Physics
Physics
3rd Edition
ISBN: 9781259233616
Author: GIAMBATTISTA
Publisher: MCG
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Chapter 20, Problem 26P

(a)

To determine

The current drawn by the motor when it is first started up.

(a)

Expert Solution
Check Mark

Answer to Problem 26P

The current drawn by the motor when it is first started up is 6.0A.

Explanation of Solution

Write an expression for the current drawn by the motor when it is first started up.

  I=εextR                                                                                                             (I)

Here, I  is the current drawn by the motor when it is first started up, εext is the external emf and R is the resistance.

Conclusion:

Substitute 12V for εext and 2.0Ω  for R in equation (I) to find I.

  I=12V2.0Ω=6.0A

Thus, the current drawn by the motor when it is first started up is 6.0A.

(b)

To determine

The current draws at normal operating speed.

(b)

Expert Solution
Check Mark

Answer to Problem 26P

The current draws at normal operating speed is 0.551A.

Explanation of Solution

Write an expression for Kirchhoff’s voltage law.

  εextI'Rεback=0                                                                                                (II)

Here, I'  is the current draw at normal operating speed and εback is the back emf.

Write an expression for back emf.

  εback=PI'                                                                                                     (III)

Here, P is the power.

Rewrite the expression (II) by using equation (III).

  εextI'RPI'=0I'εextI2RP=0                                                                                      (IV)

Rearrange (IV) to calculate I'.

  I'=εext±εext24(R)(P)2(R)                                                                   (V)

Conclusion:

Substitute 12V for εext, 2.0Ω for R and 6.0W for P in equation (V) to find I'.

  I'=12V±(12V)24(2.0Ω)(6.0W)2(2.0Ω)=12V±(12V)248V2(4.0Ω)=0.551A

Thus, the current draws at normal operating speed is 0.551A.

(c)

To determine

The back emf induced in the winding at normal speed.

(c)

Expert Solution
Check Mark

Answer to Problem 26P

The back emf induced in the winding at normal speed is 10.9V.

Explanation of Solution

Write an expression for back emf induced in the winding at normal speed.

  εback=εextIR                                                                                          (VI)

Here, εext is the back emf induced in the winding at normal speed.

Conclusion:

Substitute 12V for εext, 0.551A for I and 2.0Ω for R in equation (VI) to find εext.

  εback=12V(0.551A)(2.0Ω)=12V1.1V=10.9V

Thus, the back emf induced in the winding at normal speed 10.9V.

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Chapter 20 Solutions

Physics

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