Resistance and Ohm’s lawCopy

Learning Objectives

• State that resistance = p.d. / current and understand qualitatively how changes in p.d. or resistance affect current
• Sketch and explain the current–voltage characteristic of an ohmic resistor and a filament lamp
• Recall and use the equation R = V / I
• Describe an experiment to determine resistance using a voltmeter and an ammeter
• Relate (without calculation) the resistance of a wire to its length and to its diameter
• Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire

Contents to learn

Resistance
Factors affecting the resistance of the conductor

Length
Cross-sectional area
Temperature
Nature of material
There is particular relation between length of the conductor, its cross-sectional area and its resistance, which gives us resistivity of the conductor.

There are few resistance components which we use in IGCSE Physics

Resistors
Thermistors
Variable resistors
Light-dependent resistors
Diodes

Affect of length of the conductor on resistance
Resistance of the wire is directly proportional to its , if the length is doubled, the resistance would also be doubled

Affect of cross-sectional area on resistance
Resistance of the wire is inversely proportional to its cross-sectional area. If cross-sectional area of the wire is doubled the resistance of the wire will become half of the first. If diameter of the wire is doubled, its cross sectional area becomes four times, so resistance decreases by four times.

Affect of temperature on resistance
For metal conductors (ohmic conductors), resistance of material is directly proportional to its resistance. If temperature of the material increases, its resistance increases.

How nature of material affects the resistance
Substances that allow an electric current to flow through them are called conductors; those which do nor are called insulators. Metals are conductors. In a metal structure, the metal atoms exist as positive ions surrounded by an electron cloud. If a potential difference is applied to the metal, the electrons in this cloud are able to move and a current flows. When the electron are moving the metal structure, they bump in to the metal ions and this causes resistance to the electron flow or current. In different conductors the ease of flow of the electrons is different and so the conductors have different resistances. For instance, copper is a better conductor than iron. Here is the list of materials on the basis of their nature.

Ohm’s law
In 1826 George Simon Ohm discovered that “for all metallic conductors the electric current through a conductor is directly proportional to the potential difference mantained across the ends of the conductor provided the physical state of the conductor like pressure, temperature and tension etc. are kept constant”.

Mathematically

I V

I = (constant) X V

I = V / R

V = I R

Where V is the potential difference across the ends of the conductor, I is the electric current through the conductor and R is the proportionality constant, called the resistance of the conductor.