Electrical resistivity - The electrical resistivity of a material is defined as the resistance of a conductor of unit length and unit area of cross-section.
The constant of proportionality is given as \(\rho\) in the above equation. It is known as an electrical resistivity or specific resistance of the material of the conductor.
| Resistance | \(R\) | \(ohm\) | \(omega\) |
| Conductance | \(\frac{1}{R}\) | \(ohm^{-1}\) | \(\Omega^{-1}\) |
| Resistivity | \(\rho\) | \(ohm m\) | \(\Omega m\) |
| Conductivity | \(\frac{1}{\rho}\) | \(ohm^{-1}m^{-1}\) | \(\Omega^{-1}\) \(m^{-1}\) |
A system of resistors is a combination or a group of resistors in a circuit.
Resistance in series - \(R_s\ =\ R_1\ +\ R_2\ +\ R_3\)
Resistance in parallel - \(\frac{1}{R_eq}\ = \frac{1}{R_1}\ +\ \frac{1}{R_2}\ +\ \frac{1}{R_3}\)
The conversion of electrical energy into heat energy is known as Joule's heating.
\(H\ =\ I^2RT\), where I is current, R is resistance, T is time
The unit is Joule (J)
The heating appliances should have following properties
- It has high resistivity.
- It has a high melting point.
- It is not easily oxidised.
The fuse consists of a ceramic or porcelain body and two points for connecting the fuse wire. The fuse wire is made of a metal or an alloy containing 50% lead and 50% tin. The fuse wire can also be made with materials like aluminium, copper, iron, etc. Some properties of fuse wire are listed below.
- It has a high resistivity.
- It has a low melting point.
Electric Power:
Electric power is defined as the rate of doing work or spending energy. In other words, it is the rate of consumption of electrical energy.
\(P\ =\ VI\)
\(P\ =\ I^2R\)
\(P\ =\ \frac{V^2}{R}\)
The unit is Watt, \(W\)
Electrical energy:
The product of electric power and its usage time is used to calculate the electrical energy consumed.
\(Electrical\ energy\ =\ power\ \times\ time\)
The unit is \(watt hour\)
\(1\ watt\ hour\ =\ 3600 J\)
\(1\ kilo\ watt\ hour\ =\ 3.6\ \times\ 10^6\ J\)
Exam questions:
Exam tips:
1. Formula
2. Hint words/Laws/Definition
3. Conversion of units
4. SI unit
5. Steps involved