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**1. Homework Statement**

A series dc motor:A dc motor with its rotor and field coils connected in series has an internal resistance of 2.00 Ω. When running at full load on a 120-V line, it draws a current of 4.00 A.

A series dc motor:

a) What is the emf in the rotor?

b) What is the power delivered to the motor?

c) What is the rate of dissipation of energy in the resistance of the motor?

d) What is the mechanical power developed?

e)What is the efficiency of the motor?

f)What happens if the machine that the motor is driving jams and the rotor suddenly stops turning?

**2. Homework Equations**

Vab = E + Ir,

P = I^2r

**3. The Attempt at a Solution**

This is a worked example in my textbook so the answers are all there but I'm finding it challenging to understand what is actually going on with some of the equations.

what exactly is the back emf and why am I adding it to Ir?

why does the current become V_ab/r only when the back emf goes to zero? what would be in the equation

I=v_ab/r if the back emf was still there?

would appreciate help with any of these questions and maybe an explanation of what is actually going on here. thanks

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