hi, if I have a Nema 17-03 motor with 1.2A phase resistance 4.2 ohm, (the voltage is not specified on the motor site) and I connect a drv8825 to it, the Arduino uno and a 7.4V 2000mah battery can work?
dependent on how you connect it. but no fear - it is more than enough videos and instructables guides about such setup.
7.4V isn't enough for the DRV8825. It needs a tleast 8.2V - and that's the minimum. More is better ( up to 45V 
).
how much such batteries you have? you want for sure more than 2 minutes run, aren't you?
As @MicroBahner says you need more volts. You only need to set the coil current to provide enough torque for your application, not the full 1.4 amps. You will find that the higher the supply voltage you use the lower the actual supply current, as is explained in for example the DRV8825 setup video on the pololu site. If you only have 7.2V available then a better choice of driver is the DRV8834 which can operate from 5v up to 10.8v max though the supply current for a given coil current will be greater.
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thanks 
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