An electric bulb is marked 200 V, 100 W. Calculate electrical resistance of its filament. If five such bulbs are connected in series to a 200 V supply, how much current will flow through them ?

Rating of the bulb is given as 220 V , 100 W i . e V b = 200 V a n d P b = 100 W Thus the resistance of the bulb R = V b 2 P b = 200 2 100 = 400 Ω The equivalent resistance of 5 bulbs connected in series R s = 5 R = 5 × 400 = 2000 Ω Supply voltage V s = 200 V So, current in the circuit I = V s R s = 200 2000 = 0 . 1 A

Define current density. Write an expression which connects current density with drift speed.

Current Density: It is defined as the current flowing the conductor per unit cross-sectional area that is perpendicular to the direction of flow of current. j = i / A The relation between current density and drift velocity is J = v d n e , where v d , n , e are drift velocity , charge denisty and charge of electron.

A metallic wire having a resistance of 20 Ω is bent in order to form a complete circle. Calculate the resistance between any two diametrically opposite points on the circle.

The diameter of the circle of bent wire will divide it into two wires of equal length and hence the equal resistance. The bent wire can be taken as the parallel combination of two wires as shown. The equivalent resistance R of the parallel combination of two wires of 10 Ω each: 1 R = 1 10 + 1 10 ⇒ R = 5 Ω