# Calculating Average EMF in Two Coils on a Ferromagnetic Core

• James Holtum
In summary, the conversation discusses two coils, A and B, wound on the same ferromagnetic core and the average EMFs induced in the coils. The relevant equations are mentioned and the value for mutual inductance (M) is calculated. The conversation also includes a discussion on the use of the equation for EMF induced and clarifies that the "average EMF" may not be a meaningful concept. The conversation also addresses the calculation of final current in coil A and the importance of considering the time step in calculations.
James Holtum
Hey guys been trying this question for a while now and i just can't seem to find any equations which link the two coils to find average EMF and also the final current I2. I am new to electrical so still learning any help is much appreciated, thanks!

Two coils, A and B, are wound on the same ferromagnetic core. There are 800 turns on A and 2800 turns on B. A current of 4 A through coil A produces a flux of 1500 μWb in the core. If this current is reversed in 30 ms, calculate the average EMFs induced in coils A and B.

Relevant equations
M=N2*(Flux2)/I1=N1*(Flux1)/I2

So for I've calculated M to be 1.05 mH and L to be 0.3 (not sure what units) but I don't think its right!

James Holtum said:
Two coils, A and B, are wound on the same ferromagnetic core. There are 800 turns on A and 2800 turns on B. A current of 4 A through coil A produces a flux of 1500 μWb in the core. If this current is reversed in 30 ms, calculate the average EMFs induced in coils A and B.

Relevant equations
M=N2*(Flux2)/I1=N1*(Flux1)/I2
A and B is wound on the same core, so Flux1 = Flux2.

The equation to be used as for the Emf induced:

V = dΨn/dt , Ψn = Ψ * n = Flux * turns

So my value for N=2800+800?

Im given flux but that equation doesn't take into account the amperage and the time step?

Thanks

James Holtum said:
There are 800 turns on A and 2800 turns on B. A current of 4 A through coil A produces a flux of 1500 μWb in the core. If this current is reversed in 30 ms, calculate the average EMFs induced in coils A and B.
So the current (4A) and thus the flux (1500 μWb) will change sign within 30 ms.

Last edited by a moderator:
You don't need to compute inductances, including mutual inductance.
If 4A produces 1500 μWb in the core, and assume a linear change of current from +4A to -4A in 30 ms., what is dΦ/dt in the core? Then, what is the emf induced in each coil?
(The term "average emf" doesn't make sense to me. The "average" emf is as small as you want it to be by dividing by as long a time as you want. But if you assume a linear change in current you can use the resulting emf's as the answer).
What do you mean by I2? Current in coil B? That's obviously always zero. Final current in coil A? That's obvious.

rude man said:
You don't need to compute inductances, including mutual inductance.
If 4A produces 1500 μWb in the core, and assume a linear change of current from +4A to -4A in 30 ms., what is dΦ/dt in the core? Then, what is the emf induced in each coil?
(The term "average emf" doesn't make sense to me. The "average" emf is as small as you want it to be by dividing by as long a time as you want. But if you assume a linear change in current you can use the resulting emf's as the answer).
What do you mean by I2? Current in coil B? That's obviously always zero. Final current in coil A? That's obvious.

got 80 for A 280 for B

Last edited:
James Holtum said:
got 80 for A 280 for B
Very good, except 80 and 280 what?

rude man said:
Very good, except 80 and 280 what?
@James Holtum: And remember that when calculating with AC-current in the coil, the induced voltage is the instant value, V(t), not the effective value, Veff, or whatever.

## What is EMF and how is it measured?

EMF stands for electromagnetic field, which is a type of radiation emitted by electronic devices. It is measured in units of volts per meter (V/m) or milliGauss (mG).

## What is a two coil setup and how does it affect EMF?

A two coil setup is used to create a magnetic field. It consists of two coils of wire, one connected to a power source and the other connected to a device that measures EMF. The magnetic field generated by the first coil induces an electric current in the second coil, which can be measured as EMF.

## How does the distance between the two coils affect EMF?

The strength of the EMF produced by the two coil setup is inversely proportional to the distance between the coils. This means that as the distance between the coils increases, the EMF decreases.

## What factors can affect the accuracy of EMF measurements using a two coil setup?

The accuracy of EMF measurements using a two coil setup can be affected by the distance between the coils, the strength of the magnetic field generated by the first coil, the sensitivity of the measuring device, and any interference from external sources.

## Are there any health risks associated with EMF from a two coil setup?

There is currently no conclusive evidence that exposure to EMF from a two coil setup poses any significant health risks. However, it is recommended to use caution and limit exposure to EMF as a precautionary measure.

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