Electricity And Magnetism

[natethegrea90090]

suppose you have a transformer that provides a secondary voltage four times as great as the primary voltage. You have a cell phone that uses a 6 voltage battery. Calculate the battery you could use with the transformer to power the phone

How did you calculate the answer?
And why is this problem impossible? What would needed to be changed to make it possible?

[beso]

The primary to secondary ratio is just one factor in a transformer. The transformer must be designed for a combination of voltage and frequency, also taking into account the length, area and permeability of the material used in the magnetic circuit. Then there is the wave shape of the AC voltage which is not necessarily sinusoidal.

Furthermore transformers work on AC while the battery needs DC so the output must be rectified. This process causes current to flow only during the peaks of the wave. Just how much of the wave conducts depends on the nature of the filter associated with the rectifier. The DC output voltage from the rectifier is higher than the AC input voltage.

In any case a cell phone battery should never be charged on anything other than a specifically designed circuit. These batteries are Lithium metal hydride and will explode and burn violently if not properly regulated.

[bert_h]

For a transformer output (secondary) of 6 volts, you would need a transformer input (primary) of 1 1/2 volts, because the question tells you that secondary = 4 times primary.

So you 'could' use a 1 1/2 volt battery, but it's could, not would, because as Beso correctly explains, batteries provide DC (direct current) and transformers only work on AC (alternating current) both in and out.

In principle (but not very realistically) you can imagine an alternator of some kind converting the 1 1/2 volt DC battery input to 1 1/2 volts AC, then the transformer stepping that up to 6 volts AC, then a rectifier converting that to the 6 volts DC that the phone needs. It's possible.

[jomifl]

You cannot use a transformer on a battery as the battery produces a Direct Current. You need an AC supply.

[beso]

bert // transformer stepping that up to 6 volts AC, then a rectifier converting that to the 6 volts DC //

Sinusoidal 6VAC rectifies up to about 8.5 VDC at no load. The actual output depends on the load and the nature of the rectifier filters.