# NCERT Class X Science Class: Chapter –12. Electricity – Part-6

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**Question 24:**

What is electrical resistivity? In a series electrical circuit comprising a resistor made up of a metallic wire, the ammeter reads . The reading of the ammeter decreases to half when the length of the wire is doubled. Why?

**Answer:**

Electrical resistivity is the property of the material independent of the geometry of conductor. It is basically resistance for a given area of cross-section of conductor per unit length.

Use the formula

When length of wire is doubled, the resistance is also doubled, because resistance is directly proportional to length of wire. Hence according to ohms law for a given potential difference, the current reduces to half.

**Question 25:**

What is the commercial unit of electrical energy? Represent it in terms of joules.

**Answer:**

Commercial unit of electric energy is kilo wall hour (kWh)

**Question 26:**

A current of ampere flows in a series circuit containing an electriclamp and a conductor of when connected to a battery.Calculate the resistance of the electric lamp.

Now if a resistance of is connected in parallel with thisseries combination, what change (if any) in current flowing through conductor and potential difference across the lamp will take place? Give reason.

**Answer:**

Lets the resistance of the lamp

Resistance of conductor

Total resistance in series,

Using ohm’s law,

Thus, the resistance of electrical lamp is

(ii) Now, a resistance of is connected in parallel with the series combination. Therefore, the total resistance of the circuit is given by

Hence, Current flowing in the circuit,

Thus, 1 A current will flow through resistor and will flow through the lamp and conductor of resistance. Hence, there will be no change in current flowing through conductor. Also, there will be no change in potential difference across the lamp.

**Question 27:**

Why is parallel arrangement used in domestic wiring?

**Answer:**

Parallel arrangement is used in domestic wiring so as to provide equal amount of energy to all the appliances.

Moreover if fault or short circuit occurs in any circuit it will nor not result in the disconnection to other circuits.

This reduces the equivalent resistance and results in consumption of less energy. Although sometimes it is found that electric points in separate room has separate circuit with individual distribution switch and fuse.

**Question 28:**

are three identical bulbs connected as shown in Figure. When all the three bulbs glow, a current of is recorded by the ammeter A.

(i) What happens to the glow of the other two bulbs when the bulb gets fused?

(ii) What happens to the reading of and A when the bulb gets fused?

(iii) How much power is dissipated in the circuit when all the three bulbs glow together?

**Answer:**

Resistance od combination of three bulbs __in__ parallel__,__

If R is the resistance of each wire, then

(i) When bulb B_{1} gets fused, the currents in remain same , so their glow remains unaffected.(ii) When bulb gets fused, the current in . becomes zero and currents in and remains 1 A.

Total current

Current in ammeter

Current in ammeter

Current in ammeter

Current in ammeter

When all the three bulbs are connected

Power decapitated,