Abstract and Theory about Ohms law experiment

Description

Write 3 to 4 lines abstract then the rest of the page write theory about this experiment (Resistance and Ohm’s Law).

Abstract includes: – overview and summary of experiment.


Theory includes:

– summary of necessary physics – voltage, current, resistance, Ohm’s law

– connection between background and principles of lab procedure

– Ohmic vs. non-ohmic materials

– generic equation

Resistance and Ohm’s Law
In this lab we will explore some fundamental ideas regarding the flow of electricity
through objects.
Electricity deals with the properties and motions of electrically charged particles; in
almost every case this means electrons. Although strongly bound to the bulk metal,
the electrons in a metal are not confined to a particular atom but “hop” from atom to
atom almost freely. If an electrical force is applied to the wire, these free electrons will
quickly rearrange themselves; their motion is called electric current, represented
mathematically as “p or “?”. The unit of current is called the Ampere (A), or “amp” for
short, and it may be preceded by a prefix, most commonly milli- to give us milliamps
(mA). We’ll define the amp carefully later on, but now let it suffice to say that it is simply
proportional to the number of electrons per second that pass through a particular point
in a wire. It is essential to remember that since the electrons making up the current are
from the metal itself, they can’t “leak out” or otherwise get lost under normal circuit
operations; they are bound to the net-positive nuclei of the metal. As much charge
(electrons) flows out of the circuit (at the “-“terminal) as flows into it (at the “+” terminal).
Another quantity we will use in describing the flow of electricity is voltage, and it is
represented mathematically as “V”. In a very inaccurate, but highly intuitive, manner of
speaking, voltage is the “force” that pushes electrons through the wires or other
objects. The higher the voltage, the more electron flow, or current, will be present in
the system. We’ll see later that voltage is closely related to potential energy (another
name for voltage is “electrical potential”). Thus, it is voltage differences that are
important rather than absolute voltages; we will always measure voltage differences in
the lab. The unit of voltage is the Volt (V).
Resistance (R) is a measure of how much an object resists the flow of electricity
through it. Its mathematical definition is
V
R=
Ī
(15.1)
where R is the resistance. If volts and amps are used for the units on the right side of
this equation, then the units of resistance are “Ohms” (rhymes with “homes”). We use
the capital Greek letter omega (2) to represent Ohms.
Procedure
.
The voltage (and current) will be provided by a black box” called a power supply (PS)
that allows you to adjust the voltage by turning a dial. Some guidelines for its use:
The positive voltage terminal (electrical connection) is the red one (+) and the
negative terminal is the white one (-); you will not use the black (gnd) terminal.
The digital voltage and current displays on the unit are inaccurate and unreliable.
Your measurements will be made using separate meters connected to your circuit.
.
If the red light marked “cc” (near the current display) comes on, decrease the
voltage immediately and notify the instructor. Don’t panic, it’s not going to blow up,
but it will not function properly unless adjusted.
Turn the PS off before making any connections or disconnections.
In this and other labs involving electricity, don’t touch any metal parts that are part
of a circuit, as you may yourself become part of the circuit! (This is generally a bad
idea.)
.
Part 1: Cute little resistors
In this part you will measure the resistance of a circuit element that is called a resistor.
These adorable little items are used to control the resistance of circuits in electronic
devices. If you’ve ever torn apart an electronic gadget, then these will probably look
familiar to you. They are supposed to have a particular resistance, which is indicated
by a color code on the casing; Your instructor will help you read this code and then
you’ll know what “ballpark” your resistance measurements should be in (expect the
resistance to be 5-10% above or below its nominal value).
V
Figure 1. The circuit
used in parts 1 through
3.
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Figure 1 illustrates the circuit you must build in order to measure the resistance. The
electric current flows from the (+) terminal of the PS into and through the ammeter (A,
measuring current), then to the resistor (the zigzag line), and then back to the (-)
terminal of the PS. Solid black lines represent wires. The voltmeter (V, measuring
voltage) is connected across the ammeter and resistor combination; it draws a very
small amount of current that allows it to measure the voltage across your resistor. Your
data will consist of I and V measurements taken from these two instruments; ignore
the power supply display, except to note that it should read approximately what you
are measuring with your meters. If they don’t agree within about 10%, there’s likely a
problem with your wiring.
Be sure to record the values of the resistors you use. Check the selected resistors’
color codes against the chart provided to you in lab to make sure of those values. Read
the first three bands, starting with the band closest to an end of the resistor. For
example:
Resistor:
2002
300022
47022
Color code: red-black-black orange-black-red yellow-purple-brown
The fourth band indicates the tolerance of the resistor, i.e. how accurately it was
manufactured. A silver fourth band indicates that your resistor may have a resistance
that is as much as 10% different from the “nominal” (color code) value. A gold band
Analysis and Discussion of Part 4 data:
1. Plot V vs. I for the lamp. Is the tungsten lamp ohmic?
2. What happens to the resistance as the current increases?
Part 5: Edison’s incandescent lamp
In part 4, you used a lamp that had a filament made of tungsten. This is basically what
all modern incandescent bulbs use. Before about 1920, though, the common material
used was carbon. Edison’s first commercially successful light bulbs used carbon
filaments. We have for your use some modern reproductions of carbon incandescent
lights.
Part
A. Replace the tungsten lamp with a carbon lamp and repeat the procedure in
4.
Analysis and Discussion of Part 5 data:
1. Plot V vs. I for the lamp. Is the carbon lamp ohmic?
2. What happens to the resistance as the current increases?
C
THIS LAB PROCEDURE REQUIRES SUBMISSION OF A FORMAL LAB
REPORT.
om
ace
Part 4: Incandescent tungsten-filament lamp
In parts 4 and 5 we’ll be using standard-sized light bulbs. There is no fundamental
difference between operating these and the circuit elements you’ve already used,
however, they are designed to operate at around 100V so you’ll have to use a different
power supply. This is a variable transformer, commonly called a Variacæ due to the
popularity of one product.
100
Figure 2. A typical variac shown with the
on/off switch in the “120 V” position and
the dial set to zero. The lamp is plugged
into the outlet near the switch. The voltage
at this outlet varies from 0% of 120 (or
140) volts to 100% of 120 (or 140) volts.
The dial is a rough guide only; voltage
must be measured across the lamp with a
voltmeter.
20 V
11
140 V
The Variac produces AC (alternating current) voltage, which differs from the DC (direct
current) used in the previous section. DC is constant voltage like that produced by
batteries, but AC is constantly varying (alternating between negative and positive). As
long as you use “AC” meters to measure voltage and current, the AC circuit will behave
exactly like a DC circuit, and good old Ohm’s Law will be as reliable as ever!
A. Once you’re sure you have all of the data you need from parts 1-3, you can
disassemble your circuit and select the AC voltmeter and ammeter.
B. Plug in the Variac, making sure that the 120 V-off-140 V switch is in the off
position.
C. Screw the lamp into its base. Connect the AC meters to your lamp assembly as
described by your instructor. Plug the assembly into the Variac.
D. Turn the switch on the Variac to the “120 V” position. Crank the dial up and down;
the lamp should behave just like a house light on a dimmer switch, growing brighter
with increasing voltage.
E. Set the dial to zero. The dial reading is not equal to voltage; it is a rough
percentage of maximum output voltage (120 in this case). You need not record the
dial position, as it is not useful data, just a rough guide.
F. You may vary the dial from zero to 100. Obtain at least 10 data pairs (V, ) over the
range of 0-20V. Extend to the maximum voltage (120V) with several more pairs,
but be sure to record the voltage from your voltmeter, not the dial setting.
G. Make (and record and report!) careful qualitative observations of the lamp as you
make your measurements.
+ In most household dimmers, the voltage isn’t actually controlled but rather the “duty cycle” of the current
is reduced. Oh, you want to know what that means? Basically, the dimmer shuts the current on and off
rapidly, so that less current gets to the light. Why not just change the voltage like the variac? Note the
size of the variac compared to a common dimmer switch; the electronics for the variac must be much
larger.
indicates a 5% tolerance. Absence of a fourth band indicates a really poor quality
resistor, only 20% tolerance.
A. Select a resistor, set up the circuit as described and have it checked by the
instructor before you turn on the power supply.
B. Begin with the voltage knob turned all the way ccw, your voltmeter should read
zero. Carefully and slowly increase the voltage to about 1.0V and record your
voltage and current values in your notebook.
C. Continue increasing the voltage approx. 1V increments, recording the
corresponding currents, up to a maximum voltage of 12.0V.
D. Select another resistor and repeat this procedure.
Analysis and Discussion for Part 1 data:
1. On graph paper, OR on a computer, make a plot of V vs. I for your resistors
(they can both be on the same graph if the resistors’ values aren’t too
different) and calculate the slope of each best-fit line.
2. Ohm’s law actually states that resistance is constant (independent of the
current) and resistors that satisfy this condition are said to be ohmic. Are
your resistors ohmic?
3. What slopes do you expect for your graphs? (And why?) Remember, a
slope is a number. How do your measured slopes compare to the expected
values?
robes
4. How do your measured resistances compare to the value expected from
the color-coded stripes?
JOYS
Part 2: Super-Secret Mystery Resistor
Snea
A. Select an unknown and measure five V, 1 pairs using your circuit.
B. You need not make a graph this time; just calculate the resistances using Ohm’s
Law.
C. Average these and report the unknown resistance as R error, where “error” is
maximum difference between a single value and the average.
Part 3: Get the LED out
A. Remove the resistor and replace it with an LED (Light Emitting Diode). Note that
the LEDs can conduct electricity only in one direction (that’s what a “diode” is for).
So it’s important that the red-colored wire on the LED be connected to the (+) side
of the circuit (the part that is closest to the (+) terminal of the power supply; the side
from which the current comes).
B. For this part of the lab, you will limit yourself to voltages between 0 and 5V, and
make sure you get plenty of data points in the 0-3 V range (voltage steps of
0.25 or smaller are suggested). Note that a current of zero is still valid data!
C. Make careful qualitative observations of the LED as you make your measurements.
What do you see and when do you see it?
Analysis and Discussion of Part 3 data:
1. Plot V vs. I for the LED; is it ohmic?
2. In what way is this curve different from the other resistance curves you plotted?

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