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Resonance in AC circuits is analogous to mechanical resonance, where resonance is defined to be a forced oscillation—in this case, forced by the voltage source—at the natural frequency of the system. The receiver in a radio is an RLC circuit that oscillates best at its f 0 size 12{f rSub { size 8{0} } } {} . A variable capacitor is often used to adjust f 0 size 12{f rSub { size 8{0} } } {} to receive a desired frequency and to reject others. [link] is a graph of current as a function of frequency, illustrating a resonant peak in I rms size 12{I rSub { size 8{"rms"} } } {} at f 0 size 12{f rSub { size 8{0} } } {} . The two curves are for two different circuits, which differ only in the amount of resistance in them. The peak is lower and broader for the higher-resistance circuit. Thus the higher-resistance circuit does not resonate as strongly and would not be as selective in a radio receiver, for example.

The figure describes a graph of current I versus frequency f. Current I r m s is plotted along Y axis and frequency f is plotted along X axis. Two curves are shown. The upper curve is for small resistance and lower curve is for large resistance. Both the curves have a smooth rise and a fall. The peaks are marked for frequency f zero. The curve for smaller resistance has a higher value of peak than the curve for large resistance.
A graph of current versus frequency for two RLC series circuits differing only in the amount of resistance. Both have a resonance at f 0 size 12{f rSub { size 8{0} } } {} , but that for the higher resistance is lower and broader. The driving AC voltage source has a fixed amplitude V 0 size 12{V rSub { size 8{0} } } {} .

Calculating resonant frequency and current

For the same RLC series circuit having a 40.0 Ω resistor, a 3.00 mH inductor, and a 5.00 μF capacitor: (a) Find the resonant frequency. (b) Calculate I rms size 12{I rSub { size 8{"rms"} } } {} at resonance if V rms size 12{V rSub { size 8{"rms"} } } {} is 120 V.


The resonant frequency is found by using the expression in f 0 = 1 LC size 12{f rSub { size 8{0} } = { {1} over {2π sqrt { ital "LC"} } } } {} . The current at that frequency is the same as if the resistor alone were in the circuit.

Solution for (a)

Entering the given values for L and C into the expression given for f 0 size 12{f rSub { size 8{0} } } {} in f 0 = 1 LC size 12{f rSub { size 8{0} } = { {1} over {2π sqrt { ital "LC"} } } } {} yields

f 0 = 1 LC = 1 ( 3 . 00 × 10 3 H ) ( 5 . 00 × 10 6 F ) = 1 . 30 kHz . alignl { stack { size 12{f rSub { size 8{0} } = { {1} over {2π sqrt { ital "LC"} } } } {} #" "= { {1} over {2π sqrt { \( 3 "." "00" times "10" rSup { size 8{ - 3} } " H" \) \( 5 "." "00" times "10" rSup { size 8{ - 6} } " F" \) } } } =1 "." "30"" kHz" {} } } {}

Discussion for (a)

We see that the resonant frequency is between 60.0 Hz and 10.0 kHz, the two frequencies chosen in earlier examples. This was to be expected, since the capacitor dominated at the low frequency and the inductor dominated at the high frequency. Their effects are the same at this intermediate frequency.

Solution for (b)

The current is given by Ohm’s law. At resonance, the two reactances are equal and cancel, so that the impedance equals the resistance alone. Thus,

I rms = V rms Z = 120 V 40 . 0 Ω = 3 . 00 A. size 12{I rSub { size 8{"rms"} } = { {V rSub { size 8{"rms"} } } over {Z} } = { {"120"" V"} over {"40" "." "0 " %OMEGA } } =3 "." "00"" A"} {}

Discussion for (b)

At resonance, the current is greater than at the higher and lower frequencies considered for the same circuit in the preceding example.

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Power in RLC Series ac circuits

If current varies with frequency in an RLC circuit, then the power delivered to it also varies with frequency. But the average power is not simply current times voltage, as it is in purely resistive circuits. As was seen in [link] , voltage and current are out of phase in an RLC circuit. There is a phase angle     ϕ size 12{ϕ} {} between the source voltage V size 12{V} {} and the current I size 12{I} {} , which can be found from

cos ϕ = R Z . size 12{"cos"ϕ= { {R} over {Z} } } {}

For example, at the resonant frequency or in a purely resistive circuit Z = R size 12{Z=R} {} , so that cos ϕ = 1 size 12{"cos"ϕ=1} {} . This implies that ϕ = 0 º size 12{ϕ=0 rSup { size 8{ circ } } } {} and that voltage and current are in phase, as expected for resistors. At other frequencies, average power is less than at resonance. This is both because voltage and current are out of phase and because I rms size 12{I rSub { size 8{"rms"} } } {} is lower. The fact that source voltage and current are out of phase affects the power delivered to the circuit. It can be shown that the average power is

Questions & Answers

Mass of air bubble in material medium is negative. why?
Hrithik Reply
a car move 6m. what is the acceleration?
Umaru Reply
depends how long
What is the simplest explanation on the difference of principle, law and a theory
Kym Reply
how did the value of gravitational constant came give me the explanation
Varun Reply
how did the value of gravitational constant 6.67×10°-11Nm2kg-2
A steel ball is dropped onto a hard floor from a height of 1.50 m and rebounds to a height of 1.45 m. (a) Calculate its velocity just before it strikes the floor.
Kris Reply
0.5m* mate.
0.05 I meant.
Guess your solution is correct considering the ball fall from 1.5m height initially.
How can we compare different combinations of capacitors?
Prakash Reply
find the dimension of acceleration if it's unit is ms-2
Happiness Reply
b=-2 ,a =1
M^0 L^1T^-2
what is botany
it is a branch of science which deal with the study of plants animals and environment
what is work
Sunday Reply
a boy moving with an initial velocity of 2m\s and finally canes to rest with a velocity of 3m\s square at times 10se calculate it acceleration
6.6 lol 😁😁
show ur work
sorry..the answer is -10
your question is wrong
If the boy is coming to rest then how the hell will his final velocity be 3 it'll be zero
re-write the question
men i -10 isn't correct.
using v=u + at
ya..1/10 is very correct..
how did the value 6.67×10°-11Nm2kg2 came tell me please
Work is the product of force and distance
what is longitudinal wave
Badmus Reply
A longitudinal wave is wave which moves parallel or along the direction of propagation.
longitudinal wave in liquid is square root of bulk of modulus by density of liquid
Is British mathematical units the same as the United States units?(like inches, cm, ext.)
Nina Reply
We use SI units: kg, m etc but the US sometimes refer to inches etc as British units even though we no longer use them.
Thanks, just what I needed to know.
What is the advantage of a diffraction grating over a double slit in dispersing light into a spectrum?
Uditha Reply
can I ask questions?
Boniface Reply
hello guys
when you will ask the question
anybody can ask here
is free energy possible with magnets?
you could construct an aparatus that might have a slightly higher 'energy profit' than energy used, but you would havw to maintain the machine, and most likely keep it in a vacuum, for no air resistance, and cool it, so chances are quite slim.
calculate the force, p, required to just make a 6kg object move along the horizontal surface where the coefficient of friction is 0.25
Yes ask
if a man travel 7km 30degree east of North then 10km east find the resultant displacement
Ajali Reply
disagree. Displacement is the hypotenuse length of the final position to the starting position. Find x,y components of each leg of journey to determine final position, then use final components to calculate the displacement.
1.The giant star Betelgeuse emits radiant energy at a rate of 10exponent4 times greater than our sun, where as it surface temperature is only half (2900k) that of our sun. Estimate the radius of Betelgeuse assuming e=1, the sun's radius is s=7*10exponent8metres
James Reply
2. A ceramic teapot (e=0.20) and a shiny one (e=0.10), each hold 0.25 l of at 95degrees. A. Estimate the temperature rate of heat loss from each B. Estimate the temperature drop after 30mins for each. Consider only radiation and assume the surrounding at 20degrees
Is our blood not red
Aditya Reply
If yes than why when a beam of light is passing through our skin our skin is glowing in red colour
because in our blood veins more red colour is scattered due to low wavelength also because of that scattered portion comes on skin and our skin act as a thinscreen.
so you saying blood is not red?
blood is red that's why it is scattering red colour!
like if u pass light frm red colour solution then it will scatter red colour only.. so same it is with our skin..red colour blood is moving inside the veins bcz of thinkness of our fingers.. it appears to be red.
No I am not saying that blood is not red
then ur qtn is wrong buddy.. 😊
Blood is red. The reason our veins look blue under our skin, is because thats the only wavelength on light that can penetrate our skin.
Red light is reflected from our blood but because of its wavelength it is not seen. While in the other hand blue light has a longer wavelength allowing it to pass the our skin and to our eyes.
Thus, our veins appear blue while they are really red... THE MORE YOU KNOW...(;
So in conclusion our blood is red but we can only see blue spectrum because of our skin. The more longer a wavelength is the more durable it is to reflection, so blue light cant pass thew skin completely causing a reflection which causes veins to appear blue. While the red light is scatter around.
the reason why when we shine a light at our skin it appears red is because the red light is increased and more goes to your eyes. So in other words it increases the amount of red light vs it being scatterd around everywhere.
I think the blood is only a mixture of colors but red is predominant due to high level of haemoglobin.
As a side note, the heme part of hemoglobin is why blood is red
Practice Key Terms 4

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