Hello everybody, am confronted with this question: A transformer with 5500 turns in its primary is used between a 240V a.c supply and a 120V kettle. Calculate the number of turns in the secondary. The equation Es/Ep = Ns/Np can be applied in solving the problem. Where Es = e.m.f of secondry. Ep = e.m.f of primary, Ns = number of turns in secondary coil, Np number of turns in primary coil Using the formular: Es/Ep = Ns/Np Where Np = 5500turns Ns = ? becuase am required to calculate for it. For Es and Ep, I don't know the value to substitute in for Es and Ep respectively. Whether is the 240V or 120V. I don't know between 240V and 120V the one that is Ep or Es respectively. I can't go further with the calculation since am unable to substitute in the values into the equation correctly. Can sombody help me here please?

Isn't the ratios of the EMFs just the ratios of the voltages, in ideal circumstances? Isn't the volt the unit of EMF?

It's nothing but a simple ratio. The ratio of the two voltages equals the ratio of the turns in the windings. Since the ratio of the voltages - going from primary to secondary is 2:1 (which can also be expressed as 1/2) just do the same with the windings. Your secondary will have exactly half as many turns as the primary.

The ratio of the two voltages equals the ratio of the turns in the windings. Sure. But conservation of energy kicks in. If you reduce the output voltage, then ameragec SHALL increase. And some devices may not be able to tackle the increased currents. So a way has to found to decrease the currents too. Nothing special, battery emilinators have been around much longer than you realise. Your mobile charger does exactly that ie reduce voltage AND amperage.

TOTALLY wrong!!! The available amperage increases, that's all. Seems your understanding of basic electricity is very poor. I suggest you hit the books again - and pay attention this time. :bugeye:

It's a simple 2:1 ratio, so N[sub]s[/sub] = 2750. The secondary winding will need to have a current-carrying capacity capable of supplying the current drawn by the load. The primary must be capable of supplying half that current plus the current to account for the transformer losses.

I know is a simple ratio, is just that I can't tell between 240v and 120v, the one that is primary and secondary e.m.f respectively. Who can give me a clue about that?

You have done well by defining those terms. Let me be more specific these time; how do you know that 240v is for primary and 120v is for secondary? Or suppose am faced with similar problem in future, how do I know the primary and the secondary e.m.f respectively.

In the OP you stated a 240 v.a.c. supply. The supply voltage will be used across the primary winding. The problem states that you will want 120 v.a.c. to be provided by the secondary winding.

That has nothing at all to do with the post that you made which I replied to. And I'm pretty sure that most people are aware of the wasted power in those devices.

Here is the working: E[sub]p[/sub]/E[sub]s[/sub]=N[sub]p[/sub]/N[sub]s[/sub] =240/120=5500/N[sub]s[/sub] :. N[sub]s[/sub]=5500*120/240 =2750 My thanks goes to all who contributed immensly to this thread.