I don't Understand electricity ... ._.

[xentropa]

22 hours ago, Evann said:

i'm dumb i know, but i still dont really get it.

 

earlier we said U = R*I . Therefore we can say R = U/I.

 

let's say motor:

 

a) 240volts @ 2 amps --> R = 240/2 --> R= 120 Ohm

 

then u said I^2R - 2A^240Ohm = lot of heating loss.

 

 

b) 12volts @ 40 amps --> R = 12/40 --> R= 0.3 Ohm

 

I^2R - 40A^0.6Ohm = way less heat loss.

 

I meant I^2 * R.  

 

So I think you meant to write

 

Motor a) 2^2 * 120 = 480

Motor b) 40^2 * 0.3 = 480

 

Though you bring up a good point that I should clarify a bit.

 

The equation shows that the I^2 * R is the same so why did I say that high voltage is more efficient?  The reason deals mainly with the fact that wires have non-zero resistance, and how the power is delivered to each device is dependent on the relative resistance of the wires to the machine.

 

Let's say you have a 1 ohm wire, 1 ohm motor and another 1 ohm wire connected in series to a 6V power source.  The total current is 2A because 6V / (1 + 1 + 1).  Applying the I^2 * R equation to each component, we see that the first wire dissipates 4W (2^2 * 1 ohm), the motor dissipates 4W, and so does the last wire.  In the case that the motor is equal resistance to the wire, the power is then evenly distributed to each component.  Adding the total power of each component 4W + 4W +4W, we get 12W which is consistent with 6V * 2A.

 

Now we use those two same 1 ohm wires and connect it to a 10 ohm motor.  The total resistance is 12 ohms ( 1ohm wire + 10 ohm motor + 1 ohm wire) and apply 12V to the system.  Now the current is 1A and the total is the same as before 12W (12V * 1A).  But this time using the I^2 * R to each component, we find that the wires dissipate 1W each and the motor dissipates 10W.  In this case, the higher voltage and resistance of the motor relative to the wires increases efficiency.  This is why high voltage is preferred when wire resistance is non trivial as in the case with powerful motors and long distance power transmission.

 

OK so technically motors don't really have resistance, they have impedance and reactance, but we'll discuss that later.