Voltage/Current Questions

Factual Questions
1

I am confused by voltage and current.

  1. What is the difference between voltage and current? I know that voltage is a potential difference between two points in a circuit, and current is the number of electrons passing a point in a second, but how do the two relate to each other? e.g. Increase in voltage=Increase in current? Must there always be voltage and current? i.e. one cannot exist without the other.

  2. If a circuit has 230V at 13A, and the current is reduced to 6.5A using a resistor (If V=IR , I suppose that the resistance is 35.38 ohms), how does this affect an appliance powered by it? If it is 115V at 13A, how does this affect the appliance?

  3. Alternating Current is a common method of transmitting current. Is this because there is a loss of power over a long line if DC is transmitted? Can voltage alternate?

  4. How is charge (Q) related to voltage and current?

  5. A computer draws 230V at 13A from a socket, but it only uses DC at several volts. What equipment uses 230V at 13A?

  6. In an AC (sinusoidal) waveform, AC reaches a peak and then falls back to zero, before reaching the same peak reverse-biased. At the beginning of the cycle the voltage is zero, but at the peak it is approximately 339.4V. If it is Alternating Current why does the voltage change?

It would find it helpful if someone could answer these questions.

Thanks.

Dunmurry

2

Here’s an answer for #3:

There is a loss of power when ever there is a current flow through a wire. The loss is proportional to the current, while power is a function of voltage and current. The same amount of power can be transmitted by increasing voltage, thereby having a lower current. This provides for lower loss through heating of the lines.

AC allows the voltage to be increased and decreased using transformers. Transformers don’t work with straight DC, so it’s more difficult to acheive the lower transmission losses.

3

In #5 you say “a computer draws 230V at 13A from a socket”. That’s almost 3000 watts. Upon what do you base this statement? Certainly not a desktop PC?

The Dell on my desk has a data plate claiming it needs 6A supply at 115V and 3 A at 230V. That’s for the CPU.

4

You can have voltage without current, and you can have current without voltage. A fully-charged 9V battery that isn’t hooked up to anything is an example of the former. (I’m neglecting leakage currents and stuff.) A current induced in a ring made from a superconductor is an example of the latter.

Introducing a series resistor has two effects: 1) It decreases the overall current, no matter what, 2) It decreases the voltage at the appliance when the appliance is turned on.

It is much more efficient to transfer power using high voltage and (relatively) low current. This is because insulation is much cheaper and less bulky than copper or aluminum. The problem is that the end user cannot use a high voltage. Therefore we need a convenient and efficient method for increasing/decreasing voltage. Transformers do a wonderful job of this, but there’s one catch: transformers only work with AC. Thus we use AC for power distribution.

Charge is a fundamental quantity, sort of like mass. Electrons and protons “have” charge, and electrical current is simply defined as “charge/time.”

Lots of things do, especially things with motors and/or heaters.

Both the voltage and current are alternating.

5

Current is the rate of change of charge with time. Its units are coulombs of charge per second.

Voltage is the work done in moving a charge in an electric field. Its units are joules of work per coulomb of charge.

Voltage and current are always related to each other by the impedance. V = I*Z

It depends on the nature of the appliance. Appliances like an electric heater or a motor have a starting current that is high as compared to the running current. The heater will be very slow to warm up and the motor might not start. An electric light bulb would be slow to attain max brightness. By the way, in your example the resistor would have to dissipate 459 W. so you had better not touch it.

Transmission over long distances is most efficient when the voltage is high and the current low because line loss is I[sup]2[/sup]*line resistance. There are some other losses as well such as radiation but at 60 Hz. the main loss is from the circuit resistance. So you want as high a voltage as possible. At the same time you don’t want 550 K volts running around people’s houses. Alternating current can be transformed from low to high and high to low voltages with a simple device called a transformer, direct current can’t.

Yes. voltage is alternately plus and minus during a cycle. That is first one terminal is positive with respect to the other for half a cycle and then the polarity reverses for the next half cycle.

See the answer to 1).
5) A computer draws 230V at 13A from a socket, but it only uses DC at several volts. What equipment uses 230V at 13A?

Things like arc welders, electric motors, electric furnaces etc.

“Alternating current” is just shorthand for circuits in which both voltage and current alternate in polarity and direction. Voltage and current are always related in a circuit by the impedance. Both reverse although the reversals are not necessarity exactly in phase, i.e. at exactly the same time.

6

You are correct, but also keep in mind that AC and DC equally suffer from I[sup]2[/sup]R heating losses. In other words, AC is not inherently more efficient than DC when you’re considering just the conductors. In fact, it is slightly less efficient, since AC also has inductive losses. But when you look at the complete picture AC is much, much more efficient.

7

I’ll see what I can do.

  1. You already described the difference between voltage and current. Voltage can certainly exist without current. If there is no charge path between Point A and Point B, you can have a voltage difference between the points and no current will flow. If the voltage gets high enough, a current path may appear. For example the air can be ionized in the presence of the high voltage causing sparks or lightning.

NB: Current is not quite the number of electrons passing a point per second, but it is related. Current (in amperes) is the number of coulombs of charge passing a point per second. The charge on an electron is -1.6021892 10-19 coulomb.

Current without voltage is a little harder. You can achieve this with superconductors, in which you get the current started and it just keeps going on its own.

The usual relationship between current and voltage is V = IZ where Z is the impedance of the device or devices being supplied with the current and voltage. For resistors, Z is R, the resistance. If you have AC power and the circuit is not entirely resistive (and no real circuits are), the impedance is best expressed as a complex (real and imaginary) number, with the voltage and current also being expressed as complex functions. What this boils down to is that in a real AC circuit, the voltage and current will be out of phase with each other.

  1. I’m just guessing here, that you have a power supply that is specified for 13 amps at 230 volts. If you hook it up to the thing it is supposed to power and, in parallel, a resistor that will draw off 6.5 amps, most likely the power supply will not be able to maintain the voltage of 230 volts. In this case, the device that requires the 13 amps will probably not function correctly.

If the “power supply” is a wall socket in your house, it will probably manage the 230 volts, put out more current than the fuse or circuit breaker can handle, and blow the fuse or breaker. If you don’t have an adequate breaker, it may melt the wires in the wall and start a fire.

  1. In AC power transmission, both voltage and current alternate. There are a number of reasons for this choice. One is that it’s easy to design a motor to work on AC. Another is that Tesla won and Edison lost.

  2. The relationship of charge to current is given in (1) above. The relationship of charge to voltage is more complicated since it depends on how the charge is arranged. A capacitor of capicatance C with a static charge Q on it has a voltage given by V = Q / C.

  3. The 230V, 13A is consumed by the computer’s power supply which consists of one or more transformers and associated circuitry which produce the various DC voltages required by different parts of the computer circuitry.

  4. Both the current and the voltage alternate because they are related as described in (1) above. You can’t have an alternating thing on one side of the equation and a DC thing on the other. The impedance Z does not normally change with time.

If you want more complete answers, I suggest that you take a physics or electrical engineering course. The physics course will probably be the second semester of a 2- or 3- semester sequence, so you will have to take mechanics first. The EE course will most likely be the introductory semester.

Chava

8

True, if assume there is impedance in the circuit.

An ideal battery not hooked up to anything definitely has a voltage, but no current (since Z = infinity). And a ring made from a superconductor can have a current with no voltage drop (Z = 0). These “circuits” don’t do any work, which (obviously) makes them useless. But they are instructive for academic purposes.

9

If you think of electricity as a flow of electrons through a wire, the current is the volume of the flow (how many electrons are moving) while the voltage is the force pushing the electrons. An electric current needs a force to push it through a load. The force is provided by generating an electric potential in the source - either through a chemichal reaction in a battery or as the result of rotating a conductor in a magnet field in a generator. The amount of current which flows is determined by the resistance of the load, by ohm’s law: V=IR, or I=V/R. For example, a 5V source connected to a 1K resistor will generate a current flow of 5/1000=0.005 amps. Increasing the voltage while keeping the load the same will increase the current. Increasing the load resistance while keeping the voltage the same will decrease the current.

Decreasing the current will be accomplished by putting a resistance in series with it, which will increase the total load resistance. This will cause the effective voltage across the load to decrease - when you have two resistances in series, the voltage is split between them.

Alternating current is also acternating voltage. AC typically comes from sources which have a rotating conductor in a magnetic field, like a generator, although AC can also be generated electronically from a DC source. An AC generator is actually creating an alternating voltage, which with a fixed load generated an alternating current flow in the line. AC is used because it is very easy to convert AC from one voltage to another. It is more efficient to transmit electricity at high voltages, then step it down to the 115V or 230V needed at the load. DC is more difficult to convert from one voltage to another.

Charge is the amount of stored electrons in an electric storage device like a capacitor. When a storage device is charging or discharging, the rate of change is proportional to the current flow in our out.

Inside the computer is a transformer to reduce the 230V to something lower, a rectifier to convert the AC to DC, then several voltage regulators to generate the specific DC voltage the computer’s circuits need. Some devices can use the AC power directly, such as AC motors and lights.

The voltage is what is “pushing” the current through the load. If the voltage didn’t alternate, the current wouldn’t alternate.

10

[QUOTE]
*Originally posted by Dunmurry *
**I am confused by voltage and current.

  1. What is the difference between voltage and current? I know that voltage is a potential difference between two points in a circuit, and current is the number of electrons passing a point in a second, but how do the two relate to each other? e.g. Increase in voltage=Increase in current? Must there always be voltage and current? i.e. one cannot exist without the other.**

[QUOTE]

Voltage is the potential difference between two points. Current is rightly defined by you. You can think of voltage as potential energy, that is a number of balls raised to an elevation. How much the balls are raised is the voltage, how many fall per second from that height (when you allow them to fall) is the current.

Increase in AC or DC voltage results in increase of current for resistors (neglecting heat resulting in change of the resistance itself), Increase of AC voltage results in increased currents in Capacitors and Inductors (however there are transients), Increase of DC voltage for capacitors results in a spike for a capacitor followed by an exponential decay, Inductors resist change in voltage and put out high currents initially which decay exponentially. Semiconductors may show a nonlinear trend with voltage depending on what the voltage is - less than cut in or more than breakdown or if saturation current has been achieved.

You can have voltage without current. The battery in your flashlight has a voltage when you are not using it. However, current results in voltages in most cases. In superconductors, currents can be present without measureable voltages.

[QUOTE]
2) If a circuit has 230V at 13A, and the current is reduced to 6.5A using a resistor (If V=IR , I suppose that the resistance is 35.38 ohms), how does this affect an appliance powered by it? If it is 115V at 13A, how does this affect the appliance?.

[QUOTE]

The relationship between current and voltage is not so simple for AC. In your example, the current is not only decided by the resistor you put in but also the resistance (or more proper word would be impedance) of the appliance. So the current flowing the circuit (assuming your resistor and appliance are in series) is I, where I = 230V/(35.38 + Impedance of appliance). Now 230 - I*35.38 is the voltage across your appliance.

[QUOTE]
3) Alternating Current is a common method of transmitting current. Is this because there is a loss of power over a long line if DC is transmitted? Can voltage alternate?.

[QUOTE]

The loss in any resistor is I^2 R. That is if you can reduce the current you can reduce the power lost in the resistor (or long wire transmission lines). To do this you use Transformers which work with AC only, to increase the Voltage, thereby decreasing the current I. You can think of a transformer as a lever, A lever will increase the force , but reduce the displacement (keeping the work done the same). Similarly the transformer will increase the voltage but reduce the current.

DC voltages cannot be changed easily, because a device like the transformer is not feasible for higher DC voltages and current.

Voltage can alternate. Voltage in sinusoidal AC is produced from rotating Generators . To simplyfy, think about looking at a particle doing a circulator motion from perpendicular to the plane of the circle. The particle will seem like doing SHM simple harmonic motion or Sinusoidal motion.

[QUOTE]
4) How is charge (Q) related to voltage and current?.

[QUOTE]

Charge (Q) coulombs flowing in one seconds in 1 ampere of current. The gradient in the potential difference is the is the electric field.Details Here

[QUOTE]
5) A computer draws 230V at 13A from a socket, but it only uses DC at several volts. What equipment uses 230V at 13A?.

[QUOTE]

13A?? Are you sure - what computer are you using ? Just because it says that the power plug is rated for 230V and 13A, does’nt means that the computer actually draws 13A. I suspect the computer draws current in the neighborhood of 1V.

230V at 13A means around 3 kW (neglecting power factor). 3 kW is around 4 HP. So anything in that range, your Oven, your Air Conditioner, etc. maybe using that. However, when it comes to more than 1.5 KW power, it is customary to use a 3 - phase supply. So I suspect nothing in your house would be using 13A at 230V AC single phase.

[QUOTE]
6) In an AC (sinusoidal) waveform, AC reaches a peak and then falls back to zero, before reaching the same peak reverse-biased. At the beginning of the cycle the voltage is zero, but at the peak it is approximately 339.4V. If it is Alternating Current why does the voltage change?.

[QUOTE]

The voltage commonly quoted for AC power is the RMS voltage or the root mean squared voltage. It is the integral of voltage * dt over the complete cycle divided by the cycle time. It is precisely because that it is an alternating current, that the voltage is changing.
I am sure the above answers will give rise to more questions from you. It is almost like teaching electrical circuits course. :smiley:

11

Damn - looks like all the Dilberts jumped on this one.

12

Just to clarify a bit on voltage/current ratings for power supplies: Most power supplies (including the wall socket in your house) are designed to produce a constant voltage (or a constant RMS voltage, if it’s AC). What the current is will depend on what you have it hooked up to. When I have nothing plugged into the socket, it’s still at 120 volts, but the current is 0. If I plug in a 1000 ohm resistor, then the voltage is also 120 volts, but by Ohm’s Law, the current is now 0.12 amps. The current rating of a power supply just tells you the limit of current that that supply can handle: If I plug a .001 ohm resistor into my wall socket, it’ll try to maintain 120 volts, but that would be 120000 amps, which is a heck of a lot more than the socket can supply. The voltage will probably end up being a lot lower than the nominal 120 volts, and the current will be high enough to (hopefully) blow a fuse, or if I’ve stupidly bypassed my fusebox, might cause the house wiring to overheat and burn the place down. The amperage rating of the circuit just tells how much I can try to do with it before I overload.

13

In regards the transmission of power (question 3 in the OP):

Transmission of electricity at higher voltages minimizes the resistive losses in the cable, whether it be AC or DC. AC was originally used because there was not an economical method to raise DC voltages to transmission levels. This has since changed and the use of DC for transmission is gaining favor. In order to achieve the voltage necessary for long distance transmission of DC, rectifiers are used on the source end and inverters are used on the receiving end.

A couple of the benefits of DC transmission:

  • Only 2 conductors are necessary instead of the 3 conductors needed for 3Ø transmission. Thus savings are realized in construction of the line from 1 less conductor, associated insulators and hardware, and lighter structures.
  • Line loses from induction are kept to a minimum. Due to the nature of AC, voltages are induced on the lightning protection conductors (commonly known as ‘statics’) from the constant expanding and collapsing magnetic fields. As the statics are at ground potential, this is seen as load and is a considerable ‘loss’ inherent in AC transmission lines. While various schemes have been used with varying success to minimize these loads, such as insulating and segmenting the statics, the losses are greater than with a DC line of the same voltage.

The cost of rectifiers and inverters however, are such that the construction savings mentioned above are cancelled out with all but the longer lines. After about 400 miles, the DC line becomes progressively more econimical.

For a quick overview, read This

14

Just to be sure this isn’t misunderstood. There is still no way to easily change voltage on DC circuits. In order to step the voltage up or down the DC has to be converted to AC for the transformation.

There is a 1.5 million volt DC transmission line that goes down the eastern edge of the Sierra Nevada and Cascades from somewhere up in Washington or Oregon and terminates at a converter station in Sylmar, CA. So far, DC transmission hasn’t quite taken over and seems not to have lived up to its promise. Not many such transmission lines are being built as compared with AC systems.

15

Understood. However, once the voltage is transformed to the desired level, through rectification it is converted to DC and transmitted. As i mentioned, until the length of the line reaches 400+ miles, it is not economically sound to build a DC line because of the offsetting costs of the rectifiers and inverters.

There are two 500KV DC lines in proximity to where i live although i think that they are more prevalent in the Eastern US and Europe.

I’ve also read that there is an experimental line somewhere (maybe California) wherein they doubled the effective size of the center conductor of an AC line and are running 2 DC circuits on 3 conductors. I’ll have to dig around to see if i can find the article.

16

I come from several generations of electricians, but I’m not one myself (engineer). As a kid, I was always the electricians helper, and I was always trying to understand this stuff.

The way my dad/grandfather explained it to me was making the analogy with water.

Water and electricity behave much the same way. They both always flow to the area of least resistance, and they both need stimulus (potential difference) in order to flow at all.

Think of the wire as a water pipe. Voltage is the differnce in height of the two ends of the pipe. If the pipe is level, no flow. The steeper the pipe, the more flow. Current is the amount of water flowing thru the pipe. Dad was always telling me not to freak about high voltage, because it’s current that kills. A huge pipe pointing straight down at your head can’t hurt you unless the volume of water coming out is great enough. A trickle, even if it’s fast, can’t hurt you. (no water jet comments, the analogy only goes so far)

17

Ok - i am not able to understand this part. When you rectify an AC power (unless you are using voltage regulators which are not feasible for high power circuits), you end up getting a waveform with double the frequency. Double the frequency means increased capacitive losses between wire and ground. So using a rectified AC voltage will mean more losses, plus if there are other synchronous generators supplying/consuming power from the grid it will be a new problem.

So how is this a better solution ?

18

for double the frequency, i assumed full wave rectification.

19

I can’t believe how easily you answered this guy’s homework and/or exam questions.

20

The generators that provide the input to transmission lines are 3-phase machines. A full-wave, three-phase rectifier has 6 half cycles per cycle of the basic 60 Hz. wave. This gives an rms ripple of less 9% without any filtering, so the alternating component of the output is small.