# 0. What is Current ? 1. What is Voltage? 2.What is Frequency? Answered

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My Dout is :

0. What is Current ?

1. What is Voltage?

2.What is Frequency?

I know Definition. But i need practical example for this thing OK.

So what is my dout is

1. When voltage increased current is also increased !
But in some case When voltage increased current decreased OR voltage decreased current increased.

How it is possible Current is depend on voltage so
When Voltage is zero then current is also zero only know

For Example:

When 100 Watt bulb receive high voltage then high electron will flow through tungsten of bulb then bulb will give high brightness ok

Then same 100 Watt bulb receive low voltage then low amount of electron only flow through the bulb then bulb give low brightness

This is the Rule or Nature
So Current is depend on voltage only.

Then how voltage decreased when current increased.

This is my basic dout not only this and many dout about frequency also

And Thank U for Replying ME.

Thank U

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## Discussions

Electric Charge:

The definitions of current and voltage are both based on the assumption of electric charge,
http://en.wikipedia.org/wiki/Electric_charge
and the idea that there are these little microscopic things, like electrons, ions, etc, that carry this charge stuff.

Current:

If you are willing to believe in electric charge (measured in coloumbs), then electric current is simply a flow of electric charge.  If you can imagine a sort of river of electrons, existing in a piece of wire,and you could (somehow) see (or measure, or detect) these electrons flowing by, then a current that moves one coulomb of charge into one end of the wire,  and out through the other end,  every second in time, would be an electric current of one ampere; or mathematically:  1 ampere = (1 coulomb)/(1 second) = 1 A = 1 C/s

By the way, this flow of charge does not have to be moving electrons.  It could be a flow of zinc ions, which carry a charge of +2e per ion, compared to electrons themselves which carry a charge of -e, where e = 1.6*10^-19 coulombs.  Part of the electric current inside an alkaline battery actually is a current of zinc ions.

However, for most purposes, the electrical engineer does not actually care what the current is made of of microscopically. Is it 6*10^18 electrons flowing to the left? Or is it 3*10^18 zinc ions flowing to the right?  Macroscopically, it does not matter, and the engineer, he or she just think of as electric current as an abstraction of its own.

When a battery discharges, positive current flows from the terminal labeled (+), then through the load (e.g a flashlight bulb) and then to the terminal labeled (-) , then through the battery, from the (-) terminal back to the (+) terminal, in a loop,  called a "circuit".

Voltage:

If you are willing to believe that electric charges move, in the form of electric currents, you might ask:  What causes them to move?   Or what  compels them to move, in one direction or the other.

Voltage is essentially an explanation for why charges move, and also an explanation for how moving charges can absorb, carry, and dissipate energy.

What voltage is, by definition,  is a difference in potential energy, per unit charge.  The SI units for this are joules of energy, and coulombs of charge, so that:

1 volt = (1 joule)/(1 coloumb) = 1 V = 1 J/C

So you can imagine there is a tiny amount of positive charge sitting at the positive (+) terminal of a battery, and this charge wants to move, feels a force compelling it to move, towards the negative (-) terminal.  Why does this small piece of charge want to move? Because it has potential. Because there is a voltage difference between the (+) terminal and the (-) one. If a wire, or flashlight bulb, is connected between the terminals, then a current will flow. Charge will move where, in the direction that voltage is telling it to move.

The Water Analogy:

If charge "wanting to move" seems like too much of an abstraction, consider a tank of water, on top of a tall tower.  The water in the tank on the top of the tower wants to move to the bottom of the tower.  Why does it want to move this way?  Because the force of gravity is pulling the water downwards.  Or another way of saying that is that the water at the top of the tower has more potential energy than water at the bottom of the tower.  If a pipe is connected between the top of the tower and the bottom, then water will flow, and it will flow downward, in the direction that potential is telling it to move.

That's part of the water analogy.  However one important difference between water and electric charge, is that it is somewhat difficult to pile up a large amount of one kind of charge in one place, because like charges repel one another.  For example you can fill a bucket with water, but you cannot fill a bucket with one kind of electric charge, even if it is a metal (conductive) bucket.  Or that is to say, if you try to do this, to fill a bucket with electrons, they all just smear themselves into a thin layer on the outside surface of the bucket.

What this means for the water analogy, is there is a rule that says whenever water flows out of some thing (eg. the water tank on the tower) then the same amount of water must flow into it.  So if water is flowing out of the tank, downwards through a pipe, then there must also be water flowing into the tank; i.e there is also a pump moving water up from a reservoir of water at the bottom of the tower.  In doing this, the pump does work, and puts potential energy into the water at the top of the tower.  Water flowing downward from the top of the tower to the bottom loses this potential energy, and give it up in the form of heat.  The pipes carrying the water get warmer due to friction from the water moving through them.

In terms of a battery connected to a flashlight bulb, the battery does work, it puts energy into the charges.  Then these charges lose energy when they move through the flashlight bulb filament, giving it up in the form of heat (friction).

In fact just from the relative direction of voltage and current, you can tell which circuit elements are doing work, and which are absorbing work.  When a current flow and voltage increase are in the same direction this indicates that a component is putting energy into the circuit, like the discharging battery.  When current flow and a voltage drop are in the same direction, this indicates a component that is recieving energy from the circuit, like the flashlight bulb.  Mathematically this is P = I*V, and in the usual way it is defined, P, as power taken from the circuit, like the energy dissipating flashlight bulb, is positive. In accordinace with this definition, a device that dissipates negative power, like the battery,  is actually supplying power to the circuit.

Also notice that for a rechargable battery:  Under discharge conditions, positive current flows out of the (+) terminal into the (-) terminal.  However when the same battery is charging, positive current flows into the (+) terminal and out of the (-) terminal. And this is consistent with the idea that the relative direction of voltage and current tell you which way energy is flowing.  Energy flows out of the battery when it discharges, and energy flows into the battery when it re-charges.

Somewhere on the internet here, there must be some pictures for this water analogy, because this is one of the ways voltage and current are commonly explained.
Um... here's a good one:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html

Resistance and Ohm's Law:

A resistor is a circuit element which dissipates power, and does so in a predictabale way.  Specifically the current which flows through it directly proportional to the voltage across it; i.e. I= V/R and P = I^2*R = V^2/R.  Note that if you were to make a graph of current (I) versus voltage (V) for a resistor, it would be straight line that intercepts the origin (0V,0A) and has a slope of 1/R.

In contrast, if you plot current (I) versus voltage (V) for a dissipative load that is not a resistor, like a light bulb, or a diode, or a LED, or a recharging battery, you will, in general get a curve that is not straight line intersecting the origin.

And the moral of that story is that not every load responds linearly, drawing a current that is directly proportional to the applied voltage.  The only load that actually responds that way, is a resistor.  So not everything is a resistor, and I am sorry if that makes the math harder.

By the way, this idea of a graph defining current (I) versus voltage (V) for a particular circuit component is called an I-V characteristic, and you can read more about those here:
http://en.wikipedia.org/wiki/Current%E2%80%93voltage_characteristic

Causality between voltage and current:

As I have told the story so far, it is voltage which causes current to flow; i.e voltage is the reason why the charges wanted to move (cause), and current is the result of charges following their desire to move (effect).

Usually cause precedes effect in time.

However the amount of cause-effect time delay between the voltage and current in electric circuits is so small, and the relationship so predictable, that you might as well just assume that current can cause voltage too.  Or that it is not really one causing the other, but that they are linked.  They just always happen together.

In this sense,  a current can be said to cause a voltage.  For example a resistor can be said to have a voltage drop across it, as a result of the current flowing through it.

Frequency:

An AC power source is one for which the voltage changes direction periodically; e.g. at one instant in time the voltage difference between its terminals is +300 V, then 0.01 seconds later the voltage difference is -300V,  then another 0.01 seconds after that it is back to +300V again, and it just does this, rapidly switching back and forth.

The period of such a signal, or AC source,  is the amount of time it takes to complete one cycle.  For the example given above this is T= 0.02 seconds, the amount of time it takes the voltage to get back to the value it started at.

Frequency f is the inverse of the period T.   (f= 1/T) It is the number of cycles that occur per unit time, e.g. number of cycles in one second.  For the example given above that would be 50 cycles per second (since 1s/0.02s =50).  There is even a unit for frequency called "hertz"  or Hz

50 cycles per second = 50 Hz

You can read more about AC power lots of places, but just to pick one, I'll link you that hyperphysics place I linked to before:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html

Current depends upon both voltage and resistance; this is Ohm's Law: V = I*R. An incandescent light bulb has a fixed resistance (it depends on the size of the filament). So higher voltage means higher current, because R is constant.

Frequency has nothing to do with Ohm's Law. Electricity is supplied to your home in the form of alternating current (look it up). The direction of the current flow switches back and forth at some frequency set by your national power grid (60 Hz in North America, 50 Hz in Japan, and so on).

For electronics more complicated that simple resistors or light bulbs, the effective "resistance" of the system (called impedance, look it up) depends on frequency.

Terrible example: A light bulb is HORRIBLY non-linear ! The bulb's hot resistance is a LOT higher than its cold.

It all looks rather complicated doesn't it? Just think of the ohms law equation (voltage=amperage times resistance) E=I X R with this analogy of WEIGHT GAIN. If you eat a lot or exercise very little, it will affect your weight. so your
weight= exercise times calories eaten. If you eat a lot, but dont exercise, your weight will go up. Now to prove this, just do several sample calculations using various values of amps and ohms to see how it affects the voltage. Those calculations are based on a steady flow of electricity called "DC" (direct current). Just like the voltage out of a battery, it flows steady and does not turn off and on hundreds of times a second. The flow is steady without interruption, so it has a frequency of ZERO. If the electricity turned off and on 100 times a second, then we would say it has a FREQUENCY of 100 Hertz. If someone slapped you 23 times in one second, that is a frequency of 23 hertz.

If a 12 volt battery is just sitting by itself.... ZERO amperes are flowing. If a light bulb is connected then current flows because the bulb has resistance (R). If two similar bulbs are connected(parallel to the battery), then TWICE as much current flows.
If however the 2 bulbs are connected in SERIES (in a row) with each other to the battery, then the resistance doubles and current is cut in half. In this way, you should understand that the battery is a fixed value in most circuits and therefore will always be 12 Volts for this example ohms law equation. The value of "R" or resistance of the bulbs will determine how much current flows in the circuit. Zero amps flow if no bulbs are connected. One amp may flow if ONE bulb is connected. Two amps will flow is 2 bulbs are connected.

This is no more complicated than the example of your WEIGHT = CALORIES times EXERCISE... so don't get frustrated and confused.