When is the potential difference across a capacitor an emf?
Only when the current being drawn from or put into the capacitor is zero. Capacitors, like batteries, have internal resistance, so their output voltage is not an emf unless current is zero. This is difficult to measure in practice so we refer to a capacitor’s voltage rather than its emf. But the source of potential difference in a capacitor is fundamental and it is an emf.
Phet explorations: circuit construction kit (dc only)
An electronics kit in your computer! Build circuits with resistors, light bulbs, batteries, and switches. Take measurements with the realistic ammeter and voltmeter. View the circuit as a schematic diagram, or switch to a life-like view.
Section summary
An
$\text{RC}$ circuit is one that has both a resistor and a capacitor.
The time constant
$\tau $ for an
$\text{RC}$ circuit is
$\tau =\text{RC}$ .
When an initially uncharged (
${V}_{0}=0$ at
$t=0$ ) capacitor in series with a resistor is charged by a DC voltage source, the voltage rises, asymptotically approaching the emf of the voltage source; as a function of time,
Within the span of each time constant
$\tau $ , the voltage rises by 0.632 of the remaining value, approaching the final voltage asymptotically.
If a capacitor with an initial voltage
${V}_{0}$ is discharged through a resistor starting at
$t=0$ , then its voltage decreases exponentially as given by
In each time constant
$\tau $ , the voltage falls by 0.368 of its remaining initial value, approaching zero asymptotically.
Conceptual questions
Regarding the units involved in the relationship
$\tau =\text{RC}$ , verify that the units of resistance times capacitance are time, that is,
$\Omega \cdot \mathrm{F}=\mathrm{s}$ .
The
$\text{RC}$ time constant in heart defibrillation is crucial to limiting the time the current flows. If the capacitance in the defibrillation unit is fixed, how would you manipulate resistance in the circuit to adjust the
$\text{RC}$ constant
$\tau $ ? Would an adjustment of the applied voltage also be needed to ensure that the current delivered has an appropriate value?
When making an ECG measurement, it is important to measure voltage variations over small time intervals. The time is limited by the
$\text{RC}$ constant of the circuit—it is not possible to measure time variations shorter than
$\text{RC}$ . How would you manipulate
$R$ and
$C$ in the circuit to allow the necessary measurements?
Draw two graphs of charge versus time on a capacitor. Draw one for charging an initially uncharged capacitor in series with a resistor, as in the circuit in
[link] , starting from
$\text{t}=0$ . Draw the other for discharging a capacitor through a resistor, as in the circuit in
[link] , starting at
$\text{t}=0$ , with an initial charge
${Q}_{0}$ . Show at least two intervals of
$\tau $ .
When charging a capacitor, as discussed in conjunction with
[link] , how long does it take for the voltage on the capacitor to reach emf? Is this a problem?
When discharging a capacitor, as discussed in conjunction with
[link] , how long does it take for the voltage on the capacitor to reach zero? Is this a problem?
Referring to
[link] , draw a graph of potential difference across the resistor versus time, showing at least two intervals of
$\tau $ . Also draw a graph of current versus time for this situation.
the basic formula is uncertainty in momentum multiplied buy uncertainty In position is greater than or equal to 4×pi/2. same formula for energy and time
Luke
I have this one question can you please look it up it's 9702/22/O/N/17
Question 1 B 3
Is there a formula for time of free fall given that the body has initial velocity? In other words, formula for time that takes a downward-shot projectile to hit the ground. Thanks!
Formula for for the falling body with initial velocity is:v^2=v(initial)^2+2*g*h
Mateo
i can't understand
Maxamed
we can't do this calculation without knowing the height of the initial position of the particle
Chathu
sorry but no more in science
Imoreh
2 forces whose resultant is 100N, are at right angle to each other .if one of them makes an angle of 30 degree with the resultant determine it's magnitude
The abacus (plural abaci or abacuses), also called a counting frame, is a calculating tool that was in use in Europe, China and Russia, centuries before the adoption of the written Hindu–Arabic numeral system
a load of 20N on a wire of cross sectional area 8×10^-7m produces an extension of 10.4m. calculate the young modules of the material of the wire is of length 5m
Young's modulus = stress/strain
strain = extension/length (x/l)
stress = force/area (F/A)
stress/strain is F l/A x
El
so solve it
Ebenezer
please
Ebenezer
two bodies x and y start from rest and move with uniform acceleration of a and 4a respectively. if the bodies cover the same distance in terms of tx and ty what is the ratio of tx to ty
The atoms which form the element Cesium are known as Cesium atoms.
Naman
A material that combines with and removes trace gases from vacuum tubes.
Shankar
what is difference between entropy and heat capacity
Varun
Heat capacity can be defined as the amount of thermal energy required to warm the sample by 1°C. entropy is the disorder of the system. heat capacity is high when the disorder is high.
The quantum realm, also called the quantum scale, is a term of art inphysics referring to scales where quantum mechanical effects become important when studied as an isolated system. Typically, this means distances of 100 nanometers (10−9meters) or less or at very low temperature.
i want know physics practically where used in daily life
Vinodhini
I want to teach physics very interesting to studentd
Vinodhini
how can you build interest in physics
Prince
by reading it
Austin
understanding difficult
Vinodhini
vinodhini mam, physics is used in our day to day life in all events..... everything happening around us can be explained in the base of physics.....
saying simple stories happening in our daily life and relating it to physics and questioning students about how or why its happening like that can make
revolutionary
your class more interesting
revolutionary
anything send about physics daily life
Vinodhini
How to understand easily
Vinodhini
check out "LMES" youtube channel
revolutionary
even when you see this message in your phone...it works accord to a physics principle. you touch screen works based on physics, your internet works based on physics, etc....... check out google and search for it
revolutionary
what is mean by Newtonian principle of Relativity?
definition and explanation with example