Charge • Comes in + and – • Is quantized • elementary charge, e, is charge on 1 electron or 1 proton • e = 1.602 10 -19 Coulombs • Is conserved • total charge remains constant
Charge•Comes in + and –•Is quantized
•elementary charge, e, is charge on 1 electron or 1 proton
•e = 1.602 10-19 Coulombs
•Is conserved•total charge remains constant
Coulomb’s Law•F = kq1q2/r2
•k = 8.99 109 N m2 / C2
•q1, q2 are charges (C)•r2 is distance between the charges (m)
•F is force (N)•Applies directly to spherically symmetric charges
Spherical Electric Fields
F =kqq0
r2
E = F = kq q0 r2
Why use fields?Forces exist only when two or more particles are present.
Fields exist even if no force is present.
The field of one particle only can be calculated.
Field around + charge
Positive charges accelerate in direction of lines of force
Negative charges
accelerate in opposite direction
Field around - charge
Positive charges follow lines of force
Negative charges go in opposite
direction
For any electric field
F = EqF: Force in NE: Field in N/Cq: Charge in C
Principle of Superposition
When more than one charge contributes to the electric field, the resultant electric field is the vector sum of the electric fields produced by the various charges.
Field around dipole
Caution…
Electric field lines are NOT VECTORS, but may be used to derive the direction of electric field vectors at given points.The resulting vector gives the direction of the electric force on a positive charge placed in the field.
Field Vectors
Electric Potential
U = kqq0
rV = U = kq q0 r
(for spherically symmetric charges)
Electrical Potential
V = -EdV: change in electrical potential (V)
E: Constant electric field strength (N/m or V/m)
d: distance moved (m)
Electrical Potential Energy
U = qVU: change in electrical potential energy (J)
q: charge moved (C)V: potential difference (V)
Electrical Potentialand Potential Energy
Are scalars!
Potential Difference
Positive charges like to DECREASE their potential.(V < 0)
Negative charges like to INCREASE their potential.(V > 0)
Announcements 04/19/23
Lunch Bunch WednesdayGraded quiz tomorrowExam Friday
Potential surfacespositive
negative
highhighest
medium
lowlowest
Equipotential surfaces
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Today...More with electric potential and potential energy.
Definition: Capacitor
Consists of two “plates” in close proximity.
When “charged”, there is a voltage across the plates, and they bear equal and opposite charges.
Stores electrical energy.
Capacitance
C = q / VC: capacitance in Farads (F)q: charge (on positive plate) in Coulombs (C)
V: potential difference between plates in Volts (V)
Energy in a Capacitor
UE = ½ C (V)2
U: electrical potential energy (J)C: capacitance in (F)
V: potential difference between plates (V)
Capacitance of parallel plate capacitor
C = e0A/dC: capacitance (F)
e: dielectric constant of filling
0 : permittivity (8.85 x 10-12 F/m)A: plate area (m2)d: distance between plates(m)
dielectric
Parallel Plate Capacitor
E
+Q
-Q
V1V2
V3
V4
V5
Cylindrical Capacitor
- Q
+ QE
Problem #2
Calculate the force on the
4.0 C charge due to the other two
charges.60o 60o
+4 C
+1 C+1 C
Problem #3
Calculate the mass of ball B, which is suspended in midair.
A
B
q = 1.50 nC
q = -0.50 nC
R = 1.3 m
Problem #2Two 5.0 C positive point charges are 1.0 m apart.What is the magnitude and direction of the electric field at a point halfway between them?
Problem #4Calculate the magnitude of the charge on each ball, presuming they are equally charged.
A B
0.10 kg
1.0 m40o
1.0 m
0.10 kg