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.
An
circuit is one that has both a resistor and a capacitor.
The time constant
for an
circuit is
.
When an initially uncharged (
at
) 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
, the voltage rises by 0.632 of the remaining value, approaching the final voltage asymptotically.
If a capacitor with an initial voltage
is discharged through a resistor starting at
, then its voltage decreases exponentially as given by
In each time constant
, the voltage falls by 0.368 of its remaining initial value, approaching zero asymptotically.
Conceptual questions
Regarding the units involved in the relationship
, verify that the units of resistance times capacitance are time, that is,
.
The
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
constant
? 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
constant of the circuit—it is not possible to measure time variations shorter than
. How would you manipulate
and
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
. Draw the other for discharging a capacitor through a resistor, as in the circuit in
[link] , starting at
, with an initial charge
. Show at least two intervals of
.
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
. Also draw a graph of current versus time for this situation.
Step 1: Find the mean. To find the mean, add up all the scores, then divide them by the number of scores. ...
Step 2: Find each score's deviation from the mean. ...
Step 3: Square each deviation from the mean. ...
Step 4: Find the sum of squares. ...
Step 5: Divide the sum of squares by n – 1 or N.
The sample of 16 students is taken. The average age in the sample was 22 years with astandard deviation of 6 years. Construct a 95% confidence interval for the age of the population.
Bhartdarshan' is an internet-based travel agency wherein customer can see videos of the cities they plant to visit. The number of hits daily is a normally distributed random variable with a mean of 10,000 and a standard deviation of 2,400
a. what is the probability of getting more than 12,000 hits?
b. what is the probability of getting fewer than 9,000 hits?
Bhartdarshan'is an internet-based travel agency wherein customer can see videos of the cities they plan to visit. The number of hits daily is a normally distributed random variable with a mean of 10,000 and a standard deviation of 2,400.
a. What is the probability of getting more than 12,000 hits
