1. Electric Field And Potential

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1. Electric Field and Potential

1. Electric Charge (Franklin’s Experiment):

● It had been found that if a rubbed amber rod was dangling from a string, and another rubbed amber rod was brought near, the dangling one would move away. ● If a dangling rubbed glass rod is brought near another rubbed glass rod, the dangling one would move away. ● If a rubbed glass rod and amber rod were brought near to each other, they were attracted. ● Therefore, the charge on the glass must be different from the charge on the amber! Franklin decided to say that… ● the glass rod had a positive charge ● the amber rod (or the plastic ebonite used today) had a negative charge

Why did he choose to call glass positive and amber negative?

● No reason! He knew they were different and opposite to each other so he just picked one to be positive and the other negative. 2. Electric Charge: Atomic Model • All matter consists of atoms • Atoms are made of a nucleus (neutron and proton) and electron/(s) revolving around the nucleus. (Orbital Model) • Protons are positively charged and Neutrons are neutrally charged. • Electrons are negatively charged.  󲐀 All charges follow the qualitative “fundamental law of electrostatics”:  “ Like charges repel, unlike charges attract”

Fig.1.1 Particle

Mass

ELECTRON, 𝑒𝑒 − PROTON, p NEUTRON, n

9.11 × 10−31 𝑘𝑘𝑘𝑘 1.67 × 10−27 𝑘𝑘𝑘𝑘 1.67 × 10−27 𝑘𝑘𝑘𝑘

Charge −1.6 × 10−19 𝐶𝐶 +1.6 × 10−19 𝐶𝐶 None

• Electric Charges have two properties: “Charge is Quantized” • All observable charges in nature occur in discrete g packets or in integral amounts of the fundamental unit of charge e. • Any charge Q occurring in nature can be written 𝑸𝑸 = + 𝑵𝑵𝒆𝒆 “Charge is Conserved” • When you effect a transfer of charge: If an electron goes from object A to object B, object A becomes positive and object B becomes negative. The net charge of the two objects remains constant; that is, charge is conserved. • Even in certain interactions, where charged particles are created and annihilated, the

amount of charges that are produced and destroyed is equal, so there is conservation.

2 Unit • Coulomb • Abbr. C • Type: Derived

Derivation • Derived from the concept of current which is one of the 7 SI fundamental units

Value • −1.6 × 10−19 𝐶𝐶

3. The Charging Process: 1. Charging by Friction: • Rubbing two different materials together, a process known as charging by friction is the simplest way to give something a charge. • Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths. • As they pass over each other the electrons with weaker bonds are “ripped” off one material and collect on the other material. Example 1: Rub a piece of ebonite (very hard, black rubber) across a piece of animal fur. The fur does not hold on to its electrons as strongly as the ebonite. At least some of the electrons will be ripped off of the fur and stay on the ebonite. Now the fur has a slightly positive charge (it lost some electrons) and the ebonite is slightly negative (it gained some electrons).The net charge is still zero between the two… remember the conservation of charge. No charges have been created or destroyed, just moved around. Example 2: Rub a glass rod with a piece of silk. This is the same sort of situation as the one above. In this case the silk holds onto the electrons more strongly than the glass. Electrons are ripped off of the glass and go on to the silk. The glass is now positive and the silk is negative 2. Charging by Conduction: Conduction just means that the two objects will come into actual physical contact with each other (this is why it is sometimes called “charging by contact”).

Fig.1.2 3. Charging by Induction: It is possible to charge a conductor without coming into direct contact with it.

Fig.1.3

3 4. Coulomb’s Law Introduction The magnitude of the force of attraction or repulsion between two electric charges at rest was studied by Charles Coulomb. He formulated a law ,known as "COULOMB'S LAW". STATEMENT According to Coulomb's law: •

The electrostatic force of attraction or repulsion between two point charges is directly proportional to the product of charges.

•

The electrostatic force of attraction or repulsion between two point charges is inversely proportional to the square of distance between them.

MATHEMATICAL REPRESENTATION OF COULOMB'S LAW

Fig.1.4 In Fig.1 there are two charges 𝑞𝑞1 and 𝑞𝑞2 at a distance 𝑟𝑟 in air or vacuum, then by Coulomb’s law we have 𝐹𝐹𝑒𝑒 =

1 𝑞𝑞1 𝑞𝑞2 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 4𝜋𝜋𝜖𝜖0 𝑟𝑟 2

where , 𝜖𝜖0 is known as the “permittivity of free space”, 𝜖𝜖0 = 8.86 × 10−12 𝐶𝐶 2 𝑁𝑁 −1 𝑚𝑚−2 and 1 = 9.0 × 109 𝑁𝑁𝑚𝑚2 𝐶𝐶 −2 4𝜋𝜋𝜖𝜖 0

FORCE IN THE PRESENCE OF DIELECTRIC MEDIUM If the space between the charges is filled with a non conducting medium or an insulator called "dielectric", it is found that the dielectric reduces the electrostatic force as compared to free space by a factor K called DIELECTRIC CONSTANT. This factor is also known as RELATIVE PERMITTIVITY. It has different values for different dielectric materials. In the presence of a dielectric between two charges the Coulomb's law is expressed as:

or

𝐹𝐹𝑒𝑒 =

1 𝑞𝑞1 𝑞𝑞2 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 4𝜋𝜋𝜖𝜖0 𝐾𝐾 𝑟𝑟 2

𝐹𝐹𝑒𝑒 =

1 𝑞𝑞1 𝑞𝑞2 𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 4𝜋𝜋𝜋𝜋 𝑟𝑟 2

where 𝜖𝜖 is called absolute permittivity and 𝜖𝜖 = 𝜖𝜖0 𝐾𝐾 VECTOR FORM OF COULOMB'S LAW

The magnitude as well as the direction of electrostatic force can be expressed by using Coulomb's law by vector equation:

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𝐹𝐹⃗12 =

1 𝑞𝑞1 𝑞𝑞2 𝑟𝑟̂ 4𝜋𝜋𝜖𝜖0 𝐾𝐾 𝑟𝑟 2 12

Where 𝐹𝐹⃗12 is the force exerted by 𝑞𝑞1 on 𝑞𝑞2 and 𝑟𝑟̂12 is the unit vector along the line joining the two charges from 𝑞𝑞1 to 𝑞𝑞2 .

5. Electric Field A charge produces an electric field in the space around it and this electric field exerts a force on any charge placed on it. The intensity of field is defined as ⃗

• • • •

𝐹𝐹 𝐸𝐸�⃗ = 𝑞𝑞𝑒𝑒

𝐸𝐸�⃗ is a vector field. Travels at the speed of light. Acts on tests not on its own source Does not actually require test charges to compute 𝐸𝐸�⃗

Computing for the Electric Field • When the force and test charge (𝑞𝑞0 ) [not the source charge] values are given: 𝐹𝐹⃗𝑒𝑒 𝐸𝐸�⃗ = 𝑞𝑞0 •

•

When only the source charge (𝑞𝑞) [not the test charge] value is given:

The unit of E is N/C

𝐸𝐸�⃗ =

1 𝑞𝑞 4𝜋𝜋𝜖𝜖0 𝐾𝐾 𝑟𝑟 2

Let's keep in mind that you've already studied fields when you learned about gravity. We can look at the parallels between the following two formulas to remember things about each of them. 𝑔𝑔⃗ =

𝐹𝐹⃗𝑔𝑔 𝑚𝑚

𝑔𝑔 = measurement of the gravitational field strength. 𝐹𝐹⃗𝑔𝑔 = the force acting on the small object. 𝑚𝑚 = mass of the small object (like a person), not the large object (like the earth).

This formula measures the amount of force per unit mass.

𝐸𝐸�⃗ =

𝐹𝐹⃗𝑒𝑒 𝑞𝑞0

𝐸𝐸�⃗ = measurement of the electric field strength. ⃗ 𝐹𝐹𝑒𝑒 = the force acting on the test charge. 𝑞𝑞 = the charge of the test charge, not the source charge making

This formula measures the amount of force per unit charge.

5 Just as we were able to find a connection between electrostatics and gravity as above, we can do the same thing with our new formula. 𝑔𝑔 =

𝐺𝐺𝐺𝐺 𝑟𝑟 2

𝑔𝑔 = measurement of the gravitational field strength 𝐺𝐺 = gravitational constant 𝑀𝑀 = mass of body producing gravitational field 𝑟𝑟 = distance from centre of body

6. Electric Field Lines

𝐸𝐸�⃗ =

𝑘𝑘𝑘𝑘 𝑟𝑟 2

𝐸𝐸�⃗ = measurement of the electric field strength 𝑘𝑘 = Coulomb's constant 𝑞𝑞 = charge of source charge producing electric field 𝑟𝑟 = distance from centre of body

Fig.1.5 • • • • •

•

•

We can visually represent electric fields as field lines. Caution: Electric Field lines are just imaginary – a mere representation of the electric vector field! We note that the electric field lines indicate the direction to which the force will be exerted by a positive test charge! 7. Motion of Charges in Electric Fields When a test charge ventures in an electric field, it experiences a force 𝑞𝑞0 𝐸𝐸! It will accelerate following Newton’s Second Law with acceleration ∑ 𝐹𝐹⃗ 𝑞𝑞𝐸𝐸�⃗ 𝑎𝑎 = = 𝑚𝑚 𝑚𝑚 8. Electric Field Due to Charge Distributions Two Kinds of Charge Distributions: 1. Discrete 1. Electric Dipole 2. Systems of Point Charges 2. Continuous 1. Linear: Line and Ring Charges 2. Surface: Disk and Plane Charges 3. Volume: Spheres and Cylinders a. Electric Dipole A combination of two charges +𝑞𝑞 and −𝑞𝑞 separated by a small distance of d constitutes an electric dipole.

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Fig.1.6 �⃗ describes the strength and orientation of electric dipoles. Electric dipole moment, 𝒑𝒑 𝑝𝑝⃗ = 𝑞𝑞𝑑𝑑⃗ p, points from the negative charge to the positive charge

• •

b. System of Point Charges Just as Forces are vectors, Fields are also vectors hence they also follow the superposition principle,

•

i.e.

•

•

•

𝐸𝐸 =

𝐸𝐸 =

The net field at a certain point is just the vector sum of the individual contribution of each point charges 𝐸𝐸�⃗𝑛𝑛𝑛𝑛𝑛𝑛 = 𝐸𝐸�⃗1 + 𝐸𝐸�⃗2 + 𝐸𝐸�⃗3 + ⋯ 𝐸𝐸�⃗𝑛𝑛 For Continuous Charge Distributions Just like mass corresponds to its effect in density, charge can be found to exist in three spatial forms, with corresponding densities:

Name Charge Linear charge density Surface charge density Volume charge density

c. Linear Charges: Lines and Rings We have three types of linear charges

2𝑘𝑘𝑘𝑘𝑘𝑘

𝑦𝑦�4𝑦𝑦 2 +𝐿𝐿2

2𝑘𝑘𝑘𝑘 𝑦𝑦

Symbol 𝑞𝑞 𝜆𝜆 𝜎𝜎 𝜌𝜌

: 𝐿𝐿 → 𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓𝑓

SI Unit

𝐶𝐶 𝐶𝐶/𝑚𝑚 𝐶𝐶/𝑚𝑚2 𝐶𝐶/𝑚𝑚3

𝐸𝐸 =

𝑘𝑘𝑘𝑘𝑘𝑘

�(𝑥𝑥 2 +𝑅𝑅 2 )3

: R→ Radius of ring

𝑥𝑥→distance from the center of the ring

: 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿ℎ → 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖

y→ distance from the center L→ Length of line charge Fig.1.7 d. Surface Charges: Disks and Plane We only consider circular disks and its infinite extension: the infinite plane 𝐸𝐸 =

𝜎𝜎

2𝜀𝜀 0

�1 −

𝑧𝑧

√𝑧𝑧 2 +𝑅𝑅 2

�

R is the radius of the disk 𝑍𝑍 is the distance from the center of the disk

Fig.1.8

𝐸𝐸 =

𝜎𝜎

2𝜀𝜀 0

= 2𝜋𝜋𝜋𝜋𝜋𝜋, 𝑘𝑘 = 1/4𝜋𝜋𝜀𝜀0

Field is measured normal to the plane

7 9. Electric Potential Energy & Voltage: Gravitational Potential Energy To better understand electric potential energy it is a good idea to first review gravitational potential energy and figure out similarities between them. • Understanding the parallels between (seemingly) unrelated things in physics is actually one of the best ways to learn physics. According to Newton’s Second Law, if a force acts on an object it will accelerate. • If you drop an object, the force due to gravity will cause it to accelerate down. • At the top, we can say that the object has a high gravitational potential energy... in fact, it has its greatest potential energy at this point. • While it is falling we know that the gravitational potential energy is being converted to kinetic energy, so that at the bottom (its reference point) it has no gravitational potential energy remaining. Now if you want to move the object from its position of low potential energy to high potential energy, you must do work on the object.

• •

• • • 1. 2. 3.

Fig.1.9 The work is necessary since you are adding gravitational potential energy to the object. You would the work done using… 𝑊𝑊 = 𝐹𝐹𝐹𝐹 and 𝐹𝐹 = 𝑚𝑚𝑚𝑚 𝑊𝑊 = 𝑚𝑚𝑚𝑚𝑑𝑑 ∵ 𝑎𝑎 = 𝑔𝑔 and 𝑑𝑑 = ℎ ∴ 𝑊𝑊 = 𝑚𝑚𝑚𝑚ℎ So the work you do to change the gravitational potential energy is… 𝐸𝐸𝑝𝑝 = 𝑚𝑚𝑚𝑚ℎ What is important to realize is that we are specifically looking at this in terms of how much work needs to be done to increase the object's potential energy, from an area where it has low potential to an area where it has high potential. This change in gravitational potential energy depends on… Mass of the object (𝐸𝐸𝑝𝑝 ∝ 𝑚𝑚) Gravitational field strength (𝐸𝐸𝑝𝑝 ∝ g) Height to which the object is moved (𝐸𝐸𝑝𝑝 ∝ ℎ)

Electric Potential Energy If we follow the same ideas that we did above, you might see that there are similarities between the gravitational potential energy described above and electric potential energy. Lets say you place a charge in an electric field and release it.

Fig.1.10

8 •

We expect the charge will begin to accelerate from an area of high potential energy, to an area of low potential energy. • This is because there is an electric force acting on the charge. • Notice that this is just like the object dropped in the discussion above; the difference is that here the reason is electrical in nature. If you want to move the charge from a position of low to high potential energy, you must do work on the object against the electric force. • Again, this sounds exactly like what we were talking about above when we lifted the ball back up against the gravitational force. • You would calculate it using… 𝐹𝐹 𝑊𝑊 = 𝐹𝐹𝐹𝐹 and 𝐸𝐸 = 𝑒𝑒 𝑞𝑞

𝑊𝑊 = 𝑞𝑞𝐸𝐸 𝑑𝑑 • This looks very much like the formula we used to figure out gravitational potential energy. • The change in the electric potential energy depends on... 1. Charge of the object (𝑊𝑊 ∝ 𝑞𝑞) 2. Electric field strength (𝑊𝑊 ∝ 𝐸𝐸 ) 3. Distance the object is moved parallel to the field lines (𝑊𝑊 ∝ 𝑑𝑑) Voltage • Voltage is the change in electric potential energy per unit charge.  When we were talking about gravitational potential energy, it would sort of be like saying “How much work do we have to do to lift up something against gravity per kilogram.” Something that has more mass would need more work to be done to it. • Now we are measuring the voltage... how much work is needed per Coulomb of charge. If something has more charge, it needs more work to move it. The unit for voltage could be given in 𝐽𝐽/𝐶𝐶, but instead it is a derived unit called the Volt (V) in honor of Alessandro Volta. • This means that we have a formula for voltage that looks like this... ∆𝑈𝑈 𝑉𝑉 =  𝑞𝑞 Where, 𝑉𝑉 = voltage (V) ∆𝑈𝑈 = electric potential energy (J) 𝑞𝑞 = charge (C)

10. Electron Volts Sometimes it is not convenient to measure energy in Joules. • This is quite often the case when we are dealing with charges like electrons moving through potential differences. • Instead, we can use a different unit, that although it is not part of the metric system, is still useful... the electron volt.  If we look at the formula for voltage and solve it for energy, we get... ∆𝑈𝑈 = 𝑞𝑞𝑞𝑞 • Typically we would just put in the value for the charge in Coulombs and the Voltage in Volts.  Instead, we will define one electron volt as the energy needed to move one electron through one volt of potential difference. ∆𝑈𝑈 = 𝑞𝑞𝑞𝑞 1 𝑒𝑒𝑒𝑒 = 1𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 (1𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉)  (1𝑉𝑉) 1 𝑒𝑒𝑒𝑒 = 1.60 × 10−19 𝐶𝐶 − 19 𝐶𝐶 1 𝑒𝑒𝑒𝑒 = 1.60 × 10−19 𝐽𝐽 If you need to do a calculation of energy in electron volts, you just figure out how many elementary charges you have multiplied by the voltage they moved through. Warning! When you do this, remember two things. First, “2e” does not mean 2 electrons, it mean 2 elementary charges. Second, the answer in electron volts is not a metric unit and cannot be used in any other formulas.

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•

Example : Determine how many electron volts are needed to move an alpha particle through 20V. We know that, an alpha particle has a 2e+ charge. ∆𝑈𝑈 = 𝑞𝑞𝑞𝑞 = 2𝑒𝑒(20𝑉𝑉) = 40𝑒𝑒𝑒𝑒

The potential at a distance r from a point charge q is given by 1 𝑞𝑞 𝑉𝑉 = 4𝜋𝜋𝜀𝜀 𝑟𝑟 0

11. Electric Dipole: A combination of two charges +𝑞𝑞 and −𝑞𝑞 separated by a small distance of d constitutes an electric

dipole. • Electric dipole moment: It is defined as a vector 𝑝𝑝⃗ = 𝑞𝑞𝑑𝑑⃗ where distance vector is the vector joining the negative charge to the positive charge. The line along the direction of the dipole moment is called the axis of the dipole.

12. Electric potential due to a dipole at a point P Point is at distance r from the centre of the diploe (𝑑𝑑/2) and theline joining the point P to the centre of the dipole make an angle θ with the direction of dipole movement (from – 𝑞𝑞 to +𝑞𝑞) Potential at P

1

𝑉𝑉 = 4𝜋𝜋𝜀𝜀

0

𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑟𝑟 2

where p is magnitude of electric dipole moment 𝑝𝑝 = 𝑞𝑞𝑞𝑞.

Any charge distribution that produces electric potential given by above formula is called an electric dipole. The two charge system can be expressed as 𝑝𝑝 = 𝑞𝑞𝑞𝑞. 13. Electric field due to a dipole

𝐸𝐸𝑟𝑟 =

1 2𝑝𝑝 cos 𝜃𝜃 4𝜋𝜋𝜀𝜀0 𝑟𝑟 3

𝐸𝐸𝜃𝜃 =

Resultant electric field at P

1 𝑝𝑝 sin 𝜃𝜃 4𝜋𝜋𝜀𝜀0 𝑟𝑟 3

𝐸𝐸 = √ (𝐸𝐸𝑟𝑟 2 + 𝐸𝐸𝜃𝜃 2 ) 1

= 4𝜋𝜋𝜀𝜀 (𝑝𝑝/𝑟𝑟³)√(3 𝑐𝑐𝑐𝑐𝑐𝑐²𝜃𝜃 + 1) 0

The angle the resultant field makes with radial direction OP (O is the centre point of the dipole axis and P is that at which electric field is being calculated) is α. 𝑡𝑡𝑡𝑡𝑡𝑡 𝛼𝛼 =

a.

𝐸𝐸 𝜃𝜃 𝐸𝐸𝑟𝑟

1

= 2 𝑡𝑡𝑡𝑡𝑡𝑡 𝜃𝜃 or

1

𝛼𝛼 = sin−1 �2 𝑡𝑡𝑡𝑡𝑡𝑡 𝜃𝜃�

 Special cases: θ = 0. In this case P is on the axis of the dipole. This position is called an end-on position. 1

𝑉𝑉 = 4𝜋𝜋𝜀𝜀 1

𝑉𝑉 = 4𝜋𝜋𝜀𝜀 1

0

𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑝𝑝

𝑟𝑟 2

2 0 𝑟𝑟

as 𝜃𝜃 = 0

𝐸𝐸 = 4𝜋𝜋𝜀𝜀 (𝑝𝑝/𝑟𝑟³)√(3 𝑐𝑐𝑐𝑐𝑐𝑐²𝜃𝜃 + 1) as 𝜃𝜃 = 0 0

10 1

b.

𝐸𝐸 = 4𝜋𝜋𝜀𝜀

2𝑝𝑝

3 0 𝑟𝑟

θ = 90°. In this case P is on the perpendicular bisector of the dipole axis. 1

General formula for 𝑉𝑉 = 4𝜋𝜋𝜀𝜀 1

𝑉𝑉 = 0

0

𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 𝑟𝑟 2

as θ = 90°.

General formula for 𝐸𝐸 = 4𝜋𝜋𝜀𝜀 (𝑝𝑝/𝑟𝑟³)√(3 𝑐𝑐𝑐𝑐𝑐𝑐²𝜃𝜃 + 1) as θ = 90°, Angle 𝛼𝛼 is given by

Therefore

0

𝐸𝐸 =

𝑡𝑡𝑡𝑡𝑡𝑡 𝛼𝛼 = 𝑡𝑡𝑡𝑡𝑡𝑡 90°/2 = ∞

1 𝑝𝑝 4𝜋𝜋𝜀𝜀0 𝑟𝑟 3

𝛼𝛼 = 90°.

14. Torque on an electric dipole placed in an electric field

If the dipole axis makes an angle θ with the electric field magnitude of the torque = | 𝛤𝛤| = 𝑝𝑝𝑝𝑝 𝑠𝑠𝑠𝑠𝑠𝑠 𝜃𝜃 In vector notation 𝛤𝛤⃗ = 𝑝𝑝⃗ × 𝐸𝐸�⃗

15. Potential energy of a dipole placed in a uniform electric field

dipole axis makes an angle 𝜃𝜃 with the electric field magnitude of the torque

Change in potential energy ∆𝑈𝑈 = 𝑈𝑈(𝜃𝜃) – 𝑈𝑈(90°) = −𝑝𝑝𝑝𝑝 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 = −𝑝𝑝⃗. 𝐸𝐸�⃗

If we choose the potential energy of the dipole to be zero when 𝜃𝜃 = 90° , above equation becomes 𝑈𝑈(𝜃𝜃) = −𝑝𝑝𝑝𝑝 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 = −𝑝𝑝⃗. 𝐸𝐸�⃗

16. Electric field inside a conductor

There can be no electric field inside a conductor in electrostatics. When electric field is applied from left to right some free electrons move toward the left creating a negative charge on the left surface. Due to which there will be positive charge on the right surface. Due to this charge buildup, coulomb attraction sets between these two charges and an electric field opposite in direction to the applied electric field is set up. The movement of free electrons continues till the applied electric field and the electric field due to redistribution of electrons are equal. Hence inside the conductor these two electric fields balance each other and there is no electric field inside a conductor in electrostatics. 17. Conductors, insulators, semiconductors

Conductors have free electrons that move throughout the body. When such a material is placed in an electric field, the free electrons move in a direction opposite to the field. The free electrons are called conduction electrons in this context. In insulators, electrons are tightly bound to their respective atoms or molecules. So in an electric field, they can't leave their parent atoms. They are insulators or dielectrics.

In semiconductors, at 0 K there are no free electrons but as temperature raises, small number of free electrons appear (they are able to free themselves from atoms and molecules) and they respond to the applied electric field.