1. A transformer is built inside a cell charger to change 120-V source from a wa

Question

1. A transformer is built inside a cell charger to change 120-V source from a wall outlet to a lower voltage ac source. If the charger draws an average power of 10.0 W with a current of 1.00 A in the secondary coil, find the turn ratio Np/Ns.

2. A long wire that can be wound into a round or square coil with the same number of turns. What is the ratio ?round/?square of the maximum torques experienced by the corresponding round and square coils in the same magnetic field with the same amount of current flowing through the coils?

I don't understand how to do ratios using the formulas :( help!!!!

Explanation / Answer

1)

Here , Power = 10 W

Is = 1 A

Vp =120 W

as P = Is*Vs = Vp*Ip

Vs = 10/1

Vs = 10 V

Now , for a transformer

Np/Ns = Vp/Vs

Np/Ns = 120/10

Np /Ns = 12

d the turn ratio Np/Ns is 12

2)

Here , for same circumference

2*pi*r = 4a

r = 2a/pi

maximum torque = NBA*w

Now ,

the ratio of max torque = Around/Aaquare

the ratio of max torque = pi*(2a/pi^2)/a^2

the ratio of max torque = 4/pi

the ratio of max torque   is 4/pi

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