howto:hambasics:sections:test
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howto:hambasics:sections:test [2021/01/04 21:46] – [The Complex Plane] va7fi | howto:hambasics:sections:test [2021/02/13 19:14] (current) – [Cartesian vs Polar] va7fi | ||
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\begin{align*} | \begin{align*} | ||
- | (1+i)^2 &= 1 + 2i + i^2 \\ | + | (1+i)^2 |
+ | &= (1+i)\cdot(1+i) \\ | ||
+ | &= 1 + 2i + i^2 \\ | ||
&= 1 + 2i - 1 \\ | &= 1 + 2i - 1 \\ | ||
&= 2i | &= 2i | ||
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... we can also represent a complex number graphically on a complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part. For example, \$ (1 + i) \$ would be represented as a point 45° up the horizontal axis and \$ \sqrt{2} \$ away from the origin: | ... we can also represent a complex number graphically on a complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part. For example, \$ (1 + i) \$ would be represented as a point 45° up the horizontal axis and \$ \sqrt{2} \$ away from the origin: | ||
- | {{ggb>/ | + | <WRAP center round info 80%> |
You can move the point around to look at other complex numbers on the plane. | You can move the point around to look at other complex numbers on the plane. | ||
+ | </ | ||
+ | |||
+ | {{ggb>/ | ||
To convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, | To convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, | ||
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Note that very often, we use radians instead of degrees for the angle. | Note that very often, we use radians instead of degrees for the angle. | ||
* Imagine a circle. | * Imagine a circle. | ||
- | * Now imagine the length from the centre to the circle (along the ``radius" | + | * Now imagine the length from the centre to the circle (along the "radius" |
* Take that length and lay it down on the perimeter of the circle. | * Take that length and lay it down on the perimeter of the circle. | ||
* The angle that this length covers is 1 radian (because of the length of the radius on the circle). | * The angle that this length covers is 1 radian (because of the length of the radius on the circle). | ||
- | * That's why a circle has 2π (because the circumference is 2πr) | + | * That's why a circle has 2π radians |
==== Roots ==== | ==== Roots ==== | ||
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< | < | ||
{{ complexroots.png? | {{ complexroots.png? | ||
- | So \$z = 2\$ was to be expected since \$ 2^3 = 8 \$ but it looks like there' | + | So \$z = 2\$ was to be expected since \$ 2^3 = 8 \$ but it looks like there are two more solutions. \\ \\ |
To find them, we first notice that the three solutions are spread out evenly around the circle, that is they are 120° apart. | To find them, we first notice that the three solutions are spread out evenly around the circle, that is they are 120° apart. | ||
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===== Euler Identity and Polar-Cartesian Representations ===== | ===== Euler Identity and Polar-Cartesian Representations ===== | ||
- | In the previous section, we saw that a complex number \$z = a + ib \$ could be represented as a point \$(a, b)\$ on the complex plane, which could also be viewed in polar coordinates as \$r\angle \theta \$. We saw that to convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, | + | In the previous section, we saw that a complex number \$z = a + ib \$ could be represented as a point \$(a, b)\$ on the complex plane, which could also be viewed in polar coordinates as (\$r\angle \theta) \$. We saw that to convert between the Cartesian \$(a,b) \$ and the Polar \$ (r \angle \theta) \$ representations, |
- | ^ \$$ a + ib \rightarrow r\angle \theta \$$ ^ \$$ r\angle \theta \rightarrow a+ib \$$ | | + | ^ \$$ (a,b) \rightarrow |
| \$$ r^2 = a^2 + b^2 \$$ | \$$ a = r\cos\theta \$$ | | | \$$ r^2 = a^2 + b^2 \$$ | \$$ a = r\cos\theta \$$ | | ||
| \$$ \tan \theta = \dfrac{b}{a} \$$ | \$$ b = r\sin\theta \$$ | | | \$$ \tan \theta = \dfrac{b}{a} \$$ | \$$ b = r\sin\theta \$$ | | ||
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^ Algebraic |\$$z = a + ib\$$ |\$$ z = re^{i\theta} \$$ | | ^ Algebraic |\$$z = a + ib\$$ |\$$ z = re^{i\theta} \$$ | | ||
- | This now allows us to simplify a lot of difficult mathematics. | + | This now allows us to simplify a lot of difficult mathematics. |
\$$ | \$$ | ||
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\$$z_1 = 1 + i = \sqrt{2}e^\left(i\frac{\pi}{4}\right) \quad \text{and} \quad z_2 = -1 + i = \sqrt{2}e^\left(i\frac{3\pi}{4}\right) \$$ | \$$z_1 = 1 + i = \sqrt{2}e^\left(i\frac{\pi}{4}\right) \quad \text{and} \quad z_2 = -1 + i = \sqrt{2}e^\left(i\frac{3\pi}{4}\right) \$$ | ||
- | Imagine having to add, subtract, multiply, | + | Imagine having to add, subtract, multiply, or divide these together. |
< | < | ||
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\\ | \\ | ||
- | The lesson here is that since the polar representation uses exponents, and exponents turn multiplication into addition((\$ x^a \cdot x^b = x^{a+b} \$)), the polar representation is easiest for multiplication, | + | The lesson here is that since the polar representation uses exponents, and exponents turn multiplication into addition((\$ x^a \cdot x^b = x^{a+b} \$)), the polar representation is easiest for multiplication, |
===== Important Algebraic Results ===== | ===== Important Algebraic Results ===== | ||
* Use the Euler identity to get the following two useful results: | * Use the Euler identity to get the following two useful results: | ||
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<WRAP center round info 80%> | <WRAP center round info 80%> | ||
- | A differential equation is an equation that relates a function to its derivatives in some ways and the question is: given some information about the system, what's the function (or family of functions) that satisfy the differential equation. | + | * A differential equation is an equation that relates a function to its derivatives in some ways and the question is: given some information about the system, what's the function (or family of functions) that satisfy the differential equation. |
- | In physics we often use a dot above the function to indicate a derivative with respect to time, where as in math, we'll often use an apostrophe. | + | * In physics we often use a dot above the function to indicate a derivative with respect to time, where as in math, we'll often use an apostrophe. |
\$$ \dot{x}(t) = x'(t) = \frac{dx}{dt} \quad \text{and} \quad \ddot{x}(t) = x'' | \$$ \dot{x}(t) = x'(t) = \frac{dx}{dt} \quad \text{and} \quad \ddot{x}(t) = x'' | ||
</ | </ | ||
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What we've go so far says that our test function \$x(t) = e^{rt}\$ will satisfy the differential equation if \$r\$ is given by above equation. | What we've go so far says that our test function \$x(t) = e^{rt}\$ will satisfy the differential equation if \$r\$ is given by above equation. | ||
- | To simplify the notation, let's define \$\alpha\$ and \$beta\$ as: | + | To simplify the notation, let's define \$\alpha\$ and \$\beta\$ as: |
\$$ \alpha = \dfrac{b}{2a} \qquad \text{and} \qquad \beta = \dfrac{\sqrt{|{b^2 - 4ac}|}}{2a} \$$ | \$$ \alpha = \dfrac{b}{2a} \qquad \text{and} \qquad \beta = \dfrac{\sqrt{|{b^2 - 4ac}|}}{2a} \$$ | ||
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\$r\$ then: | \$r\$ then: | ||
- | \$$ r = \left\{ \begin{array}{rl} -\alpha \pm \beta & \text{if } b^2 - 4ac > 0,\\ | + | \$$ r = \left\{ \begin{array}{ll} -\alpha \pm \beta & \text{if } b^2 - 4ac > 0,\\ |
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howto/hambasics/sections/test.1609825561.txt.gz · Last modified: 2021/01/04 21:46 by va7fi