# Current in secondary coil as current in primary increases

• TT0
In summary, the conversation discusses the relationship between the primary and secondary coils of a transformer when the primary current is increased at a constant rate by reducing the resistance. It explains that due to Lenz's law, the secondary coil will oppose the increase in flux by inducing flux in the opposite direction, but according to Faraday's law, the secondary emf will remain constant. The conversation also mentions that this process will eventually come to an end when the power supply runs out of voltage.
TT0
Hi, so the scenario is that the primary coil of a transformer is attached to a battery in a circuit with a variable resistor. The resistor is set to the maximum resistance.

As I increase the current in primary coil by reducing resistance at a constant rate, will the current in the secondary also slowly increase? Since as current increase, flux increases, and the secondary coil will oppose this increase in flux by inducing flux in opposite direction by increasing current in the secondary coil (Lenz's law). However, faraday's law is that emf is equal to the change in flux, so that means the secondary coil should have a constant current. What is the problem with my understanding of transformers?

Thanks

TT0 said:
As I increase the current in primary coil by reducing resistance at a constant rate,
You can't do that with the secondary circuit closed. Current in the primary will no longer depend only on the change in resistance. It will also depend on the secondary resistance. The two circuits are now magnetically coupled and you'll have to consider how they affect each other (at the same time).

Even if the secondary were open, you can't increase the primary current at a constant rate by reducing the resistance at a constant rate because of its magnetizing inductance (very large).

Best way to understand what happens in a circuit like this is to write the differential equation of KVL for primary and secondary and solve it.

TT0
If you attach a transformer to a battery there will be a single pulse of induced emf in the secondary and then DC current through the primary.
TT0 said:
Hi, so the scenario is that the primary coil of a transformer is attached to a battery in a circuit with a variable resistor. The resistor is set to the maximum resistance.

As I increase the current in primary coil by reducing resistance at a constant rate, will the current in the secondary also slowly increase? Since as current increase, flux increases, and the secondary coil will oppose this increase in flux by inducing flux in opposite direction by increasing current in the secondary coil (Lenz's law). However, faraday's law is that emf is equal to the change in flux, so that means the secondary coil should have a constant current. What is the problem with my understanding of transformers?

Thanks
If you increase the primary current at a constant rate, the secondary emf will be constant. So the secondary current (into a resistive load) will be constant, too.
This process has to end somewhere - when the input power supply runs out of Volts, of course.

TT0
Thanks

## 1. How does the current in the secondary coil change as the current in the primary coil increases?

The current in the secondary coil, also known as the induced current, increases proportionally to the current in the primary coil. This is due to the principle of electromagnetic induction, which states that a changing magnetic field can induce an electric current in a nearby conductor.

## 2. Is there a limit to how much the current in the secondary coil can increase?

Yes, there is a limit to how much the current in the secondary coil can increase. This is determined by the number of turns in the coil, the strength of the magnetic field, and the resistance of the circuit. Past a certain point, the induced current will reach its maximum value and will not increase any further.

## 3. What factors affect the current in the secondary coil?

The current in the secondary coil is affected by the number of turns in the coil, the strength of the magnetic field, the frequency of the alternating current in the primary coil, and the resistance of the circuit. These factors determine the amount of electromagnetic force that is induced in the secondary coil.

## 4. Can the current in the secondary coil be greater than the current in the primary coil?

No, the current in the secondary coil cannot be greater than the current in the primary coil. This is because the induced current is dependent on the changing magnetic field created by the primary coil. If the current in the secondary coil were to exceed the current in the primary coil, it would violate the law of conservation of energy.

## 5. What is the purpose of increasing the current in the primary coil?

The purpose of increasing the current in the primary coil is to increase the strength of the magnetic field, which in turn increases the amount of electromagnetic force induced in the secondary coil. This can be useful in applications such as transformers, where the induced current in the secondary coil can be used to power devices or equipment.

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