Contents
The study of Physics Topics can help us understand and solve real-world problems, from climate change to medical imaging technology.
Determining Charge with an Electroscope and earthing of a charged electroscope
The electroscope is a device for detecting electric charge on an object. By using an electroscope, we can tell whether an object is electrically charged or not. We will now describe the construction and working of a simple electroscope.
Activity 5
Take an empty jam bottle (which is a glass bottle). Cut a piece of cardboard slightly bigger than the mouth of jam bottle. Make a thin hole in the middle of cardboard piece. Now take a metal paper clip (steel paper clip) and open it up in such a way that it forms a shape of 5 (five) with a hook on the lower side. Insert this opened up metal clip in the hole made in cardboard piece. Take a thin strip of aluminium foil about 5 centimetres long and 1 centimetre wide. This strip of aluminium foil is folded in the middle and suspended from the hook on the lower side of the opened up metal paper clip.
Place the cardboard piece carrying the metal clip and folded aluminium foil strip on the mouth of jam bottle in such a way that the hook carrying the folded aluminium foil is inside the bottle [as shown in Figure (a)]. If we look at the folded aluminium foil strip kept on the metal hook, it appears like two thin leaves of aluminium foil attached to the metal hook. At this stage, the two aluminium leaves are uncharged and lie close to each other [see Figure (a)]. Our electroscope is ready.
We will now describe the working of electroscope. Charge a glass rod by rubbing its one end with a piece of silk cloth. Touch the charged glass rod with the top end of metal clip [see Figure (b)]. We will see that the two aluminium leaves move away from each other [see Figure (b)]. We say that the aluminium leaves diverge (or open up). We will now explain why the aluminium leaves diverge (or open up) when touched with a charged glass rod.
When we touch the top end of metal clip with the positively charged glass rod, then some of its positive charge is transferred to the top end of metal clip. Now, since the metal clip is a good conductor of electricity, it conducts the positive electric charge to the two aluminium leaves held on its other end. In this way, the two aluminium leaves get charged with the same kind of electric charge—positive charge. We know that similar charges (or like charges) repel each other. So, the two aluminium leaves having similar charges (positive charges) repel each other due to which they move apart or diverge [see Figure (b)].
Let us now charge a plastic comb with negative electric charge by rubbing it with dry hair. When the top end of metal clip of the electroscope shown in Figure (a) is touched with the negatively charged plastic comb, even then the aluminium leaves of the electroscope diverge (or open up). In this case, the two aluminium leaves get negatively charged and hence repel each other. This repulsion causes the aluminium leaves to diverge.
From the above activity we conclude that whether an object is positively charged or negatively charged, it will cause the aluminium leaves of the electroscope to diverge when touched with the metal top of the electroscope. Please note that greater the amount of electric charge on an object, greater the aluminium leaves will diverge. If, however, an object is uncharged (having no electric charge) then the aluminium leaves will not diverge at all. In the above activity, the electric charge of a charged glass rod (or charged plastic comb) has been transferred to the aluminium leaves through a metal paper clip. So, in general we can say that an electric charge can be transferred from a charged object to another object through a metal conductor.
Detection of Charge With Electroscope
We will now describe how the charge on an object can be detected by using an electroscope. In order to detect the electric charge on an object, we touch the metal top of the electroscope with that object and observe the aluminium leaves :
(i) If on touching the metal top of electroscope with the given object, the aluminium leaves of the electroscope diverge (or open up), then the given object has an electric charge on it (or the given object is electrically charged).
(ii) If on touching the metal top of electroscope with the given object, the aluminium leaves of the electroscope do not diverge (do not open up), then the given object has no electric charge on it (or the given object is electrically uncharged).
Earthing of a Charged Electroscope
When an electroscope is charged, its aluminium leaves are diverged. If we touch the metal top of a charged electroscope with our finger, it gets discharged and its aluminium leaves collapse (or fold up). Actually, when we touch the metal top of a charged electroscope with our finger, then the electric charge present on charged electroscope flows to the earth through our hand and body. The electroscope loses all the electric charge, becomes uncharged and hence its diverged aluminium leaves collapse (or fold up). The process in which the metal top of a charged electroscope is touched with finger and its charge flows into earth through our hand and body, is called earthing.
In general we can say that: The process of transferring an electric charge from a charged object to the earth is called earthing. Earthing can also be done by connecting the metal top of the charged electroscope to the earth directly by means of a metal wire (called earth wire). In that case the electric charge of electroscope will flow to the earth through the metal wire.
The electric current in the wiring of houses and other buildings is due to the flow of electric charges (called electrons) through them. Earthing is provided in the wiring of houses and other buildings to protect us from electric shocks which may occur due to any leakage of electric current from the body of an electrical appliance. Metal wires (called earth wires) are used for this purpose.
Electric Discharge : Production of Sparks
Air is a non-conductor of electricity. So, normally air does not conduct electric charges (or electric current). If, however, the amount of electric charges on two oppositely charged surfaces is very large, then the air between them conducts electricity in the form of a spark (a spark is a flash of light which is seen for a very short time). Actually, the electric energy heats the air to such a high temperature that it glows. This glow of air is seen as a spark or flash. Thus, if the amount of opposite electric charges on two surfaces is very large, the insulation of air between them breaks and it allows electric charges to pass through it.
The passage of electric current in air due to movement of electric charges is called electric discharge.
During electric discharge, the positive and negative electric charges cancel out each other and an electric spark and a crackling sound are produced. In nature, electric discharge within a cloud during thunderstorm produces huge electric sparks known as lightning along with a loud sound called thunder. And electric discharge between a thundercloud and the earth also produces lightning followed by thunder. We will now explain the process of lightning in terms of the production of electric charges by friction in the clouds in the sky followed by electric discharge.