Q.5 It is intended to measure the temperature inside a furnace by observing the

Question

Q.5 It is intended to measure the temperature inside a furnace by observing the 25 current flowing through a coil of nickel wire placed in the furnace, as shown in the Figure. If an emf of 1.5 V is to make the ammeter indicate 10 mA when the furnace is at 20°C, calculate Ammeter502 Fumace 1.5 V Nickel a Coil (a) (b) (c) (d) (e) the length of nickel wire required in the coil if the diameter of the wire is 0.2 mm. the temperature in the furnace when the ammeter indicates 5 mA. the current indicated by the ammeter when the temperature is 520°C. the current density in the nickel wire at 20°C. the number of electrons per second flowing in the wire at 20°C.

Explanation / Answer

(A) V = I Req

1.5 = (10 x 10^-3) (50 + R)

R = 100 ohm

and R = rho L / A

100 = (7.8 x 10^-8) L / ( pi (0.2 x 10^-3 / 2)^2)

L = 40.3 m

(B) when I = 5 mA  

then R + 50 = 1.5/0.005 = 300

R = 150 ohm

R = R0 [ 1+ alpha T]

150 = 100 [1 + (0.006)(T - 20)]

T = 103.3 deg C

(C) R = 100 [ 1+ (0.006)(520 - 20)] = 400 ohm

I = 1.5 / (400 + 50) = 3.33 x 10^-3 A Or 3.33 mA

(D) J = I / A = (10 x 10^-3) / (pi (0.1 x 10^-3)^2)

J = 3.18 x 10^5 A/m^2

(E) n = (10 x 10^-3 C) / (1.6 x 10^-19)

n = 6.24 x 10^16 electrons

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