Question

Your portable phone's 12V battery is dead. It's a holiday and all the stores are closed. You do have two 1.5V batteries from your camera and a transformer with N1 = 100 turns on teh primary and N2 = 400 turns on the secondary. Can you use the batteries and transformer to get your phone operating? If so, how would you do it? If not, why not?

Answer 1

Can we do this with batteries? Probably not. The direct current (DC) those batteries supply would maybe, for a very brief amount of time, induce a voltage over the secondary coil, then once the current in the primary coil levels out, there will be no more induced voltage in the secondary coil, and your phone wouldn't charge.

What if we used two AC voltage sources? Maybe. We could try putting those sources in series. Because they're in series, add 1.5V and 1.5V to get a maximum voltage of 3V.

We'll hook up these sources to the primary coil. Why? This equation holds for transformers:

`V_(S)`/
`V_(P)` =
`N_(S)`/
`N_(P)`

`V_(S)` = voltage of secondary coil,
`V_(P)` = voltage of primary coil,
`N_(S)` = turns of secondary coil,
`N_(P)` = turns of primary coil

Given values:

`V_(S)` = 12V (we want the transformer to output 12V to the phone)

`V_(P)` = 3V (sum of 2 AC sources)

`N_(S)` = 400

`N_(P)` = 100

Plug in and check that the math works out:

12/3 = 400/100

4 = 4

All good! This tells us that putting the two 1.5V AC sources in series, then hooking them up to the primary coil is a good idea. The circuit will transform the 3V input voltage to a 12V output voltage which will charge our phone's battery.

Answer 2

No.

Step-by-step explanation:

A transformer requires AC to work. A battery delivers DC only.