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Resistance in a Wire

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Resistance in a Wire

Aim:

To investigate how the resistance of a wire changes in relationship to
its length.

Preliminary Experiment:

I did a short preliminary experiment to investigate which type of wire
would give me the most accurate set of results. I found from my
results that nichrome wire was the best because its results were far
more consistent than the other types of wire. So I chose nichrome wire
because I knew I would be able to get accurate results whereby I could
write a firm conclusion.

Preliminary Knowledge

§ Georg Ohm Discovered that:

The current flowing through a metal wire is proportional to the
potential difference across it (providing that the temperature remains
constant)

We can easily work out the resistance of a piece of wire if we know
the current and voltage, by Ohms Law:

[IMAGE]Text Box: p.d. across the wire (V) Resistance, R = Current through the wire (I)








[IMAGE]


The symbol for voltage is V. It is measured in units of Volts V

The symbol for current is I. It is measured in units of Amps A

The symbol for resistance is R. It is measured in units of Ohms

How an electrical current and resistance occurs:

All conductors have free electrons in the outer shell of their
structure. When a potential difference is put across the material, the
free electrons become charged. The electrons then form in a line, all
moving in the same direction, this forms an electrical current.
Resistance occurs when these charged particles collide with fixed
particles in the conductor or wire. As the resistance increases so
must the force (voltage) to push the same amount of current around the
circuit.

§ The larger the cross sectional area of the wire is the less the
resistance will be, because the free electrons have more room to move
around, and so less collisions will take place.

§ The higher the temperature of a piece of wire or the surrounding
temperature then the atoms in the wire will start to vibrate more
rapidly. This will cause more collisions between the electrons and the
atoms. This increase in collisions means that there will be an
increase in resistance

§ The longer a piece of wire is, the more particles there are inside
and the charge electrons have further to travel. Therefore there will
be far more collisions, and higher resistance.

The resistance of a wire increases with length and temperature; it
decreases as the cross sectional area is increased. It also depends on
the substance.

Resistance, Current and Voltage are all directionally proportional to
each other:

The HIGHER the VOLTAGE the HIGHER the CURRENT

The HIGHER the RESISTANCE the LOWER the CURRENT

Ammeters are connected in series in a circuit and voltmeters are
connected in parallel.

Prediction:

I predict that the longer the wire becomes the higher the resistance
will be. I think that the resistance will be proportional to the
length of the wire. I predict that the graph to show this should look
something like this:

[IMAGE]

Resistance

(ohms)

Length of wire

(cm)




This is because in a piece of wire there are free electrons in the
outer shell. These then become charged when a potential difference is
passed across them. These electrons then arrange themselves in a line,
and all start to move in the same direction, forming an electrical
current. The electrons are moving but the other atoms are in fixed
positions. As the charged electrons start to move, they collide with
the fixed atoms. As they collide the electrons slow down and the
current becomes less efficient, this is called resistance.

So the more collisions there are the higher the resistance will be.

So therefore the longer the piece of wire is, the more charged
electrons and atoms there are, so therefore there will be more
collisions, resulting in a increase in resistance. Also in longer
piece of wire the electrons have to travel further and will collide
with more atoms on their journey around the circuit.

For example; if there were ten collisions every 5cm's of wire, then a
piece of wire

50 cm's long would have one hundred collisions. I m trying to explain
that the longer the wire the more collisions it will have and
therefore resulting in a higher resistance.

Prediction Diagram:

I predict that this is what is going to be happening inside the wire:

[IMAGE]

[IMAGE]

Charged Electrons= [IMAGE]

Fixed Atoms = [IMAGE]

[IMAGE]


Apparatus:

Equipment:

Reason for Use:

Power Pack

A variable power supply. To supply the circuit with power.

Experimental Wire

A variable to be tested, the length will be changed.

Crocodile Clips

To connect the nichrome wire to the plastic coated wire.

Ammeter

To measure the amps in the circuit.

Voltmeter

To measure the volts in the circuit.

Ruler

To measure the length of wire.

Plastic Coated Wire

To connect equipment together, such as power pack to the ammeter

Stick of Wood and clips

The nichrome wire will be attached to the wood, then the crocodile
clips onto the wire.

Variables:

Independent:

§ I will change the length of wire deliberately so I can see how a
length of a piece of wire can affect its resistance. I intend to have
the length of wire at 10 cm intervals,

(e.g. 10, 20, 30.)

Dependant:

The size of this variable, will depend on the independent variable:

§ The current and voltage will be shown on the ammeter and voltmeter
and I will record these. These readings will change as the length of
the wire lengthens, which will enable me to find the resistance.

Controlled:

§ The length of other wire, e.g. plastic coated will not change,
because this wire is not being tested and may affect my results if
changed.

§ The temperature: if the temperature of the nichrome wire changes
then this may affect my results. So I intend to only have the power
pack on for 25 seconds at a time and then let to cool for 2 minutes.
That may the wire wont over heat and give inaccurate results.

§ I intend to keep the thickness of the wire the same, because I am
investigating the length of the wire not its thickness.

§ I will use the same amount of voltmeters and ammeters in the circuit
because that may affect the resistance.

§ I am going to use the same piece of wire throughout the experiment.
Because different pieces of wire may have different properties.

Fair test:

The following points must be kept to obtain accurate and fair results:

§ The room temperature must be kept constant, because is the
temperature increases it causes the particles in the wire to vibrate
rapidly and there will be more collisions and a higher resistance.

§ The same wire must be used, because different wires have different
properties and may have a different resistance.

§ In each experiment the circuit set up must be the same, to be able
to collect accurate results.

§ All measurements must be measured to 2 decimal points. Because the
experiment is being carried out with low quality equipment, which only
measures to 2 d.p.

§ The wire must be left to cool after the experiment, because from my
preliminary work and my knowledge, the heat of the wire will affect my
results. I know when the wire has cooled because the ammeter reading
will stop changing.

§ The experiment should be repeated two to three times in order to
collect accurate results.

Safety:

The following safety aspects must be carried out whilst doing the
experiment to enable the safety of the pupil, other people and their
surroundings:

§ When there are large currents going through small sections of a
wire, there are could be large overheating. So it must be made sure
that no one touches the wire during and up too 2 minutes after the
current has run through it. The wire will go orange and smoke will be
given off this can burn the table and can cause injury. This danger
can be limited by not sending a large amount of current through the
circuit.

§ The wire can also give off an electric shock. So I must make sure
that no one touches the wire because they will get electrocuted. To
prevent this the power pack must be un-plugged, when the crocodile
clips and the circuit is being re-configured.

§ Keep your circuit away from water sources and taps, because water is
a good conductor of electricity. Also never handle the equipment or
touch the power supply while having wet handle, encase of an electric
shock.

§ Do not lay or obstruct the wire with other equipment or objects.
Because the wire can get extremely hot and could cause things to melt
or catch fire.

§ Always ask a qualified member of staff to check your experiment
before turning on the power, to check that it is set up safely and
properly.

§ A mat should be placed underneath the wire to stop the wire burning
the table or work surface. Also, if the wire comes in contact with
something else, it may affect my results.

Errors:

There are some things that I cannot stop to make it a fair test, but I
can try and limit these as best I can. These errors may occur:

§ I may get errors in my results if the wire becomes overheated from
too much current running through it, and will therefore affect my
graph.

§ It is quite easy for me to make a wrong measurement, and make the
wire too short or too long. It may only be 1mm either way but, that
will still affect my overall results. So, I will measure the wire
precisely from the inside edge of the crocodile clips, making sure the
wire is straight when measuring.

§ Errors may occur through broken or damaged wires. These may increase
the resistance and affect my results.

§ I must also make sure that the wire is straight during the
experiment, because if not, short circuits may occur. I must also
straighten out any bends because this could cause extra-unwanted
resistance.

§ Since my experiment is spread over 2 weeks, the room temperature
between those two weeks may differ. This may slightly affect the heat
of the wire and my results. Therefore I would not have carried out a
fair test, and my results, would not be accurate enough to support a
firm conclusion.

Range Measurements:

The lengths of the wire will be 10, 20, 30, 40, 50, 60, 70cm across
three different power pack readings 4, 6, 9 p.d. I decided to use
these measurements because they were systematic, and from my
preliminary work I can tell that these measurements of the independent
variable will give me an accurate range of results, whereby I can
write a firm conclusion.

Diagram:

[IMAGE]Circuit:

[IMAGE]




[IMAGE]


Wood and Wire:

[IMAGE]




On the wood, measure out 10cm intervals up to and including 70cm. This
is too let you know where to connect the crocodile clips. The nichrome
wire should never get cut or become unattached from the wood.
----------------------------------------------------------------------

Method:

1. Set up the equipment as shown in the two diagrams.

2. Make sure the voltmeter is connected in parallel to the wire. Now
spread the crocodile clips so they are 10cm apart on the wire.

3. Turn the power pack on to 4 p.d and record the ammeter reading in
amps and the voltmeter in volts.

4. Only have the power pack on for a maximum of 30 seconds or the wire
becomes too hot. Once the power pack has been turned off, wait 2
minutes before turning back on.

5. Now move the crocodile clips 20cm apart along the wire and repeat
as before. Now repeat at 30, 40, 50, 60, 70cm at 4 p.d and then repeat
the whole experiment at 6 p.d. and 8 p.d.

6. Pack all of the equipment away; do not move the wire until it has
completely cooled. Turn of the power pack at the mains, before
disconnecting the circuit equipment.

Find the resistance of the wire in ohms ( Ω ). Refer back to
Preliminary Knowledge

Results:


The current and Voltage of the wire at different lengths:
---------------------------------------------------------

4 p.d

6 p.d

8 p.d

Length

Current

Volts

Current

Volts

Current

Volts

10

2.21

1.45

4.00

2.65

5.6

3.65

20

1.94

1.88

2.75

3.52

4.1

4.32

30

1.17

2.26

2.10

4.00

2.71

5.15

40

0.95

2.43

1.69

4.29

2.15

5.45

50

0.80

2.58

1.41

4.51

1.85

5.91

60

0.69

2.66

1.22

4.69

1.60

6.18

70

0.62

2.78

1.10

4.88

1.42

6.38

Resistance of the wire at different Power Pack Readings:

Length of wire

4 p.d

6 p.d

8 p.d

10

0.66

0.66

0.65

20

0.79

1.28

1.29

30

1.93

1.90

1.90

40

2.56

2.54

2.53

50

3.23

3.20

3.19

60

3.86

3.84

3.86

70

4.48

4.44

4.49

Average Resistance for the power pack readings:

Length of wire (cm)

Resistance (ohms)

10

0.66

20

1.29

30

1.91

40

2.54

50

3.21

60

3.85

70

4.47

Conclusion:

From my results and graph I can clearly see one very clear a pattern
forming between the length of the wire and the resistance of it. I can
say that the length of the wire is directionally proportional to its
resistance. That is, as the piece of wire increases in length, the
resistance of the wire also increases. Another significant aspect of
the experiment is that this increase is constant, throughout the
results.

Different power pack readings (p.d) had no affect on the resistance at
all.

From the graph I can see a very clear pattern forming. The length is
directly proportional to the resistance on every point on the graph.
It clearly shows that as the length of wire increases, so does the
resistance. There is a positive correlation, and another significant
thing is that; the increase remains constant. I also found the
gradient of the line and that remains constant throughout the line
graph.

My predictions were very accurate; these were based on my preliminary
work. I was predicted that as the length of wire increases and the
resistance would also increase. I was completely right, my graph
prediction was also very accurate.

As the power pack reading (p.d) increased, so did the electric
current. I know that current is the flow of electrons in a circuit. In
the short lengths of wire (i.e. 10cm and 20cm) the current was high.
This tells us that there was a good strong flow of electrons with very
few collisions with atoms, and they were working at a efficient rate.
I can verify this because the short lengths of wire had a very low
resistance. As the power pack was turned up the current increased and
the electrons were moving faster and had few collisions. But as the
length of the wire increased the current decreased. This tells us that
electrons were colliding a lot more with the other atoms. When the
electrons collided they slowed down and this caused the current to
decrease. So as the current decreased and the electrons kept colliding
in the longer pieces of wire, the resistance was increasing.

The wire's resistance increases as the length of the wire increases
due to Electrons and Atoms colliding more often. The free electrons in
the wire became charged when the potential difference was passed
across them, from the power pack.. The electrons then arrange
themselves in a line, and all started to move in the same direction
(electric current). As they started to move they started to collide
with other atoms, which were in fixed positions. As the electrons
collided they slowed down and the current decreased, this is called
resistance.

In the shorter lengths of wire, there were few electrons or atoms,
which meant very few collisions. That why from my results I can see
that the current was high and resistance was low. But as the length of
wire increased there were more atoms and electrons, which meant more
collisions, which resulted in a lower current and higher resistance.
Also the electrons had to travel further, if the wire was longer,
which meant more collisions on its journey.

A diagram of the 10cm Wire:

(This is an estimate of figures, just to prove my theory)

In the 10cm wire there were very few atoms or electrons, and therefore
were very few collisions, and the resistance was low and the current
was high.

[IMAGE]

[IMAGE]

Charged Electrons= [IMAGE]

Fixed Atoms = [IMAGE]

[IMAGE]


A diagram of 70cm Wire

(This is an estimate of figures, just to prove my theory)

In the 70cm wire there were lots of atoms and electrons, and therefore
there were a lot of collisions, resulting in the resistance was high
and the current was low.

.

[IMAGE]

[IMAGE]

Charged Electrons= [IMAGE]

Fixed Atoms = [IMAGE]

[IMAGE]

How to Cite this Page

MLA Citation:
"Resistance in a Wire." 123HelpMe.com. 17 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=120935>.




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