You may also like

problem icon

Roots and Coefficients

If xyz = 1 and x+y+z =1/x + 1/y + 1/z show that at least one of these numbers must be 1. Now for the complexity! When are the other numbers real and when are they complex?

problem icon

Target Six

Show that x = 1 is a solution of the equation x^(3/2) - 8x^(-3/2) = 7 and find all other solutions.

problem icon

8 Methods for Three by One

This problem in geometry has been solved in no less than EIGHT ways by a pair of students. How would you solve it? How many of their solutions can you follow? How are they the same or different? Which do you like best?

Impedance Can Be Complex!

Stage: 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Part 1:

$P = IV$

I = $\frac{E}{Z_1 + Z_2} = \frac{E}{Z_1cos\theta + Z_2cos\phi + i(Z_1sin\theta + Z_2sin\phi)}$

|I| = $\frac{E}{\sqrt{(Z_1cos\theta + Z_2cos\phi)^2 + (Z_1sin\theta + Z_2sin\phi)^2}}$

The real power transferred to $Z_2$: P = I^2 R

P = $ I^2 Z_2cos \phi = \frac{E^2 Z_2cos \phi}{Z_1^2 + Z_2^2 + 2Z_1Z_2(cos \theta cos \phi + sin \theta \phi)}$= $ \frac{E^2 Z_2cos \phi}{Z_1^2 + Z_2^2 + 2Z_1Z_2cos( \theta - \phi )} $

Part 2:

To find the maximum power; we can differentiate the power expression with respect to $Z_2$ and set the derivative equal to zero.

$\frac{dP}{dZ_2} = \frac{d}{dZ_2}\frac{E^2 Z_2cos \phi}{Z_1^2 + Z_2^2 + 2Z_1Z_2cos( \theta - \phi )}$

Let U = $E^2 Z_2cos \phi$

Let V = $ Z_1^2 + Z_2^2 + 2Z_1Z_2cos( \theta - \phi ) $

$\frac{dP}{dZ_2} = \frac{VU' - UV'}{V^2}$

V' = $ 2Z_2 + 2Z_1 cos ( \theta - \phi)$

U' = $E^2 cos \phi$

Substituting thses values into the expression and setting it equal to zero we find that:

$Z_1 = Z_2$

For maximum power transfer between load and source we must therefore match the internal impedance of the load with the impedance of the source