O Level Physics Chapter :10: Electrical Resistance

Prepared By: Shakil Raiman

10.1: Resistor and Resistance

Resistors is a device which can reduce the current. The resistance of a wire is calculated by R=V/I

(which comes from Ohm’s law) Unit: ohm, ( ) A variable resistor or rheostat is used to vary the

current in a circuit.

10.1: Resistance

10.2: Variable Resistor

A variable resistor or rheostat is used to vary the current in a circuit.

Example: volume controller of music player, speed controller of fan.

10.3: Special Resistor - Thermistors

A thermistor is a resistor whose resistance changes with temperature.

It is made from a semiconducting material such as silicon or germanium.

At room temperature the number of free electrons is small and so the resistance of a thermistor is large. If it is warmed the number of free electrons increases, its resistance decreases.

Thermistors are often used in temperature-sensitive circuits is devices such as fire alarms and thermostats.

10.3.1: Special Resistor - Thirmistors

10.4: Special Resistor – Light Dependent Resistor (LDR)

The resistance of a light dependent resistors changes if the light intensity changes.

In dark conditions LDR contain few free electrons and so have a high resistance.

In light (bright condition) more electron are freed and the resistance decreases.

LDR is used in light sensitive circuit in devices such as photographic equipment, automatic lighting controls and burglar alarms

10.4: Special Resistor – Light Dependent Resistor (LDR)

10.5: Special Resistor – Diode

Diode is a semiconductor device which let the current flow only in one direction.

When the current flow in allowed direction the resistance is low.

When the current flow in not allowed direction the resistance is very high.

Diode is used in rectifier circuits that convert alternating current into direct current.

Light emitting diodes (LED) are diodes that glow when a current is flowing through them.

10.5: Special Resistor – Diode

10.6: Ohm’s Law

The current that flows through a conductor is directly proportional to the potential difference across its ends, provided its temperature remains constant.

10.6.1: Ohm’s Law: Experiment

The circuit is arranged as shown in the figure.

The rheostat to adjust the current to a convenient value and note the readings on the ammeter and voltmeter.

Move the rheostat and take more readings.

Now use formulae R=V/I or follow the following steps: Draw a graph V against I. The gradient is the resistance of

the resistor.

10.7: Factors affecting the Resistance:

Length: For a wire of uniform cross section, Resistance is directly proportional to lengths, i.e. R∞L ie. R1/L1=R2/L2

Area: For a wire or uniform length, its resistance is inversely proportional to area. i.e. R∞1/A i.e. R1A1=R2A2

Material: Resistance depends on kinds of substance. Copper is good conductor and used for connecting wires. Nichome has more resistance and is used in the heating element of electric heater.

Temperature: The resistance of a wire also changes as the temperature changes. For metallic wires, as temperature increases, the resistance increases. But for some materials like silicon and germanium, as temperature increases, the resistance decreases.

10.8: I-V Graph:

Thank You All

Wish you all very good luck and excellent result.