Electricity and Magnetism

Topic under the Chapter : Electrostatics

1. Short Introduction

To understand this topic, we need to know about ‘free electrons’. These free electrons behave very similar to the gas atoms in a container. Both of them are under continuous random motion throughout the space given to them.

Free electrons are nothing but some loosely bonded valence electrons which come out of the atom very easily ; just as the loosely stitched button comes out of the shirt very easily : )

Note that :

• A neutral body has equal number of positive charges and negative charges. Presence of free electrons doesn’t disturb the neutrality of the body as free electrons are also negative charges but the only difference being that ; they are free to move inside the body

2. Induction in Metal and Non-metals

When we talk of bodies, we classify them as –

• Metals – The ones in which there are a lot of free electrons
• Non-metals – electrons are bounded to the Atom tightly (Atom loves them !)

2.1 Charge Induction in Metals

Now suppose you have a metal conductor placed in a region. And you bring a positive charge ‘+q’  in that region ‘externally’.

Important Note : 

• excess of electrons implies negative charge
• deficiency of electrons implies positive charge

These charges are ‘induced’ on the conductor due to the external charge. This phenomenon of separation of charges in a body by some external factor is called ‘Charge Induction’

2.2 Charge Induction in Non-Metals

As discussed, the basic difference between non-metal and metals is the absence of free electrons in case of non-metals.

Setup – Let’s consider the same condition. An non-metallic body has been placed in a region. Now, we bring a positive charge in vicinity of this body.

Important Note :

• Atom is made up of a positively charged nucleus and an negatively charged electron cloud surrounding it. In a neutral, undisturbed atom, the negative center and the positive center, both, coincide.

This alignment in dipoles in a non-conducting body due to an external charge is called induction in non-conducting bodies or ‘Polarization’. The separation between positive and negative charges is very very small. So, usually, we ignore it in our problem solving, etc.

Special Note :
This is why a Charged body always attracts a Neutral body. We can also conclude that :
“Attractive force on neutral conducting body will be more due to any external charge”

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FAQ section :

What is charge induction ?

In presence of an external electric field set up by an charge placed outside a body; charge separation happens within the body. This phenomenon is called charge induction.

What is Polarization in Electrostatics ?

The formation of dipoles within the atoms of non-metal due to the external electric field set up by the charge placed outside the body is called Polarization

Topics Covered :

• What is Galvanometer ? & it’s Types
• Conversion to Ammeter
• Conversion to Voltmeter
• Some Examples

1. What is Galvanometer ?

Galvanometer is a deflection type meter which is used to measure the current value. The needle present in the Galvanometer gets deflected when a current passes through it and the amount of deflection produced is proportional to the current passing through the device.

There are two types of Galvanometers : 

• Uni-Directional
• Bi-Directional

• Uni-Directional : In this case, the markings on the dial start from 0 till the maximum range. It has a red terminal which indicates that it has to be given high potential connection and the other one is black terminal for lower potential connection.

• Bi-Directional : The zero of the dial is in the center and we have the maximum on either side. So, from the direction of deflection, we get to know the direction of current in the conductor and the amount of deflection gives us the magnitude. 

So, the basic difference between the two is that, Unidirectional can give information only about magnitude while Bi-directional can tell direction as well as magnitude.

The inner setup of Galvanometer has something known as Coil Resistance ‘G‘ and at maximum deflection, the safe current which flows through the galvanometer is ‘ig‘ . The symbol for Galvanometer is :

2. Conversion to Ammeter

The purpose of an Ammeter is also to measure current but the range for current measurement is much higher. 

• How to Convert ? – Just add a resistor with very small resistance (Shunt ‘S’) in parallel to Galvanometer

What happens because of this ?

• Now suppose, ‘I‘ (I > ig) is the current flowing in conductor. Since S and G are connected in parallel, ‘I‘ will be divided in ‘ig‘ and ‘I-ig‘. 
• The shunt resistance S, being very small in magnitude will attract a lot of current (since current always prefers least resistance path). The shunt resistance S, is the reason why we are able to supply a larger current than ig.
• This helps us to measure a larger current, resulting in increase in the range of the galvanometer

How to calculate this ‘I‘ ?

3. Conversion to Voltmeter

The Voltmeter is used specifically to measure the potential difference across the given terminals.

• How to Convert ? – Add a very high ‘Load’ resistance R in series to Galvanometer

What happens because of this ?

4. Examples

(Question from Arihant – Electricity and Magnetism)

Solution :

(Question from Arihant – Electricity and Magnetism)

Solution :

Conclusion :

So, we have learnt about the Galvanometer and how can we use it as an Ammeter and a Voltmeter.

• This topic is important from not only practical point of view but also theory exam point of view
• And other than marks, it’s always good to know about our electrical instruments !

All the Best !