Good Evening Bill, Curt and anyone else still following this thread.
Curt, your water analogy is spot on.
Bill, you ask a lot of good questions so let's see if I can give you qood answers.
"if 'the circuit' is the 3.7ohm resistor, wouldn't the current always be 1.62 amps, so 'drop' 6 volts? isn't the circuit actually 4.6ohms? [.9 + 3.7], making the complete circuit current 1.3amps?"
Part 1a. If the circuit was JUST the 3.7ohm resistor the total current flow would be 1.62 amps, and the voltage dropped acrossed the resistor would be equal (and opposite) to the battery voltage.
Part 1b. If the circuit was the 3.7ohm resistor + the .9ohm resistance of the motor, circuit current would be 1.3 amps. The resistor would then drop 1.3amps x 3.7ohms = 4.8 volts and the motor would drop 1.3amps x .9ohms = 1.2 volts.
Kirchoff's voltage law says the sum of all of the voltages in a circuit must equal zero.
+6v (battery) - 4.8v (resistor) - 1.2v (motor) = zero and Kirchoff's voltage law is satisfied
Kirchoff's current law says that the sum of the currents entering and leaving a node (device) in a circuit must equal zero.
The Battery has 1.3 amps out from (+) terminal to the resistor.
The Resistor has 1.3 amps in from battery, 1.3 amps out to the motor.
The Motor has 1.3 amps in from resistor, 1.3 amps out to the battery (-) terminal.
Each node (device) has a sum of zero amps (In - Out) and Kirchoff's current law is satisfied.
This scenario requires one specific condition to remain true. That condition is that the shaft of the motor MUST BE MECHANICALLY RESTRAINED FROM MOVING (called a locked rotor). As soon as the motor begins to turn, Ohm's law is useless as far as helping us determine current flow and voltage drop in the motor. As I did in the last post, you can make an assumption about the current and calculate information about the motor but it is impossible to calculate motor current based on what we know at this point.
Rotating electrical machines are energy CONVERSION devices. They convert electrical energy to mechanical energy (motor) and vice versa (generator).
In a motor, we supply electrical power in (Volts * Amps)= (Watts) and have mechanical power (Torque x Angular Momentum) = (HorsePower) coming out on the shaft. We also lose some of the electrical power to the resistance of the windings and the brushes/commutator.
In a generator, we supply mechanical power in on the shaft and take electrical power out on the terminals. Again, some of the mechanical power that is converted to electrical power is lost to the resistance of the windings and the brushes/commutator.
DC motors and generators are essentially the same machine. The 1948-1951 Chevy Truck shop manual has instructions on how to run your generator as a motor.
In the next post, I'll attempt to explain what's going on in these DC machines that gives Ohm's law (and many of us Stovebolters) such heartburn.
John
