So simple, yet its confusing me? (Electricity)

[Lars]

Ok, I am having a hell of a hard time trying to figure this one out.

I used to know it, but now that its exam time, it seems to have vacated my brain. Perfect timing.

 

 

Ok, So go easy on me, I know this is simple, I was looking at my old test, and I cant remember how I've done this. Its all to do with Ohms Law and Electricity, so if you remember, you may have saved me some points on my end of year exam.

 

"The table on your answer sheet lists information about various globes when the voltage across each of them is 9V. Complete the table"

 

Current (A) | Resistance (Omega Sign)

| 2 | Blank

| Blank | 1.5

| Blank | 0.5

| 0.9 | Blank

 

Sorry if my table gets formatted I tryed my best

 

So can you help me?

 

EDIT: I think I got it, I have as my current reading in order;

 

2,6,18,0.9

 

And as my Resistance;

 

4.5, 1.5, 0.5 and 10

 

Is this right?

[YT2095]

it looks all correct to me

 

just remember that 1 volt across 1 Ohm will draw 1 Amp, then it`s all just a question of plugging in the numbers

[martianxx]

v = IR you can rearrange to solve all the problems.

[Fred56]

Also current is described by Kirchoff's Law (q.v.). Ohm's Law describes voltage and resistance (ask any Physics teacher). After that there's inductance and capacitance, and so on. When you get into time-varying voltage and current, it gets more interesting...

[ydoaPs]

Also current is described by Kirchoff's Law (q.v.). Ohm's Law describes voltage and resistance (ask any Physics teacher). After that there's inductance and capacitance, and so on. When you get into time-varying voltage and current, it gets more interesting...

What about Kirchoff's voltage law? And you're going to mention his current law but not explain it?

 

Kirchoff's current law basically says the sum of the currents entering a junction equals the sum of the currents leaving the junction. It's almost like a conservation law(what comes in must come out).

 

Kirchoff's voltage law says the sum of the voltages in a closed loop equals zero. So, basically, if you add up the voltage drops across the loads, the equal the voltage of the battery or whatever voltage source you are using.

 

It should be noted that these laws are for DC circuits. I'm not sure why Fred brought it up, though.

[NeonBlack]

YD: Ohm's laws and Kirchoff's laws are applicable for any circuit, including AC or any other time varying voltages or currents.

[ydoaPs]

YD: Ohm's laws and Kirchoff's laws are applicable for any circuit, including AC or any other time varying voltages or currents.

 

I could have sworn that Kirchoff's current law was only valid for circuits where the current density is constant with respect to time.

[NeonBlack]

As you said, it's just conservation. What goes in must come out. That doesn't change just because the amount of stuff going in and out changes.

[ydoaPs]

You're probably right. I'm just a nuclear mechanic, so I know almost nothing about electricity.

 

edit: Wikipedia defines KCL as

At any point in an electrical circuit where charge density is not changing in time, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point.

 

Wouldn't the whole "charge density not changing in time" rule out AC circuits?

[NeonBlack]

no- it only rules out capacitor plates, I believe.

Since the individual plates can gain a net charge, the current in does not have to be equal to the current out.

This is the only case I can think of where KCL is violated (and even the entire capacitor is charge neutral) and the voltage rule will always apply.

[MrMongoose]

Kirchoff's law is just a special case of Faraday's law when magnetic flux density is zero.

[TonyMcC]

Kirchoff's current and voltage laws do apply to a.c. circuits but only instant by instant. You have to find the instantaneous values of voltage and current at a specific instant. This takes care of inductive and capacitive effects. Google " instantaneous voltage kirchoff " (without the inverted commas) for more information.

Edited June 4, 2010 by TonyMcC