A resistor determines the amount of energy capable of passing through, because the resistors material restricts the movement of electrons in the material. The resistance, R of a material is defined as the ratio V/I, where V is the potential difference across the material and I is the current flowing through it.
R = V/I
Any conductor which provides a known value of resistance in a circuit is called a resistor. There are variable resistors (rheostats) and fixed resistors. Types of fixed resistors are :
Carbon fibre
Carbon composition
Tin oxide
Wire wound
The resistance of a load can be found using a voltmeter and ammeter. The ammeter is used to find the current through the load and the voltmeter the potential difference, with whose values resistance can be calculated.
1) Set up the apparatus according to the circuit diagram. 2) Adjust the variable resistor to allow the smallest possible current to flow in the circuit. 3) Not the ammeter reading and the voltmeter reading 4) Adjust the variable resistor to allow a larger current to flow in the circuit. Not the values. 5) Repeat the above for five sets. 6) Find the average resistance; or plot a graph for V against I and find the gradient which gives the resistance of the load.
Ohm’s Law
The SI unit for resistance is the ohm (Ω). One ohm is the resistance of a material through which a current of one ampere flows when a potential difference of one volt is maintained.
R = V/ I, I ohm (Ω)= 1 volt (V) / 1 ampere (A)
Ohm’s Law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided that the physical conditions (e.g. temperature) are constant.
The resistance ( R = V / I) of a metallic conductor is steady under constant physical conditions. Non-metallic conductors which obey ohm’s law are called ohmic conductors. Materials which do not obey ohm’s laws are ionic solutions, gases, superconductors and thermionic devices. These are called non-ohmic conductors.
Resistivity: The resistance of a conductor depends on its temperature, length l, its crosssectional area A and the type of material. Thinner and/ or longer wires have a higher resistance compared to short and /or thick wires.
The larger the cross-sectional area of a wire, the smaller the resistance it has. Therefore: R is inversely proportional to A when the length and type of the material are constant, R ∝ 1/A.
The longer the wire, the greater the resistance. Therefore, R is directly proportional to the length when A and the type of the material are constant, R ∝ 1. Therefore, R = I /A or R = where (a constant) is a property of the type of the conductor called its resistivity.
Filament Bulb
A filament bulb is a non-ohmic conductor, so the current is not directly proportional to the potential difference. The resistance of a bulb increases as the current increases, which means with the increase of temperature the resistance increases. Beacuse the ions vibrate more in the metal filament they resist the passage of electrons.
LDR
Resistance decreases with increase of light intensity.
Experiment:
To determine the resistance of a resistance using a voltmeter and an ammeter.
Apparatus: voltmeter, ammeter, variable resistor, accumulator