Question

What two quantities are changed by a transformer? a) Power b) Potential Difference c) Current d) Resistance

can you explain why it is not the other two

Answer 1

For an ideal transformer, the answers are:
b) Potential Difference
c) Current

Explanation:

For an ideal transformer with a turns ratio from primary to secondary of N:1

The equation for potential difference on the secondary in terms of the potential difference primary:

`V_"secondary" = V_"primary"/N" [1]"`

The equation for the current is:

`I_"secondary" = N(I_"primary")" [2]"`

Please observe that, if we multiply equations [1] and [2], we obtain the equation:

`V_"secondary"(I_"secondary") = V_"primary"/N N(I_"primary")`

This simplifies to:

`V_"secondary"(I_"secondary") = V_"primary"(I_"primary")`

Using the equation the fact that `Power = VI`, we obtain the equation:

`Power_"secondary"=Power_"primary"`

NOTE: A real transformer has power losses but they are generally small such that an efficiency of `>90%` is typical.

Resistance is not a significant characteristic of transformers.