Capacitance
Electricity and Magnetism

Episode 129: Discharge of a capacitor

Lesson for 16-19 IOP TAP

Students will have already seen that the discharge is not a steady process in episode 125, but it is useful to have graphical evidence before discussing the theory.

You need to build up your students’ understanding of exponential processes, through experiments, and through graphical and algebraic approaches, all related to the underlying physical processes involved. For the more mathematically able students, you may even be able to use calculus.

This episode is a long one, and may spread over several teaching sessions.

A spreadsheet is included as part of the student questions for activity 129-7.

Lesson Summary

  • Student experiment: Exponential discharge (30 minutes)
  • Discussion: Characteristics of exponentials (20 minutes)
  • Student activity: Spreadsheet model (20 minutes)
  • Student experiment: Varying R and C (30 minutes)
  • Discussion: Deriving exponential equations (30 minutes)
  • Worked example: Using the equations (20 minutes)
  • Discussion: Time constant (15 minutes)
  • Student activity: Analysing graphs (15 minutes)
  • Student questions: Practice with equations (30 minutes)
  • Discussion: Back to reality (10 minutes)

Student experiment: Exponential discharge

The suggestion to look for a pattern by measuring halving times is worth pursuing. It forms a basis for further discussion, and shows that the patterns for current and voltage are similar. (They can be related to the idea of exponential decay in radioactivity, episode 513, if students have met this previously.)

Even though some specifications require the use of data logging for this, it is worth collecting data manually from a slow discharge and then getting the students to plot the graphs of current against time and voltage against time for the decay.

The specifications do not require details of the charging process but data for this is easily collected in the same experiment.

Episode 129-1: Slow charge and discharge (Word, 31 KB)


Discussion: Characteristics of exponentials

Draw out the essential features of the discharge graphs. Sketch three graphs, for Q, I and V against t. All start at a point on the y-axis, and are asymptotic on the t-axis. All have the same general shape. How are they related?

The Q graph is simply the V graph multiplied by C (since Q = C × V ).

The I graph is the V graph divided by R (since I = VR ).

The I graph is also the gradient of the Q graph (since I =  d Q d t ).

Add small tangents along the Q graph to show this latter pattern. A large charge stored means that there is a large pd across the capacitor; this makes a large current flow, so the charge decreases rapidly. When the charge is smaller, the pd must be lower and so a smaller current flows. (Students should see that this will result in quantities which get gradually smaller and smaller, but which never reach zero.)

Student activity: Spreadsheet model

Students can use an iterative approach, with the help of a spreadsheet, to see the pattern of potential difference across the capacitor while it is discharging (top graph), and charging (bottom graph).

Episode 129-2: One step at a time (Word, 33 KB)


Student experiment: Varying R and C

The previous experiment produced graphs of the discharge for a particular combination of resistor and capacitor. This can be extended by looking at the decay for a range of values of C and R. If a datalogger is available, this can be done quickly and can include some rapid decays. If a datalogger is not available, measurements can be taken with the apparatus used earlier in episode 129-1.

Before the experiment, ask your students how the graphs would be affected if the value of R was increased (for a particular value of pd the current will be less, and the decays will be slower); and if the value of C was increased (more charge stored for a given pd; the initial current will be the same, but the decay will be slower, because it will take longer for the greater quantity of charge to flow away.)

Episode 129-3: Experiment analyzing the discharge of a capacitor (Word, 41 KB)


Discussion: Deriving exponential equations

At this point, you have a choice:

  • you can jump directly to the exponential equations and show that they produce the correct graphs
  • alternatively, you can work through the derivation of the equations, starting from the underlying physics

We will follow the second approach.

To explain the pattern seen in the previous experiment you will have to lead your pupils carefully through an argument which will call on ideas about capacitors and about electrical circuits.

Consider the circuit shown:

When the switch is in position A, the capacitor C gains a charge Q0 so that the pd across the capacitor V0 equals the battery emf.

When the switch is moved to position B, the discharge process begins. Suppose that at a time t, the charge has fallen to Q, the pd is V and there is a current I flowing as shown. At this moment:

I = VR (equation 1)

In a short time Δ t, a charge equal to Δ Q flows from one plate to the other so:

I = - Δ Q Δ t (equation 2)

(with the minus sign showing that the charge on the capacitor has become smaller)

For the capacitor:

V = QC (equation 3)

Eliminating I and V leads to:

Δ Q = -QCR  ×  Δ t (equation 4)

Equation 4 is a recipe for describing how any capacitor will discharge based on the simple physics of equations 1 – 3. As in the activity above, it can be used in a spreadsheet to calculate how the charge, pd and current change during the capacitor discharge.

Equation 4 can be re-arranged as:

Δ QQ = 1CR

(Showing the constant ratio property characteristic of an exponential change i.e. equal intervals of time give equal fractional changes in charge.)

We can write equation 4 as a differential equation: 

d Q d t = -  1CR

Solving this gives:

Q = Q0e- t/_CR_

where Q0 = C × V0

Current and voltage follow the same pattern. From equations 2 and 3 it follows that

I = I0e- t/_CR_

where I0 = V0R

and V = V0e- t/_CR_

Worked examples: Using the equations

A 200 mF capacitor is charged to 10 V and then discharged through a 250 kW resistor. Calculate the pd across the capacitor at intervals of 10 s.

(The values here have been chosen to give a time constant of 50 s.)

First calculate CR, which is 50 s

Draw up a table and help students to complete it. (Some students will need help with using the xy function on their calculators.) They can then draw a V-t graph.

t / s 0 10 20 30 40 50 60etc
V / V 10 8.2 6.7 5.5 etc
I / mA 40
Q / mC

Explain how to calculate I0 = V0R and Q0 = C × V0 , so that they can complete the last two rows in the table.

It is useful to draw a Q-t graph and deduce the gradient at various points. These values can then be compared with the corresponding instantaneous current values.

Similarly, the area under the I-t graph can be found (by counting squares) and compared with the values of charge Q.

Discussion: Time constant

For radioactive decay, the half life is a useful concept. A quantity known as the time constant is commonly used in a similar way when dealing with capacitor discharge.

Consider: Q = Q0e- t/_CR_

When t = CR , we have Q = Q0e-1

(i.e. this is the time when the charge has fallen to 1e = 0.37 (about ⅓ ) of its initial value. CR is known as the time constant – the larger it is, the longer the capacitor will take to discharge.)

The units of the time constant are seconds. Why? ( F  ×  W = C V-1  ×  V A-1, which simplifies to C A-1, and then again to C C-1 s, so just s)

(Your specifications may require the relationship between the time constant and the halving time T ½ :

T ½  = ln(2) × CR

or,

T ½  = 0.69 × CR)

Student activity: Analysing graphs

Students should look through their experimental results and determine the time constant from a discharge graph. They should check whether the experimental value is equal to the calculated value CR. Why might it not be? (Because the manufacturer’s values of R and C, are only given to a specified range, or tolerance, and that range is rather large for C.)

Student questions: Practice with equations

Questions on capacitor discharge and the time constant, including a further opportunity to model the discharge process using a spreadsheet.

Episode 129-4: Discharge and time constants (Word, 31 KB)


Episode 129-5: Discharging a capacitor (Word, 56 KB)


Episode 129-6: Capacitors with the exponential equation (Word, 30 KB)


Episode 129-7: Discharge of high-value capacitors (Word, 69 KB)


Episode 129-8: Spreadsheet for 129-7 (Word, 54 KB)


Discussion: Back to reality

After a lot of maths, there is a danger that students will lose sight of the fact that capacitors are common components with a wide range of uses.

Some of these can now be explained more thoroughly than in the initial introduction. Ask students to consider whether large or small values of C and R are appropriate in each case. Some examples are:

Back-up power supplies in computers, watches etc, where a relatively large capacitor (often > 1 F) charged to a low voltage may be used.

Some physics experiments need very high currents delivered for a very short time (e.g. inertial fusion). A bank of capacitors can be charged over a period of time but discharged in a fraction of a second when required.

Similarly, the rapid tranfer of energy needed for a flash bulb in a camera often involves capacitor discharge. Try dismantling a disposable camera to see the capacitor.

Capacitance
appears in the relation C=Q/V
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