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A long, inexpensive extension cord is connected from inside the house to a refrigerator outside. The refrigerator doesn’t run as it should. What might be the problem?

In [link] , does the graph indicate the time constant is shorter for discharging than for charging? Would you expect ionized gas to have low resistance? How would you adjust R size 12{R} {} to get a longer time between flashes? Would adjusting R size 12{R} {} affect the discharge time?

An electronic apparatus may have large capacitors at high voltage in the power supply section, presenting a shock hazard even when the apparatus is switched off. A “bleeder resistor” is therefore placed across such a capacitor, as shown schematically in [link] , to bleed the charge from it after the apparatus is off. Why must the bleeder resistance be much greater than the effective resistance of the rest of the circuit? How does this affect the time constant for discharging the capacitor?

An electrical circuit with a capacitor has an extra resistor R sub b l, called a bleeder, installed in parallel with the capacitor.
A bleeder resistor R bl size 12{R rSub { size 8{"bl"} } } {} discharges the capacitor in this electronic device once it is switched off.

Problem exercises

The timing device in an automobile’s intermittent wiper system is based on an RC size 12{ ital "RC"} {} time constant and utilizes a 0 . 500-μF size 12{0 "." "500-"μF} {} capacitor and a variable resistor. Over what range must R size 12{R} {} be made to vary to achieve time constants from 2.00 to 15.0 s?

range 4 . 00 to 30 . 0 M Ω size 12{"range 4" "." "00 to 30" "." "0 M" %OMEGA } {}

A heart pacemaker fires 72 times a minute, each time a 25.0-nF capacitor is charged (by a battery in series with a resistor) to 0.632 of its full voltage. What is the value of the resistance?

The duration of a photographic flash is related to an RC size 12{ ital "RC"} {} time constant, which is 0 . 100 μs size 12{0 "." "100" μs} {} for a certain camera. (a) If the resistance of the flash lamp is 0 . 0400 Ω size 12{0 "." "0400" %OMEGA } {} during discharge, what is the size of the capacitor supplying its energy? (b) What is the time constant for charging the capacitor, if the charging resistance is 800 size 12{"800"" k" %OMEGA } {} ?

(a) 2 . 50 μF size 12{2 "." "50 "mF} {}

(b) 2.00 s

A 2.00- and a 7 . 50-μF size 12{7 "." "50"-mF} {} capacitor can be connected in series or parallel, as can a 25.0- and a 100-kΩ size 12{"100""-k" %OMEGA } {} resistor. Calculate the four RC size 12{ ital "RC"} {} time constants possible from connecting the resulting capacitance and resistance in series.

After two time constants, what percentage of the final voltage, emf, is on an initially uncharged capacitor C size 12{C} {} , charged through a resistance R size 12{R} {} ?

86.5%

A 500-Ω size 12{"500"- %OMEGA } {} resistor, an uncharged 1 . 50-μF size 12{1 "." "50"-mF} {} capacitor, and a 6.16-V emf are connected in series. (a) What is the initial current? (b) What is the RC size 12{ ital "RC"} {} time constant? (c) What is the current after one time constant? (d) What is the voltage on the capacitor after one time constant?

A heart defibrillator being used on a patient has an RC size 12{ ital "RC"} {} time constant of 10.0 ms due to the resistance of the patient and the capacitance of the defibrillator. (a) If the defibrillator has an 8 . 00-μF size 12{8 "." "00"-mF} {} capacitance, what is the resistance of the path through the patient? (You may neglect the capacitance of the patient and the resistance of the defibrillator.) (b) If the initial voltage is 12.0 kV, how long does it take to decline to 6.00 × 10 2 V ?

(a) 1 . 25 k Ω size 12{1 "." "25 k" %OMEGA } {}

(b) 30.0 ms

An ECG monitor must have an RC size 12{ ital "RC"} {} time constant less than 1.00 × 10 2 μs size 12{"100" ms} {} to be able to measure variations in voltage over small time intervals. (a) If the resistance of the circuit (due mostly to that of the patient’s chest) is 1 . 00 kΩ size 12{1 "." 00" k" %OMEGA } {} , what is the maximum capacitance of the circuit? (b) Would it be difficult in practice to limit the capacitance to less than the value found in (a)?

[link] shows how a bleeder resistor is used to discharge a capacitor after an electronic device is shut off, allowing a person to work on the electronics with less risk of shock. (a) What is the time constant? (b) How long will it take to reduce the voltage on the capacitor to 0.250% (5% of 5%) of its full value once discharge begins? (c) If the capacitor is charged to a voltage V 0 size 12{V rSub { size 8{0} } } {} through a 100-Ω size 12{"100"- %OMEGA } {} resistance, calculate the time it takes to rise to 0 . 865 V 0 size 12{0 "." "865"`V rSub { size 8{0} } } {} (This is about two time constants.)

A parallel circuit with a switch, an embedded electronic circuit, a capacitor, and a resistor is shown. The embedded circuit, capacitor, and resistor are connected in parallel with each other: the electronic circuit on the left, the capacitor in the middle, and the resistor on the right. The capacitor has a capacitance of eighty micro farads. The resistor has a resistance of two hundred fifty kilohms. The switch is on the top, between the electronic circuit and the capacitor leg.

(a) 20.0 s

(b) 120 s

(c) 16.0 ms

Using the exact exponential treatment, find how much time is required to discharge a 250-μF size 12{"250"-mF} {} capacitor through a 500-Ω size 12{"500"- %OMEGA } {} resistor down to 1.00% of its original voltage.

Using the exact exponential treatment, find how much time is required to charge an initially uncharged 100-pF capacitor through a 75 . 0 -M Ω size 12{"75" "." 0"-M" %OMEGA } {} resistor to 90.0% of its final voltage.

1 . 73 × 10 2 s size 12{1 "." "73" times "10" rSup { size 8{ - 2} } " s"} {}

Integrated Concepts

If you wish to take a picture of a bullet traveling at 500 m/s, then a very brief flash of light produced by an RC size 12{ ital "RC"} {} discharge through a flash tube can limit blurring. Assuming 1.00 mm of motion during one RC size 12{ ital "RC"} {} constant is acceptable, and given that the flash is driven by a 600-μF size 12{"600"-mF} {} capacitor, what is the resistance in the flash tube?

3 . 33 × 10 3 Ω size 12{3 "." "33"´"10" rSup { size 8{-3} } %OMEGA } {}

Integrated Concepts

A flashing lamp in a Christmas earring is based on an RC size 12{ ital "RC"} {} discharge of a capacitor through its resistance. The effective duration of the flash is 0.250 s, during which it produces an average 0.500 W from an average 3.00 V. (a) What energy does it dissipate? (b) How much charge moves through the lamp? (c) Find the capacitance. (d) What is the resistance of the lamp?

Integrated Concepts

A 160-μF size 12{"160"-mF} {} capacitor charged to 450 V is discharged through a 31 . 2-k Ω size 12{31 "." "2-k" %OMEGA } {} resistor. (a) Find the time constant. (b) Calculate the temperature increase of the resistor, given that its mass is 2.50 g and its specific heat is 1 . 67 kJ kg ºC size 12{1 "." "67" { {"kJ"} over {"kg" cdot "deg"C} } } {} , noting that most of the thermal energy is retained in the short time of the discharge. (c) Calculate the new resistance, assuming it is pure carbon. (d) Does this change in resistance seem significant?

(a) 4.99 s

(b) 3 . 87ºC size 12{3 "." "87"°C} {}

(c) 31 . 1 k Ω size 12{"31" "." "1 k" %OMEGA } {}

(d) No

Unreasonable Results

(a) Calculate the capacitance needed to get an RC size 12{ ital "RC"} {} time constant of 1.00 × 10 3 s with a 0 . 100-Ω size 12{0 "." "100"- %OMEGA } {} resistor. (b) What is unreasonable about this result? (c) Which assumptions are responsible?

Construct Your Own Problem

Consider a camera’s flash unit. Construct a problem in which you calculate the size of the capacitor that stores energy for the flash lamp. Among the things to be considered are the voltage applied to the capacitor, the energy needed in the flash and the associated charge needed on the capacitor, the resistance of the flash lamp during discharge, and the desired RC size 12{ ital "RC"} {} time constant.

Construct Your Own Problem

Consider a rechargeable lithium cell that is to be used to power a camcorder. Construct a problem in which you calculate the internal resistance of the cell during normal operation. Also, calculate the minimum voltage output of a battery charger to be used to recharge your lithium cell. Among the things to be considered are the emf and useful terminal voltage of a lithium cell and the current it should be able to supply to a camcorder.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Berger describes sociologists as concerned with
Mueller Reply
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Source:  OpenStax, College physics ii. OpenStax CNX. Nov 29, 2012 Download for free at http://legacy.cnx.org/content/col11458/1.2
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