KPK Board 10th Class Physics Ch 5 Current Electricity Short Questions Answers

As water is made of atoms having protons (charge + e) and electrons (charge –e). Does the water flowing through pipe carry an electric current? Explain.

Ans.     No,2 water does carry any electric current because water molecule as a whole is neutral having equal number of protons and electrons.

Explanation:   Water molecule contains one oxygen atom and two hydrogen atoms. All these atoms are neutral (same number of protons and electrons), that’s why water molecule is a neutral molecule. The motion of water molecule is just like motion of an ordinary uncharged particle. Therefore, motion of water does not carry any electric current.

A car has two headlights, when the filament in one headlight burns out, the other headlight stays on Are the headlights connected in series or in parallel?

Ans.     The headlights are connected in parallel that’s why when the filament in one headlight burn out, the other headlight stays on.

Explanation:   In the parallel circuit, when any of the component in circuit damages or stops working, other component perform their work without any hurdle. This is not the case with series combination where if anyone the circuit component damages then the whole circuit becomes useless.  

Qurat-ul-Ain needs a 100 - resistor for a circuit, but she only has a box of 300

resistors. What can she do?

Ans.     This is possible, if she take three resistors of 300 Ὠ and connect them in parallel. The equivalent resistance in this case will be exactly equal to 100 Ὠ. We know that the equivalent resistance of three resistors connected in parallel is given by.

1/Req = 1/R1 + 1/Req

= 1/300 +  1/300  +   1/300

===>      1/Req   =    (1+1+1)/300   =  3/300   =  1/100

===>      1/Req  =     100Ὠ

Thus , the parallel combination of three resistors each of 300 Ὠ is equivalent to 100 Ὠ as calculated above.

A number of light bulbs are connected to a single power outlet. Will they provide more illumination when connected in series or in parallel? Why?

Ans.     The bulbs should be connect in parallel for feting more illumination. It is because,

  1. In parallel combination the potential difference across each bulb is the same. i.e. the potential difference remains undivided.
  2. The equivalent resistance decreases in parallel combination.
  3. By switching OFF any bulb, there is no effect on the brightness of the other bulbs.
Explain why light bulbs almost always burn out just as they are turned on and not after they have been on for some time.

Ans.     Incandescent light bulbs are most likely to burn out when they have just been turned ON. The metallic filament of an incandescent bulb works by resisting the flow of electric current. This resistance causes the filament to get very hot. Most conductive material carry current better when they are cold. Thus, when we first turn an incandescent bulb ON, the initial flow of electrically called the in-rush current is quite high. Due to this high current, more power is dissipated in the form of heat and light and the bulb filament burns out. Once the bulb is shiny, the temperature is staying relatively constant and the filament is un-likely to break.

Explain why is it possible for birds to perch safely on high tension wires without being electrocuted?

Ans.     It is possible for a bird to perch on high voltage wire without being electrocuted. It is because, the whole body of the bird sitting on a high voltage bare wire is at the same potential as that the wire. Due to zero potential difference, no current passes through the body of the bird. Therefore, the bird is not electrocuted.

An electrician working on ‘live’ circuits wears in stated shoes and keep one hand behind his or her back. Why?

Ans.     Wearing insulated shoes increase the overall resistance of the worker which reduces the dangers of electric shock. The electrician working on live wire keeps his other hand behind for safety purposes. If he keeps one hand on live wire and the other hand on neutral or on other conducting material (e.g. earth)he will get an electric shock. The possibility of touching ground or other conductor, the electrician keep his one hand on his back. In this way, he can work safely without any danger of electric shock.

Explain why is it dangerous to turn a light bulb when you are in a bath tub?

Ans.     The human is a good conductor of electricity i.e. heavy current may pass through his  / her body when touches a live wire.

Therefore, when a person in a bath tub, tries to turn ON the switch of a bulb, he may get an electric shock. Thus, for safely measures, he should not do so.

Why circuit breaker, fuses and switches are installed to ‘live wire’?

Ans.     Circuit breakers and fuses are used to protect an electrical circuit from damage caused by an excess current flow due to a fault or short circuit.

As basic function is to stop the current flow after a fault is detected. As in case of fire, if we want to control the fir we have to control it from its source. In the similar way, if we want to have a better control over electricity we have to control it from its source, i.e. live wire.

Live wire have a potential difference of 230 volt with respect to neutral wire of zero potential. Electric current enters our homes through live wire and leave through neutral. Therefore, circuit breakers and fuses should be installed on the live wires to control current at the very beginning .

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