Positive and Negative Charges
physics, electricity, charges, "charging by friction" # Landsberg - Elementary Textbook of Physics Vol 2 - Mir - 1988
Chapter 1 Electric Charges
1.4.1. An electroscope charged by a sealing-wax rod is touched by a charged glass body. How will the deflection of the electroscope leaves change?
The sealing-wax rod when charged by rubbing with wool gives a positive charge to the sealing-wax rod. On the other hand a glass-rod rubbed with silk acquires a negative charge. An electroscope has leaves that deviate when it is charged with either charge. The deflection of the leaves is a measure of the the quantity of the charge on the electroscope. When the electroscope is charged with glass rod (negative charge), and then touched with wax rod (positive) the net charge on the electroscope will decrease and hence the deflection leaves will decrease as well.
1.4.2. If a brass rod held in hand is rubbed against silk, it is not charged. If, however, we make this experiment after insulating the rod from the hand by wrapping it in rubber, electric charges will appear on it. Explain the difference in the results of these experiments.
Brass rod is a conductor, which means that it has a lot of free charges. If we are holding the rod with our bare hands while charging the extra charges accumulated on the brass rod will transfer to the hand as human body is a conductor (though not as good as metals). But when we hold the brass rod in rubber (an insulator), the charges accumulated by the rubbing stay on the rod (are retained) and are not transferred to another body as in the previous case.
1.4.3. How can a burner help in removing electric charges from an insulator, say, a charged glass rod?
The glass rod is an insulator. When it is charged, the glass rod stays charged. When a glass rod is touched with a conductor like a metal the charges are transferred from the glass to the conductor. A burner heats the gas, and hot gas (air) is a good conductor of electricity. Hence when a burner is held near a glass rod, it creates conductors in contact with the rod which removes the charges from the rod.
1.4.4. Stand on a wooden board placed on four insulating supports (like porcelain), take a piece of fur in your hand and strike it repeatedly against a wooden table. Your mate can observe a spark from your body by bringing his hand close to it. Explain the processes occurring in this experiment.
When we are standing on an insulated support (in this case a wooden board with porcelain supports) we are an insulated body. When we are striking fur on the wooden table the floor gets charged with negative charges. Since we are standing on the insulated board, our body (a conductor) also gets charged and retains the charge. When our mate approaches us, and brings the hand close to ours, charges are transferred from us to the mate. This transfer of charge causes the spark.
1.4.5. How can you prove experimentally that silk rubbed against glass is charged negatively?
Various experiments prove that there are two type of electric charges. We have arbitrarily named them positive and negative. The charges which are formed by rubbing glass are called as negative, while the charge formed by on wax by rubbing fur or wool are called as positive. There are two ways in which we can prove that silk rubbed against glass is charged negatively. There are two fundamental properties of electric charges that can be used for the required proof. The first property is that when positive and negative charges are brought in contact with each other, they cancel out or neutralise each other. The second property is that like charges repel each other, while unlike charges attract each other.
Using an electroscope: Let us charge an electroscope with a wax rod charged by rubbing with wool. According to our convention, the electroscope will be charged positively. The charging of the electroscope is attested by the diverging of the leaves. Now we know from experiments that the divergence of the leaves of the electroscope is directly proportional to the charge on the leaves. This is so because of the second property mention above. The two leaves have the same type of charge as they are made of conductors (metals) and hence they repel each other. Now if the touch the electroscope with a glass rod, we observe that the divergence of the electroscope leaves decreases or they close out completely. The explanation for this observation is that the positive charge on the leaves has been reduced when we transfer charges from the glass rod to the electroscope. It means that the other charges must be negative, for them to reduce the positive charges on the electroscope. This would be true if the negative charges on the glass rod are less than the positive charges on the electroscope. If the negative charges on the glass rod are exactly equal to the positive charges on the electroscope, it will be completely discharged and the leaves will collapse. If the glass rod was positively charged it would have increase the deflection of the electroscope leaves.
Using a wax rod: We know that opposite charges attract each other. Let us rub a wax rod with wool to create a positively charged rod. Now we charge a glass rod by rubbing with silk. Next we bring these two rods close to each other. We will find that the two rods attract each other. Thus proving that the glass rod is negatively charged. If it were positively charged, it would have repelled the wax rod.