Download Chapter 16: Electric Charge and Electric Field

Chapter 16: Electric Charge and Electric Field
Static electricity - Discuss charges that are at rest.
Electric charge
 positive (+ attracting electrons to stabilize)
 negative ( - repelling, loosing electrons to stabilize)
 defined by Benjamin Franklin (1706 – 1790)
 Whenever a certain amount of charge is produced on one body in a process, an equal
amount of the opposite type of charge is produced on another body.
 Unlike charges attract; like charges repel.
 Any charge is plus or minus – so many coulombs (C), in SI units.
 Law of conservation of electric charge: net amount of electric charge produced in any
process is zero.




The magnitude of the charge on protons and electrons is exactly the same, but their signs are
opposite.
An atom with extra or lacking electrons is an ion.
Water molecules are polar – the charge is not distributed uniformly
Metals are good conductors (contain a lot of free electrons) – other materials are insulators
Charging:
 Charging by conduction – by contact (two objects end up with same charge)
 Charging by induction – bringing up close (one positive, other negative)
 Electroscope: device used for detecting charge.
Coulomb’s Law
QQ
 F  k 12 2
r
 Coulomb’s law gives the force between two point charges q1 and q2, a distance r apart.
 k is a proportionality constant.
 k = 8.988 * 109Nm2C-2 = 9.0 * 109Nm2C-2
 applicable for point charges
 if several charges are present, the net force on any one of them will be the vector sum of
the forces due to each of the others.
The Electric Field
 developed by Faraday (1791 – 1867)
 Electric field, E, at any point in space is defined by the magnitude of the test charge q
F
 E
: force per unit charge
q
 E is defined as the limit of F/q as q is taken smaller and smaller, approaching zero.
Q
 E  k 2 (N/C or V/m)
r
Field Lines
 The number of lines starting on a positive charge, or ending on a negative charge, is
proportional to the magnitude of the charge.
 The closer the lines are together, the stronger the electric field in that region
 Electric field lines start on positive charges and end on negative charges
Chapter 17: Electric Potential and Electric Energy; Capacitance
Electric Potential and Potential Difference
 Electric potential: the potential energy per unit charge.
 Only differences in potential energy are physically measurable (voltage)
W
 Vab   ba
q
 Unit: J/C = V
 The ground of a conductor connected directly to the ground is often taken as zero
potential
 Change in potential energy: PE  qVba
Relation Between Electric Potential and Electric Field
W  qVba

W  qEd

Vba  Ed
The electric field in a given direction at any point in space is equal to the rate at which
the electric potential changes over distance in that direction
Equipotential Lines
 An equipotential surface must be perpendicular to the electric field at any point.
 A conductor must be entirely at the same potential in the static case, and the surface of a
conductor is then an equipotential surface.

Electron volt: 1eV = 1.6 * 10-19 J

Electric potential due to point charges: V  k
Q
r
Chapter 18: Electric Currents
The Electric Battery
 Electrodes are two plates of rods made of dissimilar metals (one can be carbon)
 Electrolyte: the solution such as a dilute acid into which the electrodes are immersed
 Electric cell: composed of 2 oppositely charges electrodes, and electrolyte
 Battery: several cells connected together
 Terminal: the part of each electrode remaining outside the solution
Electric Current
 Circuit: a continuous conduction path between the terminals of a battery
 Electric current: a flow of charge through the wires, from one terminal to the other
Q
 I
t
 Ampere (A) = 1 C/s
 Conventional current flows from positive to negative
 Electron flow is form negative to positive
Ohm’s Law: Resistance and Resistors
 IV
 Ohm’s Law: the definition of resistance V  IR
 Ohm  = V/A
 Resistors are used to control the amount of current
Resistivity
 The resistance R of a metal wire is directly proportional to its length L and inversely
proportional to the cross-sectional area A.
L
 R
A
  = resistivity, the constant of proportionality which depends on the material used. Unit
is  m.
 Effect of temperature: T   0 [1   (T  T0 )]
Electric Power

P  power 
P  IV

energy.transformed QV

time
t
P  I 2R
V2
P
R
Microscopic View of Electric Current
 When an electric filed exists in the wire, the electrons feel a force and initially being to
accelerate. But they soon reach a more of less steady average speed (due to collisions
with atoms in the wire) know as their drift speed, vd.
Q
 neA d
 I
t
N
N
N (1mole)
 n 


V m /  m(1mole)
Chapter 19: DC Circuits
Resistors in Series and in Parallel
 Series: resistors are connected end to end
I  I1  I 2  I 3
V  V1  V2  V3  IR1  IR2  IR3
 Series circuit: V  IReq
Req  R1  R2  R3
Parallel circuit:
V
V
V
V



Req R1 R2 R3
1
1
1
1



Req R1 R2 R3
EMF and Terminal Voltage
 A device such as a battery or an electric generator that transforms once type of energy
(chemical, mechanical, light, and so on) into electric energy is called a seat or source of
electromotive force or of emf.
 A battery itself has some resistance, called its internal resistance. (r)
 Terminal voltage (Vab)
 Vab    Ir
Kirchhoff’s Rules
 Kirchhoff’s first or junction rule (conservation of charge): at any junction point, the sum
of all currents entering the junction must equal the sum of all currents leaving the
junction.
 Kirchhoff’s second or loop rule (conservation of energy): the sum of the changes in
potential around any closed path of a circuit must be zero.
 Voltage drop: decrease in a voltage between the two ends of a resistor. Because of the
decrease in a voltage, we use a negative sign when applying Kirchhoff’s loop rule
DC Ammeters and Voltmeters
 Ammeter: used to measure current (inserted into the circuit)
 Voltmeter: measures potential difference or voltage (connected in parallel)
 Galvanometer: the reading is by a pointer on a scale – works on the principle of the force
between a magnetic field and a current-carrying coil of wire
 The deflection of the needle of a galvanometer is proportional to the current flowing
through it.
 The full-scale current sensitivity, Im of a galvanometer is the current needed to make the
needle deflect full scale.
 An ammeter consists of a galvanometer in parallel with a resistor (shunt resistor)
 Voltmeter uses series resistor
 effects of meter resistance: the more sensitive the galvanometer the less effect it will have
Chapter 20 Magnetism
Magnets and Magnetic Fields
 Every magnet has a north pole and a south pole where the magnetic effect is strongest.
 Like poles repel; unlike poles attract
 Unlike electric charge, it is impossible to isolate a single magnetic pole
 Only iron and few others such as cobalt, nickel and gadolinium show strong magnetic
effects (are ferromagnetic).
 A force one magnet exerts on the other can be described as the interaction between one
magnet and the magnetic field of another.
 Magnetic field lines:
o The direction of the magnetic field is tangent to a line at any point
o The number of lines per unit area is proportional to the magnitude of the
magnetic field
o Lines point the way that a compass arrow points (always towards south)
 Magnetic field is a vector B with direction defined by compass arrow and magnitude by
the torque exerted on a compass needle
Electric Currents Produce Magnetism
 Hans Christian Oersted (1777 – 1851) found that an electric current produces a magnetic
field.
 Right hand rule: the direction of the magnetic field is the direction of thumb & fingers.
Force on an Electric Current in a Magnetic Field; Definition of B
 A magnet exerts a force on a current-carrying wire
 The direction of the force is always perpendicular to the direction of the current and also
perpendicular to the direction of the magnetic field, B (use right-hand rule, again)
F  IlB sin 

(magnetic field B is in tesla, T = N/Am)
Fmax  IlB
Force on an Electric Charge Moving in a Magnetic Field
 F  qvB sin 
 Fmax  qvB - again use a right-hand rule
Magnetic Field Due to a Straight Wire
I
B
r

- magnetic field due to current in straight wire
0 I
B
2r
  0  4  10 7 Tm / A
Force between Two Parallel Wires
F  0 I1 I 2


l 2 L
Definition of the Ampere and the Coulomb
 One ampere is defined as that current flowing in each of two long parallel conductors 1m
apart, which results in a force of exactly 2  10 7 Nm 1 of length of each conductor.
 Thus the coulomb is exactly one ampere-second: 1C = 1As
Electromagnets and Solenoids (p.610)
 Solenoid: a long coil of wire consisting of many loops of wire
 The magnetic field is large (sum of the fields due to the current in each loop)
 Acts like a magnet (north and south poles)

Ampere’s Law
