# Understanding PhET simulation (Faraday's law)

## Homework Statement

I am supposed to make a project based on this simulation, I am trying to see what is happening:

If I go to the "Transformer" tab, I have an electromagnet and a circuit consisting of a loop of wire and a light bulb. I can change the current source to alternating current to induce current in the circuit and make the light bulb shine. If I increase either the number of loops in the electromagnet or the circuit, the light bulb shines brighter. According to my understanding, the brightness of a light bulb depends on the power which is the product of the current and the voltage. However, a transformer only changes voltage and current proportionally to each other so that power remains constant.

## Homework Equations

V2 / V1 = N2 / N1 = I1 / I2;
P = IV;
B = unI;
n = N/l;

## The Attempt at a Solution

If I use a field meter and a direct current source, I can see that the magnetic field is proportional to the number of loops for the electromagnet. The number of loops shouldn't matter due to the formula of magnetic field using "loop density" instead. Is this simulation incorrect, or is my understanding of this topic really off?

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## Answers and Replies

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cnh1995
Homework Helper
Gold Member

## Homework Statement

I am supposed to make a project based on this simulation, I am trying to see what is happening:

If I go to the "Transformer" tab, I have an electromagnet and a circuit consisting of a loop of wire and a light bulb. I can change the current source to alternating current to induce current in the circuit and make the light bulb shine. If I increase either the number of loops in the electromagnet or the circuit, the light bulb shines brighter. According to my understanding, the brightness of a light bulb depends on the power which is the product of the current and the voltage. However, a transformer only changes voltage and current proportionally to each other so that power remains constant.

## Homework Equations

V2 / V1 = N2 / N1 = I1 / I2;
P = IV;
B = unI;
n = N/l;

## The Attempt at a Solution

If I use a field meter and a direct current source, I can see that the magnetic field is proportional to the number of loops for the electromagnet. The number of loops shouldn't matter due to the formula of magnetic field using "loop density" instead. Is this simulation incorrect, or is my understanding of this topic really off?
Can you show the exact circuit your're working on? Where is the current source placed?

scottdave
Homework Helper
How exactly do I run the simulation. It just downloads a file. I clicked on the Software Required... all I need is Java, which I have.

How exactly do I run the simulation. It just downloads a file. I clicked on the Software Required... all I need is Java, which I have.
I needed to open the file with an application called "Java Web Start".

Can you show the exact circuit your're working on? Where is the current source placed?
Here is an example of what I mean about the light bulb shining brighter:

Here is an example of the magnetic field increasing linearly with the number of loops in the electromagnet:

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scottdave
Homework Helper
The increased number of turns makes it easier for the magnetic flux to transfer.

The increased number of turns makes it easier for the magnetic flux to transfer.
So, let's say this simulation used a typical transformer with two coils and an iron core. If the number of turns were increased, the magnetic field that the primary current produces wouldn't increase because there is already an iron core that helps the magnetic field propagate through, right?

cnh1995
Homework Helper
Gold Member
Here is an example of the magnetic field increasing linearly with the number of loops in the electromagnet:
Well, in case of your dc circuits (last two images), I believe the axial magnetic field 'B' shouldn't increase since it depends on turns/length (or turn-density as you said) and not on number of turns alone.

But in the first two circuits with alternating voltage, the increased number of turns of the bulb-coil in the second picture induces more voltage in it and hence the bulb glows brighter. Flux linkage of the secondary is given by NsΦp, where Φp is the part of primary flux that gets coupled with the secondary. As you can see, the flux-linkage of the secondary is proportional to the number of turns of the secondary and hence, the bulb glows brighter.

scottdave