The main part of this
lesson focused on practical activity. According to the lesson plan in a
practical task resistors used with 1Ohm, 10 Ohm and 100Ohm. However these
values might be changed if in a physics lab you can find some others.
Conclusions to their observations
students do by themselves without teachers help and verify their answer by
watching the video. The important point is teacher should tell students which
part of the video they can watch. It is important to give time to discuss in
small groups and get the answer to the questions.
For teachers might be useful the
subscription of the video:
Let’s say you take a 1.5V of a
battery and directly connected to the voltmeter it reads 1.5V. Now lets say
that you connected this battery to the resistor of resistance 100Ohm. The
voltage measured by the voltmeter might be 1.49V, the voltage will decrease.
Now if you decrease the resistance to the 10Ohm, than more current will flow.
And as you decrease the resistor the terminal voltage is going to decreases,
and it would be 1.42Ohm. If you decrease the resistance to 1Ohm, voltage
reading will be less, may be 1.25V. Why this happen? If we decrease the load
resistance across the battery, the voltage across that resistance is decreases.
This has to do with the internal resistance of the battery. Every battery has
an internal resistance..
presentation teacher have to go through the slides and explain again the role
and effect of the internal resistor in voltmeter readings.
practical task: THE CHARACTERISTICS OF A SOURCE OF EMF
illustrate the behavior of a typical seat of electromotive force, emf, with
respect to: (a) the dependence of terminal voltage on internal resistance and
current (b) the condition for maximum power output.
Seat of emf with internal resistance (two 1.5V D cells with an “internal”
resistor of 30 to 40 ohms, or one dead D cell), external load consisting of a
dial-box resistor (1-10,000 ohms), 100 milliampere meter, 5v voltmeter
momentary contact switch, and 6 leads.
internal resistance of a good storage cell is too low to illustrate the point
of this experiment. Hence it is necessary to connect a resistance in series
with the battery to make a “battery” with a higher internal resistance. The
circuit diagram below shows the effective battery B connected through a switch,
S to the variable resistive load, R. A and V represent a low resistance ammeter
and a high resistance voltmeter. e and r represent the emf and internal
resistance, respectively, of the battery. (This analysis applies equally well
to DC generators.)
I be the current read on A, and let V be the voltage or terminal potential
difference of the battery read on V. Note that V is also the potential
difference across the external load, the resistance of R and A combined. Let P
be the power in units of watts delivered by the battery to the external load.
The fundamental relations for the circuit are then:
ε / (R + r) (1)
the current through the voltmeter has been treated as negligible,
= ε – Ir = I R (2)
the resistance of the ammeter has been treated as negligible, and
P = IV = I2R. (3)
sure you understand these relations. Ask questions. As a rule, the emf ε
and the internal resistance r of a generator are considered to be constant.
using Eq. (1) in Eq. (3), you may show that
= ε2 R / (R +r)2. (4)
that P approaches zero both for very small and for very large values of load
resistance R. Hence the power delivered will be a maximum for some
intermediate load resistance. To find the value of R which results in maximum
power, one differentiates Eq. (4) with respect to R and equates the derivative
to zero. The solution of this relation yields
= r, for P at maximum. (5)
these conditions, the load is said to be “matched” to the seat of emf.
P is now a maximum, the power (I2r) dissipated in the battery or generator is
as great as the power (I2R) delivered to the load. Hence this arrangement is
not ordinarily used with batteries or generators. This principle is however of
fundamental importance in many circuit applications, as in “matching” speakers
to audio amplifiers.
Connect the circuit components, except for one battery lead; see that all
contacts are firm. After the circuit has been checked by the instructor, set R
to about 300 ohms and close S. Tabulate the readings of A and V and P for a
series of decreasing values of R including a column for P. By computing P as
you proceed, the changes in R may be made in suitable steps. These should be
large at first (100 or 200 ohms) but much smaller (5or 10 ohms) as R approaches
r in value. You should change R only 1 or 2 ohms at a step in the vicinity of
Plot the following graphs, each on a full sheet.
V as a function of I.
P as a function of R for values of R up to 100 ohms.
Show that Eq. (4) follows from Eq. (1) and (3), and that Eq. (4) with Eq. (5)
gives Pmax = ε 2/4R.
Determine the experimental values for E and r using the data from the first
graph and equation (2). From the second graph, read Pmax and compare with
ε 2/4r. Read R for Pmax and compare with r.
Show that at maximum power output, the terminal voltage of the battery is half
Why is a dead battery characterized by a high internal resistance?
Explain why batteries of low current capacity have higher internal resistances
than those batteries of high current capacity, e.g., a 12 v flashlight battery
vs. a 12 v car battery.