Compare the signs of ƒ for lenses and mirrors.
2 answers:
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12 V is the f.e.m. 
of the battery. The potential difference that is applied to the motor is actually the fem minus the voltage drop on the internal resistance r:
this is equal to the voltage drop on the resistance of the motor R:
so we can write:
and using
and 
we can find the current I:
this is equal to the voltage drop on the resistance of the motor R:
so we can write:
and using