My question is about elementary series current calculations. Above you see 3 circuits. All circuits have a Vin of 24v connected to either 1 or 2 heating elements (Z1 and Z2).
When calculating the current in circuit 1 I apply the formula I = P_Z1/U_Z1 = 2W/12V = 0.17A.
When calculating the current in circuit 2 I apply the formula I = P_Z2/U_Z2, 1W/12V = 0.08A.
When calculating the current in circuit 3 I apply the formula I = P_tot/Vin, 3W/24V = 0.125A.
I can calculate I in circuit 3 by first calculating R_tot = R_Z1 + R_Z2 then using I= Vin/R_tot.
R_1 = U_z1²/P_z1 = 12²/2 =24 Ohm.
R_2 = U_z2²/P_z2 = 12²/1 = 144 Ohm.
So R_tot= 168 Ohm. with I = 24/168= 0.14A.
Question 1: How come I get 2 different answers when applying the 2 different ways to calculate I? Where am I going wrong here? AND which answer is the correct one?
Question 2: In circuit 1 (and 2) I connected a 12V heating element to a 24V power supply and calculated I throug 2W/12v = 0.17A. I get the feeling that this is wrong because the source voltage is not taken into acocunt here, where am I going wrong here?
Question 3: what does it practically mean when connecting a 12V/2W element to a 24V source?
Question 4: In circuit 3 we have I = 0.14A. this means for Z1 (and Z2) that P has changed? P_z = 12V*0.14A= 1.68W, but when we started it was 2W. Where Am I going wrong here?