For procedures, see your electrical laboratory manual.
Title: Measurement of unknown resistance
By —1. Current-Voltage measurement method.
2. Wheatstone Bridge method/calculation method using color-coded resistors.
Aim
To measure the value of an unknown resistor and to determine the characteristics of a resistor.First Method - Current - Voltage Method
Apparatus
1. Unknown Resistor2. DC Power Supply (0-15V,2A)
3. DC Voltmeter (0-30V)
4. DC Ammeter (0-2A)
Theory
This experiment works on the principle of Ohm's law. Ohm's law states that the electrical current (I) flowing in a circuit is directly proportional to the voltage (V) and inversely proportional to the resistance (R). Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change.Mathematically:
Current = Voltage / Resistance
I = V / R
More specifically, Ohm's law states that the R in this relation is constant, independent of the current. Ohm's law is an empirical relation that accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of the current.
Resistance is the opposition that a substance offers to the flow of electric current.
Mathematically
Resistance = Voltage / Current
R = V / I
Second Method - Wheatstone Bridge method/calculation method using color-coded resistors
Apparatus
1. Unknown Resistor2. DC Power Supply
3. Wheatstone Bridge
Theory
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. The primary benefit of the circuit is its ability to provide extremely accurate measurements (in contrast with something like a simple voltage divider).
The Wheatstone Bridge circuit is nothing more than two simple series-parallel arrangements of resistances connected between a voltage supply terminal and ground producing zero voltage difference between the two parallel branches when balanced. A Wheatstone bridge circuit has two input terminals and two output terminals consisting of four resistors configured in a diamond-like arrangement.
When balanced, the Wheatstone bridge can be analyzed simply as two series strings in parallel. In our tutorial about Resistors in Series, we saw that each resistor within the series chain produces an IR drop or voltage drop across itself as a consequence of the current flowing through it as defined by Ohms Law.
The Wheatstone Bridge equation required to give the value of the unknown resistance, Rₓ at balance is given as:
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