Electricity

Ammeter burns out when connected in parallel. Give reasons.

A household uses the following electric appliances :

(i) Refrigerator of rating 400W for ten hours each day.

(ii) Two electric fans of rating 80W each for twelve hours each day.

(iii) Six electric tubes of rating 18W each for 6 hours each day.

Calculate the electricity bill of the household for the month of June if the cost

per unit of electric energy is Rs. 3.00.

Two students perform the experiments on series and parallel combinations of

two given resistors R1 and R2 and plot the following V-I graphs.

Which of the graphs is (are) correctly labelled in terms of the words ‘series’ and

parallel’ Justify your answer.

A wire of length L and resistance R is stretched so that its length its doubled.

How will its (a) Resistance change (b) Resistively change ?

Given a resistors each of resistors R. How will you combine them to get the

( i ) maximum and (ii) minimum effective resistance? What is the ratio of the

maximum to minimum resistance?

Two wires of same material and same length have radii r1 and r2. Compare their

resistances.

Two wires are of the same length, same radius, but one of them is of copper and

the other is of iron. Which will have more resistance?

How does the resistance of a wire depend upon its radius?

What happens to the resistance of a circuit if the current through it is doubled?

What happens to the current in a circuit if its resistance is doubled?

How will you join three resistances, each of 2􀈍 so that the effective resistance

is 3􀈍?

Which has more resistance: 100W bulb or 60W bulb?

What is the resistance of a Voltmeter?

What is the resistance of an ammeter?

What is the resistance of connecting wire?

A wire of resistivity 􀈡 is stretched to double its length. What is its new

resistivity?

Why are copper wires used as connecting wires?

A charge of 2C moves between two plates, maintained at a p.d of 1V. What is

the energy acquired by the charge?

What is represented by joule/coulomb?

Show how you would connect three resistors, each of resistance 6 Ω, so that the combination has a resistance of (i) 9 Ω, (ii) 4 Ω.

Compare the power used in the 2 Ωresistor in each of the following circuits: (i) a 6 V battery in series with 1 Ωand 2 Ωresistors, and (ii) a 4 V battery in parallel with 12 Ωand 2 Ωresistors.

Explain the following.

(a) Why is the tungsten used almost exclusively for filament of electric lamps?

(b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?

(c) Why is the series arrangement not used for domestic circuits?

(d) How does the resistance of a wire vary with its area of cross-section?

(e) Why are copper and aluminium wires usually employed for electricity transmission?

 An electric heater of resistance 8 ÃŽÂ©draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.

Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?

Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?

:  A battery of 9 V is connected in series with resistor 0.2 ÃŽÂ©,0.3 ÃŽÂ©,0.4 ÃŽÂ©, 0.5Ω,12 ÃŽÂ©,respectively. How much current would flow through the 12 ÃŽÂ©resistor

When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.

The values of current Iflowing in a given resistor for the corresponding values of potential difference Vacross the resistor are given below -

Plot a graph between Vand I and calculate the resistance of that resistor.

 A copper wire has diameter 0.5 mm and resistivity of 1.6 x 10 ÃŽÂ©m What will be the length of this wire to make its resistance 10 ÃŽÂ© How much does the resistance change if the diameter is doubled?

How is a voltmeter connected in the circuit to measure the potential difference between two points?

Two conducting wires of the same material and of equal lengths and equal diameters are first connected in series and then parallel in a circuit across the same potential difference. The ratio of heat produced in series and parallel combinations would be -

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be -

Which of the following terms does not represent electrical power in a circuit?

A piece of wire of resistance is cut into five equal parts. These parts are then connected in parallel. If the equivalentresistance of this combination is  R/R'is 

 An electric motor takes 5 A from a 220 V line Determine the power of the motor and the energy consumed in 2 h.

What determines the rate at which energy is delivered by a current?

 An electric iron of resistance 20 ÃŽÂ© takes a current of 5 A. Calculate the heat developed in 30 s.

Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50 V.

  Why does the cord of an electric heater  not glow while the heating element does?

  What is (a) the highest, (b) the lowest totalresistance that can be secured by combinations of four coils of resistance 4 ÃŽÂ©,8Ω,12Ω,24Ω

How can three resistors of resistances 2  ÃŽÂ©,3 ÃŽÂ© and ,6 ÃŽÂ©be connected to give a total resistance of (a) 4Ω,(b) 1 ÃŽÂ©?

  What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

  Redraw the circuit of question 1, putting in an ammeter to measure the current through the resistors and a voltmeter to measure potential difference across the 12Ωresistor. What would be the readings in the ammeter and the voltmeter?

 Draw a schematic diagram of circuit consisting of a battery of three cells of 2 V each, a 5Ωresistor, an 8Ωresistor, and a 12 ÃŽÂ©resistor, and a plug key, all connected in series

Use the data in Table 12.2 to answer the following -

Table 12.2 Electrical resistivity of some substances at 20°C

Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?

  Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former

Will current flow more easily through a through a thick wire or a thin wire of the same material, when connected to the same source? Why?

On what factors does the resistance of a conductor depend?

How much energy is given to each coulomb of charge passing through a 6 V battery?

What is meant by saying that the potential difference between two points is 1 V?

Name a device that helps to maintain a potential difference across a conductor.

Calculate the number of electron constituting one coulomb of charge

Define the unit of current.

What does an electric circuit mean?