Introduction
The purpose of the experiment was to study Ohm’s law and to investigate the relationship between current and voltage in metal wire, lamp filament and diode.
Theory
Electric current (I) is defined as the rate at which charges flow through a conductor. Current flows when a conductor is connected to a power supply in a complete circuit. The battery provides the energy needed to move the charges through a conductor. The energy supplied by battery per unit charge is called voltage. Voltage is a measure of electrical potential difference between two points in a circuit. The potential difference between two terminals of a power supply is referred to as electromotive force. According to Ohm’s law, the current through a resistor is directly proportional to the voltage across the resistor. This can be written mathematically as;
V= IR,
Where V = potential difference, I = current and R = resistance. Resistance is a measure of ease of flow of current through a conductor.
Devices/resistors that obey linear relationship between voltage and current (ohm’s law) are called ohmic. Most metals are ohmic at as long as temperature is kept constant. A graph of voltage against current for ohmic conductor is a straight line graph passing through the origin. The gradient of the line is the resistance of the device/resistor.
VI characteristic of ohmic conductor
Devices/resistors that do not obey Ohm’s law are called non-ohmic conductors. The graph of voltage against current for non ohmic conductor is nonlinear. The above VI graph is produced for metals at constant temperature. The resistance of metals increases with temperature. Consequently, the VI graph will curve upwards when the temperature is increased.
VI characteristic of non-ohmic conductor
The above VI graph shows that the resistance increases with increase in current. This is the typical VI graph for filament lamp. The resistance of filament lamp increases with increase in temperature. The increase in resistance causes a decrease in current. The increase in temperature of the filament increases the rate of collisions between conduction electrons and lattice ions. This hinders flow of electrons.
Materials
- Metal wire
- Diode
- Lamp
- Battery
- Voltmeter
- Ammeter
Circuit diagram
Procedure
The battery with variable resistor, ammeter and the metal wire were connected as shown above. The voltmeter was connected across the resistor. The voltage was varied using voltage control. Different values voltages were set and their corresponding current were determined. The values of voltage and current were recorded. The wire was replaced with lamp and the procedure repeated. Voltage and corresponding current were determined and recorded. The procedure was repeated for silicon diode.
Results
IV Characteristics
Metal wire
The graph of current (I) against voltage (V) was a straight line passing through the origin. Metal wire obeyed Ohm’s law because IV graph was linear. Metal lattice consist of free or conduction electrons. When a metal conductor is connected to a battery, the battery creates an electric field that causes the conduction electrons to acquire velocity referred to as drift velocity. This velocity is proportional to the force pushing the electrons.
Filament lamp
The graph of current (I) against voltage (V) for filament lamp was nonlinear. This implies that the filament lamp was non ohmic. The steepness of the slope reduced with increase in potential difference. This was an indication that the resistance of filament lamp was increasing with temperature. The increase in the temperature of a filament leads to increase in resistance and decrease in current.
Semiconductor diode
The graph of current (I) against voltage (V) for semiconductor diode was nonlinear. Meaning that diode is non ohmic. There was no current registered when negative voltage was applied because the diode was forward bias. Diodes conduct current in one direction. The graph steepness increased with current. Generally, the steepness of the graph increased with increase in voltage. However, beyond a specific voltage, the forward current increased rapidly with a small increase in voltage. This voltage is referred to barrier voltage. The theoretical value of barrier voltage for silicon diode is about 0.7V. This is the voltage needed to make electron cross p-n junction.
Conclusion
The IV characteristic for metal wire, filament lamp and semiconductor diode was consistent with theory. The IV characteristic of the metal was a straight line through the origin. Meaning that current was directly proportional to voltage (ohmic). On the other hand, the IV characteristic of a filament lamp was a curve (non ohmic). The gradient of curve decreased with the increase in potential difference. Finally, the IV characteristic for the semiconductor was a curve in the first quadrant whose gradient was approximately constant before barrier voltage but the gradually steadily increased beyond barrier voltage.
