Checking my electricity homework answers

[Energize]

Can someone please check my answers here, there are loads of formulaes been give that I haven't used so I'm not sure if they are right or not, thanks.

 

[Doc Al]

The first problem (resistors in series) is done correctly, but the second (resistors in parallel) is not. Recalculate the total resistance in that second circuit. Hint: When resistors are in parallel, the total resistance will be less than that of any single resistor.

 

[andrevdh]

The total resistance of resistors in parallel can be calculated with

$$\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2}$$

this can be changed to

$$R_p=\frac{R_1R_2}{R_1 + R_2}$$

 

[Energize]

The total resistance of resistors in parallel can be calculated with

$$\frac{1}{R_p} = \frac{1}{R_1} + \frac{1}{R_2}$$

this can be changed to

$$R_p=\frac{R_1R_2}{R_1 + R_2}$$

When I do that I get 6x2 = 12ohm 12ohm/8 ohms = 1.5 ohms, is that right?

 

[andrevdh]

Yes. That seems to be it.

 

[Energize]

Am I right in thinking that if you have a 12v battery and short it the current would be very high?

 

[andrevdh]

Not neccessarily. The amp-hours rating is a complicated affair. It is the amount of current it can supply (chosen for "normal" [designed] operating conditions) for a certain set period (I think it is normally for 20 hours, but I might be wrong) before the battery is completely drained and dies out. So you might get more out of it for a shorter period but only to a certain limit (everyone has his limitations!).

 

[Energize]

Ok I think I'm not sure If I've got this right. I worked out I1 and I2 using fractions.

 

[Energize]

Anyone?...

 

[Doc Al]

Rethink your answer for C (the total current).

 

[andrevdh]

The current through the 2 ohm resistor is the current through all of the circuit since

$$I = I_1 + I_2$$

So $$I$$ will be be determined by the total resistance of the circuit and the voltage of the power supply.

 

[Energize]

Ok we went through that question today and I got A-F right but G was 0.9A.

 

[CPL.Luke]

12/5 does not equal 1.2, maybe you sent us the wrong version of the problem, or the teacher did it wrong

 

[Energize]

Ah sorry I put 12v instead of 6, doh.