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Questions and Answers
What happens when a conductor is placed in an external electric field?
What is the net electrostatic field in a conductor in a static situation?
What happens to the molecules of a dielectric in an external electric field?
What is the effect of the induced field in a dielectric?
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What is the characteristic of a polar molecule?
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What is the dipole moment of a non-polar molecule?
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Which of the following molecules is an example of a non-polar molecule?
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What determines the extent of the effect of the external field on a dielectric?
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What is the main reason for the development of a net dipole moment in a dielectric with polar molecules in an external field?
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What is the direction of the net dipole moment in a dielectric in an external electric field?
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What is the term for the dipole moment per unit volume in a dielectric?
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What is the electric susceptibility of a dielectric medium?
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In the absence of an external field, what is the total dipole moment of a dielectric with polar molecules?
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What is the limitation of storing charge on a capacitor?
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What is the formula for polarisation in a linear isotropic dielectric?
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What is the reason for the alignment of individual dipole moments with the external field in a dielectric?
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What is the unit of capacitance commonly used in practice?
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What is the function of a capacitor in most ac circuits?
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What is the factor that tends to disrupt the alignment of individual dipole moments with the external field in a dielectric?
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What is the configuration of a parallel plate capacitor?
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What is the effect of a dielectric medium between the plates of a capacitor?
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What is the formula for capacitance applicable to?
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What is the relationship between the area of the plates and the separation between them in a parallel plate capacitor?
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What is the charge on the two plates of a parallel plate capacitor?
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Why does electrostatic potential follow the superposition principle?
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What is the physical significance of the quantity V in equation 2.9?
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What is the significance of the quantities r1 and r2 in equation 2.9?
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Why do we take the origin at the centre of the dipole?
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What is the physical significance of the quantity ε0 in equation 2.9?
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What is the expression for the potential due to the dipole?
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What is the energy gained by an electron when accelerated by a potential difference of 1 volt?
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What is the unit of energy defined as 1.6 × 10–19 J?
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What is the work done in bringing the charge q1 from infinity to r1 in an external field?
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What is the expression for the work done on q2 against the field due to q1?
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What is the total work done in bringing the charge q2 to r2 in an external field?
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What is the relationship between 1 keV and 1 eV?
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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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Description
Learn about electric field and potential, including the superposition principle and electric potential energy. Understand how electric potential is related to work and energy.
