Ohms Law on the secondary of an open transformer

[mull982]

We know that with Ohms Law V=IR and in a circuit all 3 of these are present.

What about where a circuit has induced voltage on it? For instance the secondary of an open transformer? Across the secondary of a transformer you will read different voltages depending where on the coil you take a reading thus implying a voltage drop across the coil. Obviously the coil will have a resistance associated with it, but with no current due to an open secondary, how is this voltage drop viewed?

I understand how in concept this secondary works, just wanted to hear thoughts on applying this particular law to it.

 

[charlie b]

Ohm's Law specifically addresses the relationships between these three parameters in a conductor that is carrying current. It does not apply to an open circuit.

 

[LarryFine]

Across the secondary of a transformer you will read different voltages depending where on the coil you take a reading thus implying a voltage drop across the coil. Obviously the coil will have a resistance associated with it, but with no current due to an open secondary, how is this voltage drop viewed?

It's not really a voltage drop, it's a voltage gain. Remember, an energized transformer secondary is a source, not a load. The applied voltage and the turns ratio determine the volts-per-turn that will be developed.

No secondary load current merely means that no current will be reflected back to the primary (beyond magnetization and any losses).

A load applied to the secondary would cause current in it, and voltage drop across it. That's what a transformer's impedance rating is all about: the ability of its output to not be affected by the load.

 

[winnie]

I would argue that if you are measuring the voltage, you have a _closed_ circuit; from one point on the secondary coils, through the coils to another point, then through the meter lead, the meter, the next lead, and back to the first point on the secondary.

Very little current is flowing on this circuit; just enough to make the meter read. Because there is very little current flow, there is very little voltage drop. Additionally, there is a large and changing magnetic flux that links this circuit. A circuit linking a changing magnetic flux has a voltage induced in it.

-Jon

 

[charlie b]

I would argue that if you are measuring the voltage, you have a closed circuit. . . .

True, of course. But I think the OP was asking about the condition of an open circuit transformer, before connecting any load, and without taking a voltage measurement. Voltage is present, and resistance (impedance) is present, and current is not present, and that combination seemed to violate Ohm's Law. My point is that it doesn't violate that law, because that law was not written to address this situation.

When you connect the voltmeter, current does flow (a very small current, as you said), and it will obey Ohm's Law. Disconnect the voltmeter and current will stop flowing, and Ohm's Law will no longer apply.

 

[LarryFine]

Disconnect the voltmeter and current will stop flowing, and Ohm's Law will no longer apply.

Well, it still applies. That's how we calculate 0.0a.

 

[gar]

100501-1012 EST

mull982:

In an ideal condition a transformer is a voltage source. A voltage source by definition is a constant voltage energy source with no internal impedance. This means for any load the voltage remains constant.

In a transformer the secondary voltage is some constant times the number of turns on the secondary and the rate of change of the flux linking the secondary. With other parameters constant change the secondary turns and you change the secondary output voltage. Ohms law has nothing to do with this.

.

 

[charlie b]

Disconnect the voltmeter and current will stop flowing, and Ohm's Law will no longer apply.

Well, it still applies. That's how we calculate 0.0a.

Entertaining, but untrue. The formula V = IR is not a ?statement of Ohm?s Law.? Rather, the value of resistance of a given circuit element is defined by the formula R = V/I. Georg Ohm discovered that in a section of metal, the resistance value is constant (at any given temperature, that is). If by chance the resistance of a given circuit element changes, when you change the voltage or the current, or if it changes as time goes on, then that particular element does not obey Ohm?s Law.

The law itself is expressed in terms of the behavior of a circuit element. So if you don?t have a circuit element, you can?t speak of Ohm?s Law.