December 11, 2012

Answers to Review Questions: Ch # 13 Electricity

Karachi Board (BSEK)

Q.13.1 Electrons leave a dry cell and flow through a lamp back to cell. Which terminal ,the +ve or –ve,is the one which electrons leave the cell?In which direction is the conventional current?
Ans.Electrons leave the negative terminal of the cell and move towards the positive terminal. and the conventional current moves from positive to negative.

Q.13.2 Both PD and emf  are measured in volts.What is the difference b/w these concepts.
Ans.P.D is the work done per unit charge across a resistor in circuit. emf is the total P.D across the external and internal resistance.i.e.  E = V + IR

Q.13.3 Can you construct two wires of same length,one of copper and one of iron ,that would have the same resistance at same temperature?
Ans. Yes.Resistivity is proportional to the cross sectional area. The resistivity of iron is 7 times higher then that of copper.Hence the iron wire must be 7 times thicker than the copper wire to have same resistance  at same temperature.

Q.13.4. Why does the resistance of conductor rises with rise in temperature?
Ans.Due to the increase in temperature ,The amplitude of the vibration atoms in the lattice increases,Which increases the probability of the collision of the atoms with electrons.So the resistance increases

Q.13.5. Why is heat produced in a conductor due to flow of electric current?
Ans.When the electrons collides with the atoms of the lattice it losses some of its energy there that is converted into Heat energy.

Q.13.6 When a metal object is heated both its dimension and resistivity increases. Is the increase in resistivity likely to be consequence of the increase in length?
Ans.The Resistivity is equal to RA/L .The increase in resistivty of conductor due to heat is the consequence of increase in resistance,and not a consequence of increase in length.

Q.13.7 It is sometimes said that electrical appliance  uses up electricity.What does such actually use in its operation?
Ans.A electrical appliance,in it operation,uses the kinetic energy carried by the moving electrons and not their quantity of charge.

Q.13.8 Do bends in a wire affect its resistance?
Ans. No .The resistance depends on the Length, Area and temperature as well as the nature of material.

Q.13.9 Resistance of 10Ω, 30Ω and 40Ω are connected in series.If the current in 10Ω resistance is 0.1 A ,What is the current in other resistances?
Ans.The flow of current is always same when they are connected in series so same current will flow from all resistances.

Q.13.10 Ten resistance of different values are connected parallel.If P.D across one of them is 5V ,What is the P.D Across nine resistances?
Ans.The Potential difference  is always same when they are connected Parallel  so same P.D  Will be there from the remaining 9 resistances.

Q.13.11 For a given pd V,how will be the heat developed in resistor depand on its resistance R?Will the heat be developed at higher rate in larger or smaller R?
Ans. The heat developed due to power dissipation is H=(V2/R) x t
So smaller the resistance larger the heat
Q.13.12 Is there any electric field inside a conductor carrying an electric current?
Ans.Yes,It is in the direction of current.

Q.13.13  How does the current flowing in a conductor depend on the number of mobile charges per unit length ,on their average velocity and on the charged per carrier?
Ans.Since I= q / t=Ne / t .If L’ is the number of charges per unit length.then
            N’=N/L  so N=N’ L now I=N’Le/t or we can write as I=N’ev
N’ is the number of charges per unit length , e is Charge per carrier and v  is the average velocity.

Q.13.14 Numerical(Solve it Yourself)

Q.13.15. Numerical(Solve it Yourself)

Q13.16 Can terminal voltage of a battery be zero?
Ans. When a battery is short-circuited Now the internal resistance of conductor makes terminal voltage zero .R=0 then V=0


  1. Very Helpful .. thanks alot

  2. So muchh precious and helpful as well. thanks

  3. Its really helpful but last one is missing.

  4. Its really helpful but last one is missing the question is why the internal resistance of a cell not constant?