electronman said:
After you read my questions, you will understand why I joined this
basic electronics group.
1) Do resistors resist current or voltage or both? Does a resistor
actually use up voltage or current as it passes through the
component? or does a resistor just slow down the passage of voltage
or current? or ...
Resistors resist the passage of current by using up voltage.
Remember that ohms (the unit of resistance is just a
shorthand way to say volts per ampere. 10 ohms of
resistance uses up (drops) 10 volts per ampere that passes
through.
2) From my reading it seems that electrons pass from the negative pole
to the positive pole.
Electrons are repelled by negative voltages and attracted to
positive voltages.
Then please explain why current in a circuit
flows from positive to negative.
Current is not electrons. It is the movement of charge.
Charges come in positive and negative polarities. When a
Negative charge (electron) moves one way, the net charge
moves the other way (a negative going in a negative
direction is the same as a positive going in the positive
direction). I know it sounds stupid that hypothetical
charge is positive when we know that the charge that is
moving in metal is bits of negative charge. But there can
also really be positive charges moving in plasmas and
solutions. The reason that the charge convention does not
match electrons is because electrons had not been discovered
yet when the convention was started. Ben Franklin guessed
that the charge moving through metal was positive charges,
and had a 50:50 chance of getting it right. But as long as
you don't get into the actual particle physics (where you
have to keep track of both the sign of the particle charges
and the sign of their direction), it really makes no
difference whether current is made up of moving positive or
negative charges.
3) I thought the purpose of a capacitor was to hold a charge for a
brief instance, then release it.
I think a better way to think of capacitors is that they
store energy in an electric field by the attraction and
repulsion of displaced surface charges. The key formula is
I=C*(dv/dt) or current is proportional to the capacitance
and the time rate of change of voltage.
So when I connect a resistor then
capacitor then LED in series with a 9V battery, why doesn't the LED
blink on and off as the capacitor charges and releases?
Look at that formula again. When you first connect the
battery, there is no voltage across the capacitor, so the
LED has some voltage available and the rest is dropped
across the resistor. The voltage across the resistor
determines how much current passes through it and the LED.
But that current produces a rate of change of voltage across
the capacitor (charging it up with some of the battery
voltage). This capacitor voltage bucks against the battery
voltage, so that the resistor and LED see only the
difference between them. (Remember that the total voltage
around a loop must be zero.)
So with this lower voltage across the resistor, its current
must also be lower, which produces a slower rate of change
of voltage across the capacitor. It charges slower and
slower as its voltage approaches the entire battery voltage.
So the effect of having an initially discharges capacitor
in series with a battery, resistor and LED is to produce an
initial bright light that fades toward zero. After the LED
fades out, you might remove the capacitor from the circuit,
and measure the voltage it carries with your volt meter.
You should just a rather large capacitor for this experiment
(an electrolytic capacitor of 100 uF to 1000 uF would be
good) so that this process happens slowly enough for your
vision to keep up.
To use the capacitor to time a blinking operation, you need
something that measures its voltage rise and reacts with a
switching decision at some voltage, to short out the
capacitor, and start the charging process over, again and
again. You might Google the LM555 timer to learn about a
chip that is often used for this sort of thing.