Asnetcomp Tutorials: 2016

Thursday, October 20, 2016

China made SW radios review

1) Kaide KK-9

This radio (Kaide KK-9) looks cheap and actually very cheap, usd 5.00 in a retail shop in HK.
But the short wave reception is quite good even using the CD2003 common Am/Fm IC. The spec of this IC does not say anything about SW reception.

7 SW bands: 5.95-6.42MHz, 7.08-8.40MHz, 9.43-10.97MHz, 11.44-12.29MHz, 13.56-14.13MHz, 14.94-15.71MHz, 17.47-18.29MHz

The inside is quite messy. Many Ceramic Disc Capacitors (30pF, 50pF, 68pF, 75pF, 120pF, 150pF, 300pF) are arranged with a long Slide Switch. This may be their technique or copy of someone else.

Inside of Kaide KK-9

2) Kaide KK-2005A

This radio is an up-grade, larger, more powerful and clear sound and higher priced version of KK-9. US$15 retail price in HK. It uses CD2003 too.

7 SW bands: 5.67-6.43MHz, 6.96-7.87MHz, 9.28-10.36MHz, 11.15-12.28MHz, 13.15-14.00MHz, 14.90-15.80MHz, 17.06-17.90MHz

We can see four colorful (red + pink, orange and green) IFTs and not so many Disc Capacitors a long Slide Switch but some between the AM bar antenna and the three filters. Because of the space not so messy. And the SW reception design seems different form KK-9. Maybe because of the ample space not so messy as KK-9. SW reception is not necessarily noticeably better than KK-9, however.

Inside of KK-2005A

3) AUDIOLA RTB2051

This radio, nice outlook and color, uses the same Radio IC CD2003 and says SW reception.

2 bands: 6-12MHz, 12-18MHz

But it detects only 2-3 SW stations under the same condition as Kaide KK-9 and must change the radio direction to try to find a strong distinctive signal. The reception signals are much weaker than those of KK-9. I cannot see many ceramic disc caps like KK-9. Even it has space quite messy too.

Inside of RTB2051

4) Yima - YM-131

A so called Potentiometer Tuning Radio. Very cheap - costing about US$10 at a discount retail shop in Hong Kong. Very simple function - even no earphone jack, no LED, nothing but simple radio function with a loud speaker. The sound quality is fair. The inside is very simple. No PVC, No IFT, No Air Coils, even no Ceramaic Filters. Much less Ceramic Disc Capacitors but many resistors and two potentiometers. A long slide switch and AM Antenna remain. Tuning function is good and the turning the wheel is almost same feeling as that of PVC tuning. The radio IC is C9615 (SI4825 equivalent). The name Yima sounds like Malaysian or some other South East Asian, but China brand.

Inside of Yima - YM-131

Radio IC: C9615 (DIP 16P) (China brand)
Audio Amp IC: 8002A (DIP 8P) (China brand)

sptt

Monday, October 10, 2016

Elecric Flux, Vm or Q

Magnetic Flux (symbol

Φ) is used as in the following equation.

Φ

v(t) = ------
dt

By using analogy Electric Flux can be considered as Electric Charge (symbol

Q) as in

the following equation.

Q

i(t) = ------
dt

I find this definition in my old Penguin's Electronics dictionary and Physics dictionary.

However, an official definition seems as follows:

From wiki

"
In electromagnetism, electric flux is the measure of flow of the electric field through a given area. Electric flux is proportional to the number of electric field lines going through a normally perpendicular surface. If the electric field is uniform, the electric flux passing through a surface of vector area

S

\Phi _{E}={\mathbf {E}}\cdot {\mathbf {S}}=ES\cos \theta ,

where

E

is the electric field (having units of

V/m

E

is its magnitude,

S

is the area of the surface, and

θ

is the angle between the electric field lines and the normal (perpendicular) to

S

.
For a non-uniform electric field, the electric flux

d Φ E

through a small surface area

d S

is given by

d\Phi _{E}={\mathbf {E}}\cdot d{\mathbf {S}}

(the electric field,

E

, multiplied by the component of area perpendicular to the field). The electric flux over a surface

S

is therefore given by the surface integral:

\Phi _{E}=\iint _{S}{\mathbf {E}}\cdot d{\mathbf {S}}

where

E

is the electric field and

d S

is a differential area on the closed surface.

"

The point is ＜electric flux is the measure of flow of the electric field through a given area＞. Magnetic flux is, however, not the measure of flow of the magnetic fields (H, B) through a given area, but simply

Φ.

The unit of Electric filed is V/m. Therefore unit wise

E.dS is (V/m) x

m

2

so in ＜Vm＞. ＜Vm＞may have a meaning itself like ＜Im＞. For instance,

d

Φ

＜Vm＞ is   ------ x m.
                      dt
                                           dm
Can we change this to

Φ

x ------ =

Φ

v (v = velocity) ?
dt

This can be considered as a velocity of

Φ or

Φ is moving with velocity, but difficult to imagine.

Likewise on the otherhand

                     dq
＜Im＞ is   ------ x m.
                      dt
                                            dm
Can we change this to q x ------ = qv (v = velocity) ?
                                            dt

This can be considered as a velocity of q or q is moving with velocity and usually considered in Electric Field creating Force. Lorentz Force.

From Wiki

"
Lorentz force, the force exerted on a charged particle q moving with velocity v through an electric E and magnetic field B. The entire electromagnetic force F on the charged particle is called the Lorentz force (after the Dutch physicist Hendrik A. Lorentz) and is given by F = qE + qv × B
"

Wiki explains ＜im (Ampere-meter, A-m)＞by quoting Einstein as follows:

"

Derivation

Einstein proved that a magnetic field is the relativistic part of an electric field. This means that while an electric field acts between charges, a magnetic field acts between moving charges (as a charge moves through space more quickly and through time more slowly, its electromagnetic force becomes more magnetic and less electric). Therefore, the pole strength is the product of charge and velocity.

1~\mathrm{A \cdot m} = 1~\mathrm{C} \cdot \frac{\mathrm{m}}{\mathrm{s}}

Usefulness

Few calculations actually involve the strength of a pole in ampere-meters because a single magnetic pole has never been isolated. Magnets are dipoles which require more complicated calculations than monopoles. However, the strength of a magnetic field is measured in teslas and one tesla is one newton per ampere-meter which confirms that the unit for pole strength is indeed the ampere-meter.

" a charge moves through space more quickly and through time more slowly "

This part is a bit difficult to understand.

" Therefore, the pole strength is the product of charge and velocity."

Why " therefore " ? Some explanation is needed.

"
the strength of a magnetic field is measured in teslas and one tesla is one newton per ampere-meter which confirms that the unit for pole strength is indeed the ampere-meter.
"

Here "the strength of a magnetic field" is B (also called "magnetic flux density"). This part is clear unit wise.

Anyway ＜im＞seems to be <magnetic pole strength>. But when differentiate this ＜im＞with time, ＜q＞and ＜v (velocity) ＞come out. This transformation (after differentiating with time) is interesting. We can apply this to ＜vm＞, which is already shown above. When differentiate this ＜vm＞with time, ＜

Φ

＞and ＜v (velocity) ＞come out.

d

Φ

＜Vm＞ is   ------ x m.
                      dt
                                       dm
We can change this

Φ

x ------ =

Φ

v (v = velocity)
dt

H (field) is in I/m while E (field) is in V/m. H.dS is therefore in ＜Im＞.

μ0 H (I/m) = B (

Φ/

m

2

) - Magnetic flux density

ε0 E (V/m) = D (

Q/

m

2

) - Electric flux density. Name wise, Q is Electric flux. D is also called Displacement and D is for Displacement.

These are very symmetrical and the names represent the meaning as compared with ＜electric flux is the measure of flow of the electric field (V/m) through a given area＞. We can visualize it as a bundle of V/m lines while it is difficult to visualize H field (I/m) as a bundle of I/m lines though not impossible if you are familiar with the relation with I (or rather di/dt) and L or

μ 0 (L = m

μ 0

).

d

i(t)

v(t) = L ------
dt

This ＜v(t)＞ generally changes with time as ＜i(t)＞ changes with time. When＜i(t)＞ does not change with time (being constant), v(t) = 0.

d

Φ

v(t) = ------
dt

Combining we get

d

i(t)

Φ

v(t) = L ------ = ------
dt dt

Therefore

Ld

i(t) =

Φ

Li =

Φ

i =

Φ / L

=

Φ /

m

μ 0

Here we can find the relation between

the original＜i(t)＞ and

Φ .

＜i(t)＞ is proportionally caused by

Φ (Magnetic Flux, Im) with 1/L or inversely proportionally

caused by L. In other words

L being fixed,

＜i(t)＞ becomes larger as

Φ becomes stronger.

Φ being fixed,

＜i(t)＞ is needed less as

L becomes larger to keep this fixed

Φ.

The definition of

Φ (Weber) is

From wiki

"

A change in flux of one weber per second will induce an electromotive force of one volt (produce an electric potential difference of one volt across two open-circuited terminals).

Officially,
Weber (unit of magnetic flux) — The weber is the magnetic flux that, linking a circuit of one turn, would produce in it an electromotive force of 1 volt if it were reduced to zero at a uniform rate in 1 second.

"

which means

0v --> 1v x 1 sec =1 --> 0 Weber

but more generally

                d

Φ

v(t) = - ------
dt

(Added the minus sign)

＜Vm＞ x ＜Im＞ = VIm². VI is Electric Power.

＜

Φ

v＞ x ＜qv＞ (v = velocity) =

Φ

qv².

E x H (cross product) is called "Poynting vector". Unit wise = (V/m) x (I/m) = VI/m²

From wiki

"
Poynting vector represents the directional energy flux density (the rate of energy transfer per unit area) of an electromagnetic field. The SI unit of the Poynting vector is the watt per square metre (W/m²).

"

Watt is not Energy but Electrical Power - the rate of energy transfer per time.

sptt

Friday, September 9, 2016

Reciprocal - 3, E and H (very simple field theory)

E (Electric Field Strength) and H (Magnetic Field Strength) . H may be difficult to catch for some people.

1. Equations

                                            V (voltage)
E (Electric Field Strength) = ----------------
                                            d (distance)

                    V(oltage)
The unit is   ------------
m(eter)

V can be either constant or variable (in this case usually the small <v> is used.)

                                               I (current)
H (Magnetic Field Strength) = ------------------
                                               d (distance)

                    A(mpere)
The unit is   -------------
m(eter)

I can be either constant or variable (in this case usually the small <i> is used)

A
----- is abstract or abstracted. Difficult to imagine.
m

I (current) is time derivative of Q (Charge) = -----
dt

and Q (charges) is supposed to move in this case, not static as V and creates magnetic field.

2. Field Concept

         V
E = -----
         d

When V is constant

E changes almost infinite ( $\infty)$ to almost zero (0) as d changes almost zero (0) to almost infinite ( $\infty). Reciprocal relation. The point is that E exists at every point (every where) in space.$

          I
H = -----
         d

When H is constant

H changes almost infinite ( $\infty)$ to almost zero (0) as d changes almost zero (0) to almost infinite ( $\infty). Again reciprocal relation. The point is that H exists at every point (every where) in space.$
$Every where then Field Theory.$

$ACT$

Friday, August 26, 2016

XL, XC - Phase Shift by calculus (A part of Reciprocal- 2)

＜Electronics Tutorials＞some of which I recently read several times.They are good reading materials as tutorials as the name shows. At the beginning of their ＜Series Resonance Circuit＞ we can find the following mostly familiar formulas.

http://www.electronics-tutorials.ws/accircuits/series-resonance.html

series circuit characteristics

sptt additional explanation and comments

The first and the second equations were already introduced at the end of Post ＜Reciprocal＞.

"
Inductors act like resistors as XL (which is regarded as resistance) = 2 $π$ fL. On the other and Capacitors act as reciprocal of Resistors as XC (which is regarded as resistance) = 1/2 $π$ fC.

"
Please note that XC is a reciprocal form. So the chart becomes y = 1/x form like below.

The difference is x : 2 $π$ fC. <x> is a variable while f is variable but 2 $π$ is constant and C can be either a constant or the second variable.

＜f＞ has the unit ＜(Number) / sec＞. The unit of C is Farad but originally (from wiki "Farad")

"
A farad has the base SI representation of: s⁴ × A² × m⁻² × kg⁻¹

It can further be expressed as:

{\mbox{F}}=\,\mathrm {{\frac {A\cdot s}{V}}={\dfrac {\mbox{J}}{{\mbox{V}}^{2}}}={\dfrac {{\mbox{W}}\cdot {\mbox{s}}}{{\mbox{V}}^{2}}}={\dfrac {\mbox{C}}{\mbox{V}}}={\dfrac {{\mbox{C}}^{2}}{\mbox{J}}}={\dfrac {{\mbox{C}}^{2}}{{\mbox{N}}\cdot {\mbox{m}}}}={\dfrac {{\mbox{s}}^{2}\cdot {\mbox{C}}^{2}}{{\mbox{m}}^{2}\cdot {\mbox{kg}}}}={\dfrac {{\mbox{s}}^{4}\cdot {\mbox{A}}^{2}}{{\mbox{m}}^{2}\cdot {\mbox{kg}}}}={\dfrac {\mbox{s}}{\Omega }}} ={\dfrac {{\mbox{s}}^{2}}{\mbox{H}}}

where F=farad, A=ampere, V=volt, C=coulomb, J=joule, m=metre, N=newton, s=second, W=watt, kg=kilogram, Ω=ohm, H=henry.

"
If we simply take ＜s / ‎Ω＞, then 2 $π$ fC is in ＜/ ‎Ω＞(Reciprocal of ‎Ω). Therefore ＜1 / ‎2 $π$ fC＞ is in ‎Ω.

＜f＞can be several Hz to several Giga Hz but for AM ranging 500KHz - 1600KHz, for FM 80MHz (80,000KHz) to 120MHz (120,000KHz). C can be several pico Farad to several micoro Farad.

For AM Tuning (which uses LC Resonance and Oscillation) we can use ＜f＞as 1,000KHz and C as 100pF. If we use original units

1,000KHz ---- ＞ 1,000,000Hz

100pF ---- ＞100 / 1,000, 000, 000,000 F or 1 / 10,000,000.000 F. This is very small number as compared with Farad or even uF (1/1,000,000 F). These small numbers make differences is a remarkable thing.
We put these two numbers
1 / ‎2 $π$ fC = 1 / 2 x 3.14 x 1,000,000 (Hz in /s) x (1/ 10,000,000,000 (F in s / ‎Ω)
= 1 / 2 x 3.14 x 1 x (1 / 10,000 (Ω)
= 1 / 2 x 3.14 x 0.0001
= 1 / 0.000628 = 1,591 Ω (=XC)

We can set L as 100uH.

XL = 2 $π$ fL = 2 x 3.14 x 1,000,000 (Hz in /s) x 100 / 1,000,000 H = 6.28 x 100 (Hz in /s) H
= 628 (supposed to be in Ω).

The unit of H is Henry and originally (from wiki "Henry")

"

{\mbox{H}}={\dfrac {{\mbox{kg}}\cdot {\mbox{m}}^{2}}{{\mbox{s}}^{2}\cdot {\mbox{A}}^{2}}}={\dfrac {{\mbox{kg}}\cdot {\mbox{m}}^{2}}{{\mbox{C}}^{2}}}={\dfrac {\mbox{J}}{{\mbox{A}}^{2}}}={\dfrac {{\mbox{T}}\cdot {\mbox{m}}^{2}}{\mbox{A}}}={\dfrac {\mbox{Wb}}{\mbox{A}}}={\dfrac {{\mbox{V}}\cdot {\mbox{s}}}{\mbox{A}}}={\dfrac {{\mbox{s}}^{2}}{\mbox{F}}}={\dfrac {\mbox{1}}{{\mbox{F}}\cdot {\mbox{Hz}}^{2}}}=\Omega \cdot {\mbox{s}}

where

Wb = weber, T = tesla, J = joule, m = meter, s = second, A = ampere, V = volt, C = coulomb,
F = farad, Hz = hertz, Ω = ohm

"

We can take simply ＜s Ω ＞.

Next we must review the fith equation.

Total Circuit Reactance XT = XC - XL or XL - XC
Why these or and why not XT = XC + XL ?

We are not able to find it in this article but find it in the previous article ＜Series RLC Circuit Analysis
＞ (http://www.electronics-tutorials.ws/accircuits/series-circuit.htmlhttp://www.electronics-tutorials.ws/accircuits/series-circuit.htm), which I will not quote in this post as it is a bit long and I have already quoted the above formulas in this post. Too much copies. We must think independently here. But we need the following drawing.

series rlc circuit analysis

We discussed a much simpler case of Series of Resistors at the end of Post ＜Reciprocal＞.

"

R = R₁ + R₂ + R₃ + ...

Generally

VT = V1 + V2 +V3.

IR = V
Therefore
ITRT = I1R1 + I2R2 + I3R3
Since IT = I1 = I2 = I3

Therefore RT = R1 + R2 + R3
Inductors act like resistors as XL (which is regarded as resistance) = 2 $π$ fL. On the other and Capacitors act as reciprocal of Resistors as XC (which is regarded as resistance) = 1/2 $π$ fC.
"
These two schematics are similar in terms of the arrangement - in Series, but the difference is big. We can re-arrange R-L-C Series in terms of "resistance".

------- R --- XL(2 $π$ fL) --- Xc (1/2 $π$ fC) --------

Then use the same reasoning

Generally

VT = VR + VL +VC.

IR(X) = V
Therefore
ITR(X)T = IRR + ILXL(2 $π$ fL) + ICXc (1/2 $π$ fC)
Since IT = IR = IL = IC

Therefore R(X)T = R + XL(2 $π$ fL) + Xc (1/2 $π$ fC)
-------
So back to the same question. Why not XT = XC + XL ?
and why not R(X)T = R + XL(2 $π$ fL) + Xc (1/2 $π$ fC)

Look at the graph again.

Vs (Voltage Source) is AC not DC. And here I is also AC.

I(t) = Im sin (ωt), where ω (omega) = 2 $π$ f

The reactions of an inductor and a capacitor to AC are very interesting and they act oppositely or RECIPROCALLY or inversely in terms of frequency (cycle of up and down (voltage) or back and forth (current) as

XL = 2 $π$ fL.
XC = 1/2 $π$ fC

show.

Again back to
Total Circuit Reactance XT = XC - XL or XL - XC
Why or and why not XT = XC + XL ?

For simplicity we omit R here and use the same reasoning.

Generally

VT = VL +VC.

IX = V (please recall IR=V (Ohm's Law))

Therefore

IXT = IXL + IXc ( IT = IL = IC)
Therefore XT = XL + Xc

What is wrong ? Well very wrong. An inductor and a capacitor do not act in the same way as a resistor against or in AC (current and voltage). AC is not always sinusoidal but quite often so (like Radio signals). Some basic equations should be introduced here and used to get correct (correctly work) equations. And Calculus is needed to get the points.

1) Calculus Stage One

Inductor

v = L ----
dt

Please note that when there is no change of current (DC Current) then v = 0.

Capacitor

i = C ----
dt

Please note that when there is no change of voltage (DC voltage) then i = 0.

Above we considered

VT = VL +VC.

Unlike in the case of resistors VT, VL, VC are not constant. (should be re-write as vT = vL +vC )
di

VL = v = L ----
dt

VL (v) changes with time as i (I(t) = Im sin (ωt)) changes. The faster the change of ＜i＞becomes the higher VL (v) becomes. The faster means: the frequency becomes higher, say 50Hz to 1000Khz (1KHz) or to 1,000,000Hz (1MHz). AM Radio frequency 500KHz - 1,600KHz. FM frequency 76 - 108MHz. These are quite faster than 50Hz. When considering even at 50Hz ＜i＞ changes (up from the original current level to the maximum current level and down to the minimum current level and up again to the original current level, called one cycle) 50 times changes (cycles) per one second, 1KHz and 1MHz make extremely fast change of up and down. It is not easy to imagine these changes of the current as compared with changes of the voltage (up and down). Actually we cannot see the current flow and the change of the current flow. Just imagine. At least we can imagine the change of the direction of the current (back and forth). VL (v) changes with time though very regularly and periodically as i (I(t) = Im sin (ωt)) changes. Therefore VL (v) at 1MHz is much higher than the VL (v) at 1Hz. This is the point.

Next VC. The relation between ＜i＞ and ＜v＞ under AC or sinusoidal change is

i = C ----
dt

This is not a reciprocal of

v = L ----
dt

but looks like it. C replaces L and ＜i＞ and ＜v＞ are exchanged. Easy to remember. We want to have the value of Vc, not Ic. To get Vc we need an integration procedure.
＜Electronics Tutorials＞uses Q (electric charge on the plate on a capacitor) in ＜Series RLC Circuit Analysis＞, but does not elaborate it much.

Q
---- = v
C

Q comes from
dQ
i = -------
dt

＜What "electric current" is ? ＞ is a big issue. Quoting wiki "electric current"

"
For a steady flow of charge through a surface, the current I (in amperes) can be calculated with the following equation:

I={Q \over t}\,,

where Q is the electric charge transferred through the surface over a time t. If Q and t are measured in coulombs and seconds respectively, I is in amperes.
More generally, electric current can be represented as the rate at which charge flows through a given surface as:

I={\frac {\mathrm {d} Q}{\mathrm {d} t}}\,.

"
This explanation is very much like those found in many common text books. Please see my tutorials ＜What is Electric Current?＞and ＜What is Electric Current? - 2＞.

Q dQ dq
What is the difference between I = ------ and I = ------ (or i = ------ ) ?
t dt dt

The definition of the unit of current, Ampere is ( from wiki ＜Ampere＞)

"
. . . . . .

Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

{\rm {1\ A=1{\tfrac {C}{s}}.}}

In general, charge Q is determined by steady current I flowing for a time t as Q = It.

"
Then there is s distinctive difference. In this explanation " In general" is not a generalization. " steady current I flow" is a very special case. We can directly (not through differentiation) count Q even under the condition of "going past a given point per second", which can be imagined by an analogy of water flow through a point (or water flow through a pipe (or a hose) / its cross sectional Area.

                                      dQ                                    dq
Real generalization is I = ------ or more correctly i = ------
                                       dt                                       dt

where ＜i＞and ＜q＞ are not fixed or constant values but those changing with time. In this case when ＜q＞ is constant (not changing with time) ＜i＞ is zero (0), no flow of current ?

                 Q                   dq
Again, I = -----   and i = ------ are different things.
                  t                    dt

The latter is not a generalization of the former.

Then how to get Q to get Vc ?
This Q (which is generally defined as integration of ＜i＞), the Q related with a capacitor comprises a equation with ＜v＞. Look at the Unit ＜F＞ again.

{\mbox{F}}=\,\mathrm {{\frac {A\cdot s}{V}}={\dfrac {\mbox{J}}{{\mbox{V}}^{2}}}={\dfrac {{\mbox{W}}\cdot {\mbox{s}}}{{\mbox{V}}^{2}}}={\dfrac {\mbox{C}}{\mbox{V}}}={\dfrac {{\mbox{C}}^{2}}{\mbox{J}}}={\dfrac {{\mbox{C}}^{2}}{{\mbox{N}}\cdot {\mbox{m}}}}={\dfrac {{\mbox{s}}^{2}\cdot {\mbox{C}}^{2}}{{\mbox{m}}^{2}\cdot {\mbox{kg}}}}={\dfrac {{\mbox{s}}^{4}\cdot {\mbox{A}}^{2}}{{\mbox{m}}^{2}\cdot {\mbox{kg}}}}={\dfrac {\mbox{s}}{\Omega }}} ={\dfrac {{\mbox{s}}^{2}}{\mbox{H}}}

Here ＜C＞ is Coulomb (the unit of charge) and ＜F＞ is Farad.
                         As              C
We can see F = ----- and -----.
                          V               V

Unit wise,

FV
----   = A
s

or

CV
---- = i
t

This relates with the basic differential equation.

i = C ----
dt

Still we are not able to get Vc ?

Vc seems simply an accumulation of the charge(s) on a capacitor. And this Vc changes with Ic (= I(t) = Im sin (ωt)). The amount and direction of Ic change with time. Wait a minute. What is "the amount of Ic" ? i = dQ / dt. So the greater change of the amount of Q, not the greater the Q is, the greater i becomes. Please recall that the greater the change of of velocity (speed), not the greater the velocity (speed) is, the bigger the force is created.

Q
---- = v, or Q = Cv
C

When C is fixed, Q changes simply proportionally with v while i changes proportionally with derivative of v in a capacitor. The faster v changes the more current flows (or more correctly the faster Q changes).

This explains more correctly

First: the more current flows the faster Q changes.

The more current flows in one direction the faster Q increases.
The more current flows in the opposite direction the faster Q decreases.

This is

I={\frac {\mathrm {d} Q}{\mathrm {d} t}}\,.

And it can be applied in a capacitor circuit. In a circuit with no capacitor this cannot work. Just simply

I={Q \over t}\,,

where I and there for Q are constant.

2) Calculus Stage Two

To answer to the question. Why not XT = XC + XL but

XT = XC - XL or XL - XC

We need Trigonometric Calculus. ＜Electronics Tutorials＞ does not use Trigonometric Calculus and simply say

"
We recall from the previous tutorial about series RLC circuits that the voltage across a series combination is the phasor sum of V_R, V_L and V_C. Then if at resonance the two reactances are equal and cancelling, the two voltages representing V_L and V_C must also be opposite and equal in value thereby cancelling each other out because with pure components the phasor voltages are drawn at +90^o and -90^o respectively.
Then in a series resonance circuit as V_L = -V_C the resulting reactive voltages are zero and all the supply voltage is dropped across the resistor. Therefore, V_R = V_supply and it is for this reason that series resonance circuits are known as voltage resonance circuits, (as opposed to parallel resonance circuits which are current resonance circuits).

" We need only two Trigonometric Calculus to explain XT = XC - XL or XL - XC

d
---- sin (x) = cos (x)
dx

and
d
∫ sin (x) dx = - cos (x), this comes from more basic ---- cos (x) = - sin (x).
dx

To be convinced you read and understand some text book. But I like the following intuitive ways. From also ＜Electronics Tutorials＞

Inductor

v = L ----
dt

When ＜i＞ is sinusoidal like I(t) = Im sin (ωt) by using

d
---- sin (x) = cos (x)
dx

＜v＞becomes like V(t) = (Vm) cos (ωt), which is shown in the above nice drawing VL, exactly.

Next

Capacitor

i = C ----
dt

To integrate the both sides we use

∫ sin (x) dx = - cos (x)

∫ sin (ωt) dx = - C Vm cos (ωt), which is also shown in the above nice drawing VC, exactly.

d
This process looks like a math magic. ----- disappeared and cos (ωt) came out.
dt

But isn't it beautiful operation ?

sptt