DFV10113-CHPTR 3

INTERNAL RESISTANCE & EMF A

A

B

INTERNAL RESISTANCE OF BATTERY

1 Real batteries are constructed

from materials which possess non-
zero resistivities. They also possess
internal resistances, r. Incidentally,
a pure voltage source is usually
referred to as an emf (which stands

for electromotive force), ε
measured in units of volts.

INTERNAL RESISTANCE OF BATTERY

2
Suppose that such a battery is used
to drive a current, I through an
external load resistor, R

To measure the internal resistance,r of = +
a battery, but it can be calculated from
current and voltage data measured = -R
from a circuit. When a load is applied
to a battery the internal resistance can
be calculated from the following
equation:

EXAMPLE 1

Suppose we have a car battery with an emf
= 13.8 V, under a resistive load of 20 Ω ,the
voltage sags to 11.8 V . What is the
battery's resistance?

= The car’s battery is in series
11.8 = (20) with the load so the current
= 0.58 A is the SAME throughout the
= − + circuit.
11.8 = − (? ) + 13.8
= 3.45 Ω

INTERNAL RESISTANCE OF
MEASURING EQUIPMENT

1 To determine the internal
resistance of an instrument it is

usually only necessary to vary a

single component of an exterior

connected circuit. Enough

measurements are available 2 In this case, the oscilloscope
throughout the exterior circuit to has an internal resistance of
provide information for basic Ri. Therefore the voltage
circuit analysis techniques to across the open circuit Vi,
calculate the internal properties of when measured will be lower
an instrument. than expected.

EXAMPLE 2

If Vps is set to 8V and Rv is 1MΩ, and Vi is
measured at 1mV. Calculate the internal
resistance of the oscilloscope

= ( + ), ∴ = +

8 = 1 + 1 ,
7.999
= (1 ) = 7.999 0−6

= 1 = 0.125
= 7.999 0−6

UNIT’S SUMMARIZED

1  Electricity is a flow of electrical power or charge
2  Conversion of chemical energy in fuels or of wind, water, or

steam into electrical energy generates electricity

3  Explain the level of conductivity for different type of materials such
as conductors, insulators, semiconductors .

4  Electronics is the study of the flow of charge through various materials
and devices such as semiconductors, resistors, inductors, capacitors,
nano-structures and vaccum tubes

5  An electronic system usually consists of input devices, transducers, signal
processors and output devices.

6  Atoms have protons and electrons. They both carry an electrical charge.
An electrical charge is a force within the particle. Protons have a positive
charge (+) and electrons have a negative charge (-). The positive charge
of the protons is equal to the negative charge of the electrons. Opposite
charges attract each other.

UNIT’S SUMMARIZED

7  There are two types of magnetic poles, called North and South poles.
Like poles repel, and opposite poles attract.

8  The strength of magnetic fields is measured in units of Teslas (T).
9  Galvanometer, electric motor, generator are electromagnet

devices that interacts with a permanent magnet.

AS A CONCLUSION

√  Current is directly proportional to voltage
and is inversely proportional to
resistance.

√  Voltage is directly proportional to the
current and resistance.

√  A battery and measuring equipment have
internal resistance.

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