Lecture notes for 6-21-07
Electric charge and current, Chapter 8.
(review some of the
previous ideas, and the idea of F E W =
the take-home message)
Another fundamental quantity or base unit is introduced in
this chapter: ELECTRIC CURRENT. Others
we’ve already seen are LENGTH, TIME, MASS and the one that slipped by in
chapter 5, TEMPERATURE. Two more are coming
up later.
Electric current is the motion of electric charge. Current is much easier to measure than
charge, so current is now used as the fundamental unit. Why do we have electric charge in the
universe? Because we have protons and electrons. Protons carry positive charge, electrons
carry negative charge. Neutrons are
neutral. Atoms are made of protons,
neutrons, and electrons. The mass and
charge of each of these particles is given in Table 8.1.
The base unit of electric charge is the Coulomb. Electrons (–) and protons (+) have exactly
equal but opposite charges of 1.6 x 10–19 Coloumb. Why is that?
Not known! There is no theory
that tells why electrons have exactly the same charge (with opposite sign) as
the proton. How then do we know they are
precisely equal? If they weren’t, atoms
wouldn’t exist. A slight difference in
the charge of the proton and electron would make atoms unstable.
Electric current is the rate of flow of electric charge,
defined as
Current = charge / time, or in symbols
I = q/t . The unit of current is the ampere: amperes =
coloumbs/second.
Some materials, such as metals (gold, silver, copper,
aluminum) are good electric conductors.
The outer electrons of these atoms are loosely bound. These materials also are good heat
conductors.
Poor electric conductors have tightly bound outer
electrons—wood, glass, and plastic are examples. There are a few materials that fall in
between good and poor conductance: semiconductors. The main one used in semiconductor devices is
silicon. But there is also carbon, which
is used in resistors.
Electric
Force Like charges repel, unlike charges
attract. In terms of a force being a
push or a pull, like charges push each other away, and unlike charges pull on
each other. The book brings up Newton’s
3rd law. Why or how does it
apply to both pulling and pushing?
(Aside: We don’t
have a two-particle interaction in physics where one is attracted and the other
repelled. In life, between elementary particles known as persons (purse-AHNS),
we do have this interaction, as you may know from experience…):
What is the electric force? Coloumb’s law:
F = k q1q2
/r2
Same form as gravity, an “inverse square law,” but when
one q is negative and the other is positive, the force is attractive (has a
negative sign), and when both q’s are the same sign, the force is repulsive. Gravity is always attractive, meaning it
always exerts a pull and never a push.
Negatively charged objects have an excess of electrons, positively
charged objects have a deficiency of electrons.
How does something become charged?
Friction is a common and usually unwanted way of creating an excess or
deficiency of static charge. But in
humid weather like we usually have in summer, moisture in the air keeps excess
charge from building up.
Bringing a charged object near an uncharged one can cause
a separation of charges in the uncharged object, as in the comb and paper case
(see textbook). The plastic comb is
charged by rubbing (friction). Molecules
in the paper become polarized when
the comb is brought near, meaning they possess definite regions of separated
charge. Polarization induces the
separation of charges, so charging by polarization is called charging by induction. The case of a plastic balloon attracted to
wall or ceiling occurs because first the balloon has picked up excess negative
charge from being rubbed (static charging by friction), then the molecules of
wall or ceiling or whatever become polarized.
Opposite charges are closest to each other, so attraction occurs.
Water is a polar molecule.
Figure 8.3 shows the effect.
Also, water is a solvent, although quite slow in most cases. Plastic water bottles unopened slowly
dissolve.
Voltage and
electrical power
Work is required to separate unlike charges, and when the charges are separated, they have electric potential energy. But this is not convenient to use except in theoretical physics and plasmas. In electric circuits it’s more convenient to use the electric potential difference called voltage, which is the work done per unit charge, or equivalently the potential energy per charge.
V= W/q. The Volt is the standard unit of voltage, and is defined as a joule per coloumb.
Opposition
to the flow of charge is called resistance.
Materials naturally have some resistance, and the measurable effects of
resistance are 1) a voltage must be maintained for a current to exist, and 2) heat
is generated when a current passes through a resistance. The exception occurs for some materials when
they are cooled to near absolute zero and have no resistance, so that once current
is started it will flow even without an applied voltage, and the current
generates no heat. The name for this
process is superconductivity. But the
low, low, low temperature must be maintained, such as 4 degrees above absolute
zero. Not economical, but can be used in
certain cases, such for the high currents needed in elementary particle
accelerators.
Resistance,
on the other hand, is actually useful. It allows
control of currents and voltages, which is what allows appliances and
electronic devices to exist. Two other
current-controlling devices are the capacitor and the inductor, not covered in
text but widely used in circuits.
Ohm’s
law gives the relationship between voltage, current and resistance: V = IR hee.
Water
analogy is helpful. Circuits need an
energy source, and batteries are analogous to a water pump in that
respect. Batteries convert chemical
energy into the kinetic energy of electrons in a circuit. Light bulb produces light and heat because
work is done on electrons by the battery (or by electrical generator in the
case of AC electricity). A switch is like
valve, and the light bulb offers resistance like the water wheel being made to turn
by water in the picture in the book. The
turning of the wheel changes gravitational potential energy of water into kinetic
energy, which can be used to turn a millstone or do other work. Electric current analogy would be the turning of the armature of an electric motor.
Conventional
DC electric current flow goes from positive to negative, opposite from electron
flow.
Gotta
talk about POWER now, since electric devices are rated according to how much power
they use. Power in general is work per
unit time. Using the fact that charge
times voltage equals work, we find power can be expressed as
P =
W/t = (qV)/t = (q/t) V = IV = current
times voltage, whoopee. Also P = I(IR) = I2R, called Joule heating.
Example
8.1, the 60-watt bulb. How much current
passes thru it? P = 60 W, V=120v. Find current, and resistance of the bulb: I = P/V = 60W/120V = 1/2 A, or one-half
amp. That’s quite a bit of current, and
most of the power for the incandescent bulb goes into heat. Resistance of bulb R =
P/I2. = 60/.5squared = 240 ohms.
__________________________stopped
here__________________________________
Electric circuits and electrical safety
DC
or direct current, from batteries.
Electronic devices are inherently DC, and must have a power supply to convert AC to DC. AC is alternating current, produced by
generators. The equations above,
however, apply to DC and AC cases, where the AC is expressed in rms
(root-mean-square) form.
In
circuit analysis, devices that require electrical power are often represented
as resistances, because they do have some amount of resistance, large or small.
(Examples: hairdryer, loudspeaker, your whole house as part of the entire
electric grid.) There are two ways to connect two or more resistances or
electrical devices: series or parallel.
