Electric Field and Potential
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Questions and Answers

What happens when a conductor is placed in an external electric field?

  • The free charge carriers move and the electric field due to induced charges opposes the external field (correct)
  • The electric field within the conductor increases
  • The electric field within the conductor becomes zero instantly
  • The electric field within the conductor remains unchanged

    • What is the net electrostatic field in a conductor in a static situation?

  • Equal to the external field
  • Zero (correct)
  • Greater than the external field
  • Less than the external field

    • What happens to the molecules of a dielectric in an external electric field?

  • They become negatively charged
  • They are stretched or re-oriented to induce a dipole moment (correct)
  • They remain unchanged
  • They become positively charged

    • What is the effect of the induced field in a dielectric?

    <p>It reduces the external field</p> Signup and view all the answers

    What is the characteristic of a polar molecule?

    <p>The centres of positive and negative charges are separated</p> Signup and view all the answers

    What is the dipole moment of a non-polar molecule?

    <p>Zero</p> Signup and view all the answers

    Which of the following molecules is an example of a non-polar molecule?

    <p>Oxygen (O2)</p> Signup and view all the answers

    What determines the extent of the effect of the external field on a dielectric?

    <p>The nature of the dielectric</p> Signup and view all the answers

    What is the main reason for the development of a net dipole moment in a dielectric with polar molecules in an external field?

    <p>Alignment of individual dipole moments with the field</p> Signup and view all the answers

    What is the direction of the net dipole moment in a dielectric in an external electric field?

    <p>In the direction of the external field</p> Signup and view all the answers

    What is the term for the dipole moment per unit volume in a dielectric?

    <p>Polarisation</p> Signup and view all the answers

    What is the electric susceptibility of a dielectric medium?

    <p>A constant characteristic of the dielectric</p> Signup and view all the answers

    In the absence of an external field, what is the total dipole moment of a dielectric with polar molecules?

    <p>Zero</p> Signup and view all the answers

    What is the limitation of storing charge on a capacitor?

    <p>There is a limit to the amount of charge that can be stored without significant leaking.</p> Signup and view all the answers

    What is the formula for polarisation in a linear isotropic dielectric?

    <p>P = ε0 χe E</p> Signup and view all the answers

    What is the reason for the alignment of individual dipole moments with the external field in a dielectric?

    <p>Dipole potential energy in the external field</p> Signup and view all the answers

    What is the unit of capacitance commonly used in practice?

    <p>Millifarad</p> Signup and view all the answers

    What is the function of a capacitor in most ac circuits?

    <p>To perform important functions</p> Signup and view all the answers

    What is the factor that tends to disrupt the alignment of individual dipole moments with the external field in a dielectric?

    <p>Thermal energy</p> Signup and view all the answers

    What is the configuration of a parallel plate capacitor?

    <p>Two large plane parallel conducting plates separated by a small distance</p> Signup and view all the answers

    What is the effect of a dielectric medium between the plates of a capacitor?

    <p>It increases the capacitance from its vacuum value</p> Signup and view all the answers

    What is the formula for capacitance applicable to?

    <p>Any type of capacitor</p> Signup and view all the answers

    What is the relationship between the area of the plates and the separation between them in a parallel plate capacitor?

    <p>The area of the plates is larger than the separation between them</p> Signup and view all the answers

    What is the charge on the two plates of a parallel plate capacitor?

    <p>Q and –Q</p> Signup and view all the answers

    Why does electrostatic potential follow the superposition principle?

    <p>Because it is related to the electric field.</p> Signup and view all the answers

    What is the physical significance of the quantity V in equation 2.9?

    <p>It represents the electrostatic potential due to the dipole.</p> Signup and view all the answers

    What is the significance of the quantities r1 and r2 in equation 2.9?

    <p>They are the distances of the point P from the charges q and –q.</p> Signup and view all the answers

    Why do we take the origin at the centre of the dipole?

    <p>To make the calculation simpler.</p> Signup and view all the answers

    What is the physical significance of the quantity ε0 in equation 2.9?

    <p>It is the permittivity of free space.</p> Signup and view all the answers

    What is the expression for the potential due to the dipole?

    <p>V = q / (4πε0r1) + (-q) / (4πε0r2)</p> Signup and view all the answers

    What is the energy gained by an electron when accelerated by a potential difference of 1 volt?

    <p>1.6 × 10–19 J</p> Signup and view all the answers

    What is the unit of energy defined as 1.6 × 10–19 J?

    <p>1 electron volt</p> Signup and view all the answers

    What is the work done in bringing the charge q1 from infinity to r1 in an external field?

    <p>q1 V(r1)</p> Signup and view all the answers

    What is the expression for the work done on q2 against the field due to q1?

    <p>q1q2 / 4πεr12</p> Signup and view all the answers

    What is the total work done in bringing the charge q2 to r2 in an external field?

    <p>q2 V(r2) + q1q2 / 4πεr12</p> Signup and view all the answers

    What is the relationship between 1 keV and 1 eV?

    <p>1 keV = 10^3 eV</p> Signup and view all the answers

    Study Notes

    Electric Field and Potential

    • The electric field obeys the superposition principle, and therefore, the electrostatic potential also follows the superposition principle.
    • The potential due to a dipole is the sum of the potentials due to the charges q and –q.

    Electric Potential and Work

    • The potential energy of a charge q at a position r in an external field is given by qV(r), where V(r) is the external potential at the point r.
    • The unit of energy, 1 electron volt (eV), is defined as 1.6 × 10^–19 J, which is the energy gained by an electron when accelerated by a potential difference of 1 volt.

    Potential Energy of a System of Two Charges

    • The potential energy of a system of two charges q1 and q2 in an external field is the sum of the work done in bringing each charge from infinity to its respective position.
    • The work done in bringing q2 to its position includes the work done against the external field and the field due to q1.

    Electric Field in a Conductor and a Dielectric

    • In a conductor, the free charge carriers move and adjust themselves to cancel the external electric field, resulting in a net electrostatic field of zero within the conductor.
    • In a dielectric, the external field induces a dipole moment by stretching or re-orienting the molecules, resulting in a net charge on the surface that opposes the external field.

    Polar and Non-Polar Molecules

    • Non-polar molecules have no permanent dipole moment due to their symmetrical structure, whereas polar molecules have a permanent dipole moment due to the separation of their centers of positive and negative charges.
    • A dielectric with polar molecules develops a net dipole moment in an external electric field due to the alignment of individual dipoles with the field.

    Polarisation

    • The dipole moment per unit volume of a dielectric is called polarisation (P) and is denoted by the equation P = ε0 χe E, where χe is the electric susceptibility of the dielectric medium.

    Capacitors

    • A capacitor is a key element of ac circuits, used for storing charge and has important functions described in Chapter 7.
    • The capacitance of a capacitor is limited by the amount of charge that can be stored without significant leaking.
    • Common units of capacitance are its sub-multiples, such as 1 mF, 1 nF, and 1 pF.

    The Parallel Plate Capacitor

    • A parallel plate capacitor consists of two large plane parallel conducting plates separated by a small distance, with the intervening medium being vacuum or a dielectric.
    • The effect of a dielectric medium between the plates increases the capacitance from its vacuum value.
    • The dielectric constant of a substance can be viewed as a definition of the increase in capacitance when the dielectric is inserted fully between the plates of a capacitor.

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