How Do Transformers Work with Ohm's Law?

[Lsos]

Probably a simple question for electrical people, to which I never picked up the answer in my studies. I know I'm missing something simple, or just thinking about it the wrong way.

A transformer increases voltage, and decreases current. And yet...Ohms law states that higher voltage results in higher current. For some reason this doesn't fit into what a transformer does. So, what gives?

I should know this because a transformer is analagous to a transmission in a mechanical system, and mechanically it somehow makes sense. But electrically it doesn't. I'm missing some key piece, and I need someone to walk me through this please...

 

[vk6kro]

The power passing through a transformer to the secondary is slightly less than that entering the transformer, because transformers are efficient, but not perfect

Assume for a moment, though, that we did have a perfect transformer.

If we put a load of 500 watts on a 250 volt secondary, this would be 250 volts at 2 amps (to get 500 watts) flowing from the transformer to the load.

Now if the transformer had a 125 volt primary and 125 volts being supplied to it, then the power entering the transformer would have to be 500 watts (to get 500 watts out).
So, how much current is flowing in the primary?
125 volts times 4 amps is 500 watts.

So, the current flowing reduced from 4 amps at the input to 2 amps at the load. The transformer didn't restrict the current, though. The higher voltage just made less current necessary for the same power.

 

[oswaler]

Dear student,

Thank you for your question. It is completely understandable that you are confused about the relationship between transformers and Ohm's law. Let me try to explain it in a simple way.

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. This means that if the voltage increases, the current will also increase. However, this is true only if the resistance of the conductor remains constant.

In a transformer, the voltage is indeed increased, but the current is decreased. This may seem contradictory to Ohm's law, but it is because of the change in resistance.

A transformer consists of two coils, a primary coil and a secondary coil, which are wrapped around a core. When an alternating current is passed through the primary coil, it creates a changing magnetic field in the core. This changing magnetic field induces a current in the secondary coil, which results in a change in voltage.

The key piece you are missing is the concept of impedance. Impedance is the total opposition to current flow in an alternating current circuit. It is made up of both resistance and reactance, which is the opposition to current flow caused by a changing magnetic field.

In a transformer, the primary and secondary coils have different numbers of turns, which results in different impedances. The primary coil has a lower impedance compared to the secondary coil. This means that when the voltage is increased in the secondary coil, the impedance also increases, resulting in a decrease in current.

So, in short, the transformer does not violate Ohm's law. The change in voltage and current is due to the change in impedance caused by the changing magnetic field in the core.

I hope this explanation helps you understand the relationship between transformers and Ohm's law better. If you have any further questions, please do not hesitate to ask. Keep studying and exploring the world of science!