on
negative charges or at infinity.
2.
The number of lines drawn leaving a positive charge or approaching a negative
charge is proportional to the magnitude of the charge.
3.
No two field lines for the same field can cross each other.