How Does a Moving-Coil Galvanometer Measure High Voltages and Currents?

[Mazaraika]

Homework Statement

a moving-coil gavanometer consists of a coil that is free to rotate between the poles of a permanent magnet. the magnetic field strength between the poles is 5 x (10^-2) T. The coil consists of 250 loops of copper wire, diameter 0.5 mm, wound to form a square coil, with an average side length of 2.5 cm. At full-scale deflection, the plane of the coil makes an angle of 60(degrees) to the magnetic field and the torque on the coil is 40 uNm. You can assume that all of the resistance of the gavanometer comes from the coil. Resistivity of copper= 1.7 x (10^-8) Ohms m

i). how can this instrument be adapted so that it can be used to measure potential differences up to 20V?
ii). how can it be modified to measure currents up to 1 A?
iii). What, briefly would be the effect on the sensitivity of the instrument (i.e the current required in the coil to give full-scale deflection) of putting a soft iron core in the centre of the coil? justify your answer
b). what would be the resistance of an ideal voltmeter and an ideal ammeter? briefly explain your answers.

(Ive attached the question paper, named WOE Coursework 2)

Homework Equations

i didn't know where to start, and which equations i was meant to use, as the resistance and current, or voltage isn't given. (attached notes on it, named part 4 and part 5)

The Attempt at a Solution

again didnt know where to start :(... Neeed help with working out so i understand it, thanks a lot. (you can use the attached notes to help you understand better), and its in for tomorrow so would really be appreciated if you gave an answer, working out and explained it.

 

[Mazaraika]

bump

(also attached another set of notes which may be of further help)

 

[Mazaraika]

need help bump bump

 

[Mazaraika]

Ive managed to work out part i). and ii). just need help with part iii). and b). and questions 2 and 3 now :)

 

[rwisz]

I would approach this problem by first understanding the basic principles of a moving-coil galvanometer and its components. A moving-coil galvanometer works on the principle of the interaction between a magnetic field and an electric current, where the torque on the coil is proportional to the current passing through it. The coil is suspended between the poles of a permanent magnet, and when a current flows through the coil, it experiences a torque that causes it to rotate.

To adapt the instrument to measure potential differences up to 20V, we would need to increase the sensitivity of the galvanometer. This can be achieved by increasing the number of loops in the coil and decreasing the diameter of the wire. This will result in a higher resistance in the coil, making it more sensitive to small changes in current. Additionally, we can add a shunt resistor in parallel with the coil to increase the range of measurements. This will allow us to measure larger potential differences without damaging the instrument.

To modify the instrument to measure currents up to 1A, we would need to increase the strength of the magnetic field between the poles. This can be achieved by using a stronger permanent magnet or by increasing the number of turns in the coil. We would also need to increase the thickness of the wire to handle the higher current.

Adding a soft iron core in the center of the coil would increase the magnetic field strength within the coil and thus increase the sensitivity of the instrument. This is because the soft iron core would concentrate the magnetic field lines, resulting in a stronger interaction with the current-carrying coil.

The resistance of an ideal voltmeter would be infinite, as it would not draw any current from the circuit it is measuring. This ensures that the voltage being measured is not affected by the voltmeter. On the other hand, the resistance of an ideal ammeter would be zero, as it would allow the maximum amount of current to pass through it, ensuring accurate measurement of the current in the circuit.

In summary, to adapt the moving-coil galvanometer to measure larger potential differences and currents, we would need to increase the sensitivity of the instrument by increasing the number of turns in the coil, decreasing the wire diameter, and using a shunt resistor. We can also modify the instrument by increasing the magnetic field strength and wire thickness. Adding a soft iron core would further increase the sensitivity of the instrument. The resistance of an ideal voltmeter would be infinite, and