La Quinta Physics Lab
Ohm's Law
Introduction:
A battery or dc power supply 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.
V/I = 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 a 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, dc power supply, resistors on vernier circuit board, 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 dc power supply and the resistor. See Figure 2.
2. Introduce the ammeter (in series) so that it
measures the current flowing through the
resistor. See Figure 3.
Introduce the voltmeter so that it measures
the potential difference across the resistor.
See Figure 3.
3. Set the power supply output voltage to
zero. Slowly turn up the voltage up until the ammeter needle
deflects close to a maximum value of 0.5amps. For this
experiment do not turn the battery voltage up higher than this present setting.
4. For several ( at least 10 ) different voltages (mindful of the last statement in #3!),
measure the current that passes through the resistor (the ammeter value) and the potential
difference across the resistor (the voltmeter value).
5. Analysis:
Use an excel spreadsheet to 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?
IV.
Using a 1.5 Volt D cell battery measure the current through and voltage across
several different resistors as well as the light bulb. Be sure to include the smallest
resistor as well as the largest resistor on the board. Do the step 5 analysis.