A transformer has 500 primary turns and 9.7 secondary turns.

(a) If Vp is 120 V (rms), what is Vs with an open circuit?

(b) If the secondary now has a resistive load of 13 Ω, what is the current in the primary and secondary?

Answer: a) 2.33V b) Ip = 0.0035A, Is = 0.179A

Explanation:

The relationship formula between the Voltage, Number of turns and current in a transformer is given as

Vp/Vs = Np/Ns = Is/Ip

Vp is voltage in the primary coil

Vs is voltage in the secondary coil

Ns is number of turns in the secondary coil

Np is number of turns in the primary coil

Is is current in the secondary coil

Ip is current in the primary coil

a) substituting the values in the formula

Vp/Vs = Np/Ns

120/Vs = 500/9.7

Vs = 120*9.7/500

Vs = 2.33V

b) since secondary now has a resistive load of 13 Ω, Using ohm's law to get current in the secondary

V=IR

Is = Vs/R

Is = 2.33/13

Is = 0.179A

To get Ip, we will use the relationship Np/Ns = Is/Ip

500/9.7 = 0.179/Ip

Ip = 9.7*0.179/500

Ip = 0.0035A