Temperature and Resistance

Complexity level:	7
Project cost ($):	20
Time required:	1 hour to prepare, 1 hour for observation
Material availability:	May be obtained from hobby stores, hardware supply shops, home depot etc.
Safety concerns:	Be careful when handling electric cables and knives. Adult supervision recommended.

Hypothesis

When the temperature of a wire increases, the resistance value of the wire will also increase and therefore the conductivity of the wire will reduce.

Overview

The resistance of a conductor is a measurement of the opposition to the current flowing in it. The value of the resistance will increase as the length of the conductor is made longer and the resistance will reduce when the diameter of the conductor is increased. The value of resistance is measured in ohms.

Electrical conductivity measures the ability of a conductor to allow current to flow in it. Therefore conductivity is the inverse of resistance. The higher the resistance of the wire, the lower the conductivity will be.

Resistance = 1 / Conductivity

Ohm’s Law states that when a current I ampere flows inside a conductor of resistance R ohms, the voltage V volt across the wire will be the product of the current and the resistance.

Voltage V = Current I x Resistance R

Scientific Terms

Conductivity, resistance, current, ampere, voltage, volt, ohm

Materials

The materials required for this science fair project:

-    5 meters of varnish insulated copper wire with diameter 0.5mm (may be purchased from hobby stores, motor rewinding workshops, etc...)

-    A PVC pipe about 300mm in length

-    Three 1.5V batteries and a casing to hold the batteries

-    3 jumper wires with crocodile clips at both ends

-    An ohm meter

-    An ammeter that can measure current up to 10A

-    A paper knife

-    An infrared thermometer

-    1 roll of insulation tape

Procedure

1.    For this experiment, the independent variable is the temperature of the copper wire. The dependent variable is the resistance of the wire. This is determined by measuring the current and resistance of the wire. The constants (control variables) are the room temperature, the applied voltage, the diameter of the wire and the length of the wire.

2.    The wire of ? 0.5mm and length of 5 meter is wound around the PVC pipe. The ends of the wire are fixed to the pipe using the insulation tape. About 3 cm of insulation is removed from the wire tips using the paper knife. The resistance of the wire is measured using the ohm meter.

3.    The 3 batteries are connected in series to produce 4.5V. The ammeter, battery and wire wound around the pipe are connected as shown in figure 1. Connection is done using the jumper wires.

4. The initial current reading shown on the ammeter is recorded in the table below. As the current flows in the magnet wire, the temperature of the wire will increase. This temperature is measured using the infrared thermometer. As the temperature increases 10°C, the ammeter reading is recorded, and the wire is disconnected from the circuit and the wire resistance is immediately measured using the ohm meter.

5. The procedure 4 is repeated when the temperature of the wire increases every 10°C until the highest temperature reached and the measurements are recorded in the table below.

Results

It is observed that as the temperature of the wire increases, the resistance of the wire also increases but the current flowing in the wire reduces.

Wire temperature °C	Voltage (Volt)	Current (Ampere)	Resistance (ohm)
25	4.5	9.82	0.458
35	4.5	9.43	0.477
45	4.5	9.12	0.493
55	4.5	8.80	0.511
65	4.5	8.50	0.529

The graph below represents the results of our science project experiment.

Resistance and conductivity science fair project
Temperature and electric currents science project

Conclusion

The hypothesis that when the temperature of a wire increases, the resistance value of the wire will also increase and the conductivity of the wire will reduce is proven to be true.

The temperature of the wire increases due to its power loss, which is calculated as I²x R., where I is the current in the wire and R is the resistance. The higher the current in the wire or the higher resistance will cause the temperature of the wire to increase. Therefore it is important to choose the correct wire sizes to minimize this unwanted loss and reduce the temperature of the conductor.

Also consider

What would happen if the experiment was repeated using different wire sizes?

The experiment can be repeated using aluminum wires.

References

Electrical resistance - http://en.wikipedia.org/wiki/Electrical_resistance

Electrical conductivity - http://en.wikipedia.org/wiki/Electrical_conductivity

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