College Physics Lab
PH 144
Ohm's Law
Introduction:
A battery is an electrical device whose terminals maintain a potential difference. If the terminals are made to be
connected electrically then an electric current will flow. The flow of current generates heat, thus the loss of energy is
an indication that current is flowing. In order for a current to flow, the current requires a closed electrical path
between the terminals of the battery. The potential difference of the battery "pumps" the current through the circuit.
Resistors are electrical circuit components that resist the flow of current. Because of their connection to the battery,
the ends of a resistor are at different potentials. This potential difference "pumps" the flow of charge (current)
through the resistor. Current is generally considered to be the flow of positive charge. The motion of positive charge
is from higher potential to lower potential. Therefore as current flows through a resistor it moves to progressively
lower potential.
Ohm's Law expresses an empirical law describing the relationship between the current (I) flowing through a resistor
and the potential drop (V) across the resistor. This relationship is found to be valid for many, but not all electrical
devices.
= constant
When V is measured in volts, and I is measured in amps (coulomb/sec), then the constant, called the resistance (R),
is measured in units of ohms. A device that follows this relationship is called an ohmic resistor.
Circuit Diagrams and Circuit Elements:
An electrical circuit may include various components in a
variety of configurations. Initially we are concerned only with
simple circuits involving a battery and a component, either a
resistor or a light bulb, which provides resistance to the flow
of current. These components in some combination represent
the circuit. In addition two other electrical devices, the
ammeter and the voltmeter, are often inserted into the circuit.
Ideally, and under many real circumstances these devices, and
when inserted properly into the circuit, do not alter the
current flowing in the circuit; rather, they provide
measurements of current and voltage, respectively, at various
points of the circuit.
The standard symbols for these circuit components and devices are illustrated in Figure 1:
Circuit components are placed in contact using electrical connections, which are often but not always wires. Lines
represent electrical connections.
Use of an ammeter:
An ammeter measures the value of the current which passes through it. If you want to measure the current at some
point in a circuit, you must insert the ammeter into the point at which you wish to know the current.
Use of an voltmeter:
A voltmeter measures the value of the potential difference(change in potential) between two points of a circuit. In
order to use a voltmeter, you must connect the voltmeter leads (wires) at either end of the region of circuit across
which you wish to determine the voltage.
Materials:
battery resistors
wires light bulb
ammeter
voltmeter
Investigations:
I. Voltage and current for an ohmic resistor:
Procedure:
1.
Select a resistor whose resistance is between about 500 Ohms and 1000 Ohms.
Make a closed circuit from your battery and the resistor. See Figure A.
Figure A
Figure B
2.
Introduce the ammeter (in series) so that it measures the current flowing through
the resistor. See Figure B.
Introduce the voltmeter so that it measures the potential difference across the
resistor. See Figure B.
3.
Set the battery output voltage to zero. Slowly turn up the battery voltage up until
the ammeter needle deflects close to its maximum value. For this experiment do not turn
the battery voltage up higher than this present setting.
4.
For several different battery voltages (mindful of the last statement in #2!),
measure the current that passes through the resistor (the ammeter value) and the potential
difference across the resistor (the voltmeter value).
5.
Analysis:
Make a graph of your results. Represent your data by a "best fit" line if possible.
Is your data in agreement with Ohm's Law? Express agreement as a % error.
II. Repeat the steps 1-5 for a resistor having a different resistance, about half as large.
III. Repeat the steps 1-5 for a light bulb. How are light bulbs different from the resistors?