The circuit shown in the figure below is used to make a magnetic balance to weig

Question

The circuit shown in the figure below is used to make a magnetic balance to weigh objects. The mass m to be measured is hung from the center of the bar, that is in the uniform magnetic field of 1.43 T, directed into the plane of the figure. The battery voltage can be adjusted to vary the current in the circuit. The horizontal bar is 71.9 cm long and is made of extremely light-weight material. It is connected to the battery by thin vertical wires that can support no appreciable tension; all the weight of the suspended mass m is supported by the magnetic force on the bar. A resistor with R = 5.00 Ohm is in series with the bar; the resistance of the rest of the circuit is much less than this. Which point, a or b, should be the positive terminal of the battery? If the maximum terminal voltage of the battery is 175 V, what is the greatest mass m that this instrument can measure?

Explanation / Answer

Form the Fleming's left hand rule it is very clear that currnt must passes from a tp b so thepositive terminal of the battery be at a

current through the battery is i = V/R = 175/5= 35 A

then magnetic force = weight

B*i*L*sin(90) = m*g

1.43*35*0.719 = m*9.81

mass m = 3.69 kg

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