REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS
Voltages and Resistances in a Closed Electric Circuit
Figure A.3 shows three resistances connected in series with a battery source of 48 V. We
define a closed circuit as one in which current flows; an open circuit is a circuit in which
no current flows. Figure A.3a is a closed circuit (i.e., the switch is in the closed position)
and A.3b is an open circuit (i.e., the switch is in the open position).
Emf is measured with a voltmeter, as shown in Figure A.4, where the open circuit
voltage (emf) of a battery is measured. A voltmeter is also used to measure potential drop
as shown in Figure A.3a. Here the voltmeter is connected across
z). In later discussion we will also call this the IR drop. Remember Ohm's law: E = IR.
Regarding the potential drop across
, if we know the resistance of
and we know
the current passing through it, we know the potential or IR drop. This is one of the many
applications of Ohm's law.
In Figure A.3a, if we measure the potential drop (IR drop) in volts across each resis-
tance and sum the values, this will equal the value of the battery emf in the closed
Example of Series Resistances. Four resistors are connected in series. Their resistance
values are 250, 375, 136, and 741
. Suppose we were to replace these four with just one
resistor. The current through the circuit will be the same for one or for the four resistors
in series. What will be the value in ohms of the single replacement resistor?
A closed circuit showing three resistances in series with a battery as an emf source. A
voltmeter is placed across R
, which measures the potential difference across that resistance.
An open circuit with three resistances in series with emf source.