Voltage rating refers AC RMS voltages. For a given bulb, if the voltage drop across the bulb is greater than voltage rating, the bulb will fuse.

The electrical power P is the rate at which the electrical potential energy is delivered
\(P=\cfrac { dW }{ dt } =\cfrac { d }{ dt } \left( V.dQ \right) =V\cfrac { dQ }{ dt } \)
Since the electric current \(I=\cfrac { dQ }{ dt } \).
So the equation (1) can be rewritten as P = VI
This expression gives the power delivered by the battery to any electrical system, where I is the current passing through it and V is the potential difference across it. The SI unit of electrical power is a watt.

The resistivity of materials is
(i) inversely proportional to the number density (n) of the electrons
(ii) inversely proportional to the average time between the collisions \(\left( \tau \right) \)

When several resistances are connected in series, the total or equivalent resistance is the sum of the individual resistances
R₅ = R₁ + R₂ + .........+ Rn.

The resistanct is defined as the ratio of potential difference across the given conductor to the current passing through the conductor.
\(R=\cfrac { V }{ I } \)

By convention, the flow of current in the circuit should be from the positive terminal of the battery to the negative terminal. This current is called the conventional current or simply current and is in the direction in which a positive test charge would move.

The substances which have an abundance of free electrons are called conductors. These free electrons move at random throughout the conductor at a given temperature. In general due to this random motion, there is no net transfer of charges from one end of the conductor to other end and hence no current. When a potential difference is applied by the battery across the ends of the conductor, the free electrons drift towards the positive terminal of the battery, producing a net electric current.

Atoms in metals have one or more electrons which are loosely bound to the nucleus. These electrons are called free electrons and can be easily detached from the atoms by applying small energy.

Fuses are connected in series in a circuit to protect the electric devices from the heat developed by the passage of excessive current. It is a short length of a wire made of a low melting point material. It melts and breaks the circuit if current exceeds a certain value. Lead and copper wire melts and burns out when the current increases above 5 A and 35 A respectively.

House hold appliances are always connected in parallel so that even if one is switched off, the other devices could function properly.

The average time between successive collisions is called the mean free time denoted by \(\tau \)

1 A of current is equivalent to 1 Coulomb of charge passing through a perpendicular cross-section in Second. The electric current is a scalar quantity.

The instantaneous current I is defined as the limit of the average current \(\Delta t\rightarrow 0\) 
\(I=\underset { \Delta t-0 }{ lim } \cfrac { \Delta Q }{ \Delta t } =\cfrac { dQ }{ dt } \)

If a net charge Q passes through any cross section of a conductor in time t, then the current is defined as \(I=\cfrac { Q }{ t } \)