**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

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