3. Electrostatics
Charge is carried by subatomic particles
(protons, electrons)
• 99% of all charged effects caused by electron
transfer
Charging by Conduction
• Physical contact
Charging by Induction
• No physical contact
4. Coulomb’s Law
This law determines the force of attraction or
repulsion between 2 charged objects
• ε0 is a constant – permittivity of free space
• Positive force = repulsive, negative force = attractive
• Remember: force is a vector!
F
Q Q
r
q =
1
4 0
1 2
2π ε
5. Electric field lines
A visual
representation of
an electric field.
• More lines =
stringer force
• Point away from
positive, toward
negative.
6. Electric Fields and conductors
The electric field inside any conductor is
zero
The electric field is always perpendicular
to the surface of a conductor
7. Gauss’ Law
Electric Flux: The amount of an electric field
passing through an area
Gauss’ Law: The total electric flux passing
through a closed surface is proportional to the
charged enclosed in that surface.
Φ =
Q e n c l o s e d
ε 0
Φ = E A c o s θ
8. Electric Potential Energy
Electric Potential energy can be determined
using mechanics
Electric potential is defined as the electric
potential energy per unit charge
∆ U q E d= −
V
U
q
W
q
= = − ∆ ∆U q V= −
9. Equipotential lines or surfaces
An equipotential surface is a surface
over which all points have the same
potential.
• An equipotential surface must be
perpendicular to the electric field!
10. Potential due to a point charge
V
Q
r
=
1
4 0π ε
• Remember: potential is a scalar!
11. Capacitance
A capacitor is a device that stores electric
charge.
The capacitance of an object is defined as:
Capacitance is measured in farads.
C
Q
V
=
12. Parallel plate capacitors and
dielectrics
For a parallel plate capacitor (two conducting
plates with a vacuum between the plates)
Often, an insulator known as a dielectric is
placed between the plates to enhance
capacitance
• Dielectric constant: measures the strength of the
dielectric
C
A
d
=
ε 0
13. Capacitors and energy
A charged capacitor stores an amount of
electric energy given by
• This energy can be thought of as stored in the electric
field between the plates.
U Q V=
1
2
2
14. Electric Current
Electric current is defined as the amount
of charge that flows past a given point in
a second
15. Ohm’s Law
Ohm’s Law related the resistance of an
object to the decrease in electric potential
across a point and the current flowing
through that point.
R
V
I
=
16. Electric Resistance
Electric resistance is the innate ability of a
material to inhibit the passage of electrons.
• Measured in ohms.
• Given by the resistivity as well as the geometry of the
object.
R
L
A
= ρ
17. Circuits – emf and terminal
voltage
A device that transforms one type of energy
into electrical energy is a “source of
electromotive force”
• emf: the potential difference between the terminals of a
battery when there is no current flowing to an external
source.
• A battery has some internal resistance
• The real voltage of a battery is then
V E I r= −
18. Resistors in series
Voltage and
resistance are
additive
Current is constant
everywhere in a
series circuit
R Re q i
i
= ∑
V Vt o t a l i
i
= ∑
I I It o t a l = = =1 2 . . .
19. Resistors in parallel
Current additive
Voltage is constant
everywhere in a
series circuit
More resistors =
smaller equivalent
resistance
1 1
R Re q ii
= ∑I It o t a l i
i
= ∑
V V Vt o t a l = = =1 2 . . .
21. Kirchhoff’s rules
Junction rule: At any junction point, the
total current into the junction has to be
equal to the total current out of the
junction.
Loop rule: The sum of changes in
potential around and closed loop is zero.
23. Magnetism
Every magnet has two poles: north and
south
Magnetic field & magnetic field lines:
analogous to electric field
• Direction: points north to south
Electric current (moving charge)
produces a magnetic field!
24. Force due to magnetic fields
The force on a charged particle moving through
a magnetic field
The force in a current carrying wire immersed
in a magnetic field
F q v B= s i n θ
F I L B= s i n θ
26. Ampere’s Law
A moving charge (current) creates a magnetic
field.
• For a long wire, ∆l = 2πr
• Two wires can attract or repel due to this effect.
• A solenoid is a long coil of wire.
B l Ii
i
e n c l o s e d∆∑ = µ 0
27. Faraday’s Law
A changing magnetic field induced an emf.
• A current produced by an induced emf moves in a
direction such that its magnetic field opposes the
original change in flux (Lenz’s Law)
• A coil rotating in a magnetic field is a good example
of this.
E N
t
= −
∆ Φ
∆