Moving-coil galvanometer graph

• moenste
In summary, a square coil with N turns and side a is situated in a horizontal magnetic field of flux density B at an angle θ. The expression for the magnitude of the couple acting on the coil is T = B I a N*sinθ. In most moving-coil galvanometers, the simple arrangement is modified by making the magnetic field radial in order to have a more constant driving torque. This is achieved by providing a curvature to the magnetic poles.
moenste

Homework Statement

A square coil of side a and consisting of N turns is free to rotate about a vertical axis through the mid-points of two opposite sides. It is situated in a uniform horizontal magnetic field of flux density B so that the plane of the coil makes an angle θ with the field. Draw a diagram of his arrangement as seen from above and show the couple acting on the coil when a current I flows through it. Write down an expression for the magnitude of this couple.

Explain how and why this simple arrangement is modified in most moving-coil galvanometers.

2. The attempt at a solution
I think this should be the correct representation of the problem:

Though I don't quite understand what "couple" means. What does "show the couple acting on the coil" mean?

Last edited:
moenste said:
I think this should be the correct representation of the problem:

Yes.
moenste said:
Though I don't quite understand what "couple" means.
https://en.m.wikipedia.org/wiki/Couple_(mechanics)
moenste said:
Explain how and why this simple arrangemen is modified in most moving-coil galvanometers.
What can you say about the angle between the coil and the magnetic field as the coil rotates? How is that related to the torque acting on the coil?

moenste
cnh1995 said:
Yes.

https://en.m.wikipedia.org/wiki/Couple_(mechanics)

What can you say about the angle between the coil and the magnetic field as the coil rotates? How is that related to the torque acting on the coil?
So couple is the couple of forces which move in different directions on the graph?

T = B I a N -- an expression for the magnitude of this couple? (T = torque, B = field, I = current, a = coil side, N = number of turns.)

Well, the coil rotates with the arrow on it. So the angle decreases or increases depending on (?) current going through the magnet? Current increases and so the torque increases and the other way around (?).

moenste said:
T = B I a N*sinθ
You can see that the driving torque varies with the angle if the magnetic field is horizontal. To have an almost constant driving torque, the magnetic field is made radial by providing a curvature to the magnetic poles.

moenste
cnh1995 said:
You can see that the driving torque varies with the angle if the magnetic field is horizontal. To have an almost constant driving torque, the magnetic field is made radial by providing a curvature to the magnetic poles.
You mean that in the problem we had to draw not circled poles of the magnet but just regular ones (squared).

And in real life the magnet is modified to the one in the drawing, to make the magnetic field radial, right?

moenste said:
You mean that in the problem we had to draw not circled poles of the magnet but just regular ones (squared).

And in real life the magnet is modified to the one in the drawing, to make the magnetic field radial, right?
Yes.

moenste

1. What is a moving-coil galvanometer graph?

A moving-coil galvanometer graph is a type of graph that shows the relationship between an electric current and the deflection of a needle on a galvanometer. It is commonly used in physics and electrical engineering experiments to measure small electric currents.

2. How does a moving-coil galvanometer work?

A moving-coil galvanometer works by using a coil of wire suspended between the poles of a magnet. When an electric current flows through the coil, it creates a magnetic field that interacts with the magnetic field of the permanent magnet, causing the coil to rotate. The amount of rotation is proportional to the strength of the current.

3. What is the difference between a moving-coil galvanometer and a moving-magnet galvanometer?

The main difference between a moving-coil galvanometer and a moving-magnet galvanometer is the type of coil used. In a moving-coil galvanometer, the coil is suspended between the poles of a magnet, while in a moving-magnet galvanometer, the magnet is suspended between the poles of a coil. The principle of operation and the resulting graph are similar for both types of galvanometers.

4. What are some common applications of a moving-coil galvanometer graph?

Moving-coil galvanometer graphs are commonly used in various scientific experiments and measurements. They are often used to measure small electric currents in circuits, as well as to measure magnetic fields and forces. They are also used in instruments such as voltmeters, ammeters, and oscilloscopes.

5. How can the accuracy of a moving-coil galvanometer graph be improved?

The accuracy of a moving-coil galvanometer graph can be improved by ensuring that the coil is suspended in a stable and uniform magnetic field, and by calibrating the galvanometer regularly. The sensitivity of the galvanometer can also be adjusted by changing the number of turns in the coil or the strength of the permanent magnet. Additionally, using a higher-quality galvanometer with a finer scale can also improve accuracy.

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