Voltage / Ampere

[Eduardo Maun]

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law" but when I compare the 220/110 volts in a given load it goes the other way. For 220V given 5000 watts the current I got is 22.72A, for 110V the same load I got 45.45A. does the statemnet correct or its just a typograpical error? or there is an Theoretical / Technical answer for this? Power is the product of I squared times the resistance so the given load is not the resistance @ all. this is only the reason I see... can you explain it further to me.

Thank you!!!

Eduardo

 

[megawatt]

The statement is incorrect. If it were true, using higher voltages would be useless, because that's the main advantage of HV systems.
Volts & Amps are directly proportional. Double the voltage = half the amps

 

[infinity]

The statement is incorrect. If it were true, using higher voltages would be useless, because that's the main advantage of HV systems.
Volts & Amps are directly proportional. Double the voltage = half the amps

Your statement isn't true. The relationship between voltage and amperage is based on the type of load. With a purely resistive load the relationship between voltage and amperage is directly proportional. If one increases so does the other proportionately. With an inductive load the relationship is inversely proportional increase the voltage and you'll decrease the amperage..

 

[rattus]

The statement is incorrect. If it were true, using higher voltages would be useless, because that's the main advantage of HV systems.
Volts & Amps are directly proportional. Double the voltage = half the amps

Well not exactly.

For a fixed resistance--Ohm's Law:

I = V/R

then current is proportional to voltage--the statement is correct.

For a fixed load which is often the case:

I = P/V

here current is inversely proportional to voltage.

For example, many motors can be wired to run either on 120V or 240V. At 240V, the current is half the current at 120V.

 

[rattus]

Not so:

Not so:

Your statement isn't true. The relationship between voltage and amperage is based on the type of load. With a purely resistive load the relationship between voltage and amperage is directly proportional. If one increases so does the other proportionately. With an inductive load the relationship is inversely proportional increase the voltage and you'll decrease the amperage..

Beg to differ--for an inductor,

I = V/(2*pi*f*L)

And the current lags the voltage by 90 degrees.

 

[LarryFine]

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law" but when I compare the 220/110 volts in a given load it goes the other way. For 220V given 5000 watts the current I got is 22.72A, for 110V the same load I got 45.45A. does the statemnet correct or its just a typograpical error? or there is an Theoretical / Technical answer for this? Power is the product of I squared times the resistance so the given load is not the resistance @ all. this is only the reason I see... can you explain it further to me.

I can!

For a given resitance, if you increase the voltage, the current will increase proportionately.

However, a piece of equipment, of the same power (watts or volt/amps) desdigned for 240v, will have a resistance roughly four times that of the 120v version, ending up with twice the voltage at half the current being the same power.

Now, the same will apply to that equipment: if the voltage increases, so will the current.

(Note: we're talking resistive loads here, for theory's sake.)

 

[infinity]

Beg to differ--for an inductor,

I = V/(2*pi*f*L)

And the current lags the voltage by 90 degrees.

Is a motor an inductive load? Wouldn't a 120/240 motor @ 120 volts have a current 2X that of the same motor @ 240 volts?

 

[Smart $]

Is a motor an inductive load? Wouldn't a 120/240 motor @ 120 volts have a current 2X that of the same motor @ 240 volts?

That's a different relationship... parallel vs. series wiring. Parallel @ 120, series @ 240. The current across the windings is the same in both.

 

[mpross]

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law" ...

Eduardo

This is a very confusing statement to be made when talking about Ohm's "Law". Is this the exact quote, is there any other tid bits of information?

-Matt

 

[al hildenbrand]

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law"

Eduardo,

The key point in your quote is that the resistance (not mentioned) is unchanged.

Think of an electric heater. Say, a 120 V 1000 Watt heater. The resistance of the heating element is fixed. If you double the voltage to the heater, the current will double (the resistance doesn't change), and the 1000 Watt heater is now a 4000 Watt heater. . .

Just not for long, in the real world.

Generally, the heater will quickly burn open.

 

[infinity]

That's a different relationship... parallel vs. series wiring. Parallel @ 120, series @ 240. The current across the windings is the same in both.

You're correct, poor example. I should have said a motor that operates at 208/240. Changing the applied voltage from 208 to 240 will change the current in the same proportion only inversely.

 

[megawatt]

I thought Eduardo was looking for a simple answer to His question, using Ohm's Law. Wattage=Amps x Voltage
5000 watts @ 110 volts = 45.5 amps
5000 watts @ 220 volts = 22.7 amps
So increasing the voltage doesn't increase the amps.

 

[iwire]

I thought Eduardo was looking for a simple answer to His question,

I am sure you are correct but answers here are hardly ever simple.

I agree with Megawatt the simple answer to the question as written is it appears to be a typo in the book.

 

[Mike03a3]

I am sure you are correct but answers here are hardly ever simple.

I agree with Megawatt the simple answer to the question as written is it appears to be a typo in the book.

No, the simple answer is that the statement was in the context of Ohm's Law. In that context voltage is indeed directly proportional to current. I=E/R

 

[iwire]

No, the simple answer is that the statement was in the context of Ohm's Law. In that context voltage is indeed directly proportional to current. I=E/R

IMO voltage is inversely proportional to current when thwe context is Ohms law.

Here is the original statement with highlighting by me.

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law"

IMO that is a typo as far as Ohms Law is concerned.

 

[al hildenbrand]

I have read this " the higher the voltage the higher the current in a Electrical Estimate book under the Ohm's Law"

Eduardo,

The key point in your quote is that the resistance (not mentioned) is unchanged.

Think of an electric heater. Say, a 120 V 1000 Watt heater. The resistance of the heating element is fixed. If you double the voltage to the heater, the current will double (the resistance doesn't change), and the 1000 Watt heater is now a 4000 Watt heater. . .

Just not for long, in the real world.

Generally, the heater will quickly burn open.

iwire,

There's no typo.

It's accurate.

In the real world, if I double the volts on a fixed R, the current doubles. Ohm's Law, plain and simple.

 

[iwire]

iwire,

There's no typo.

It's accurate.

In the real world, if I double the volts on a fixed R, the current doubles. Ohm's Law, plain and simple.

Yeah your right I really missed the boat there.

 

[LarryFine]

This thread has split into two separate discussions: varying voltage across a given resistance, and current through equipment rated at different voltages. To me, they're two separate things. To wit:

Is a motor an inductive load? Wouldn't a 120/240 motor @ 120 volts have a current 2X that of the same motor @ 240 volts?

Yes, if "same motor" means the same HP/Kva/power.

That's a different relationship... parallel vs. series wiring. Parallel @ 120, series @ 240. The current across the windings is the same in both.

It would be more accurate to say that the current through (not across) each winding is the same, as well as that the voltage across each is the same. Oddly enough, the former is the result of the latter.

I thought Eduardo was looking for a simple answer to His question, using Ohm's Law. Wattage=Amps x Voltage
5000 watts @ 110 volts = 45.5 amps
5000 watts @ 220 volts = 22.7 amps
So increasing the voltage doesn't increase the amps.

Al's responses are right-on. For any fixed resistance, voltage and current are proportionate. For a given power rating, voltage and current are inversely proportionate.

 

[websparky]

Ohm's Law

Ohm's Law

Ohm?s Law. The relationship between current, resistance and voltage.

Amperes, I = E/R "This means Amps are equal to Volts divided by Resistance."

Resistance, R = E/I "This means Resistance is equal to Volts divided by Amps."

Voltage, E = I x R "This means Volts are equal to Amps multiplied by Resistance."

Now how does this work Al?

iwire,

There's no typo.

It's accurate.

In the real world, if I double the volts on a fixed R, the current doubles. Ohm's Law, plain and simple.

 

[al hildenbrand]

Dave,

How does what work?

Guessing at your meaning, . . .

Eduardo, in his OP, talks about the Ohm's Law quote and then thinks about applying the Law to constant power situations. Constant power is a red herring, , in this discussion, as that requires different values of R for different voltages and currents.

Ohm's Law, by itself, doesn't express power relationships.

I merely agreed with what Larry and Trevor had already stated before me.