16.5: Direct Calculation of Electrical Quantities from Charge Distributions (Summary)
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Key Terms
continuous charge distribution | total source charge composed of so large a number of elementary charges that it must be treated as continuous, rather than discrete |
infinite straight wire | straight wire whose length is much, much greater than either of its other dimensions, and also much, much greater than the distance at which the field is to be calculated |
linear charge density | amount of charge in an element of a charge distribution that is essentially one-dimensional (the width and height are much, much smaller than its length); its units are C/m |
surface charge density | amount of charge in an element of a two-dimensional charge distribution (the thickness is small); its units are C/m2 |
volume charge density | amount of charge in an element of a three-dimensional charge distribution; its units are C/m3 |
Key Equations
Coulomb’s law | →F12(r)=14πε0q1q2r212^r12 |
Superposition of electric forces | →F(r)=14πε0QN∑i=1qir2i^ri |
Electric force due to an electric field | →F=Q→E |
Electric field at point P | →E(P)≡14πε0N∑i=1qir2i^ri |
Field of an infinite wire | →E(z)=14πε02λzˆk |
Field of an infinite plane | →E=σ2ε0ˆk |
Dipole moment | →p≡q→d |
Torque on dipole in external E-field | →τ=→p×→E |
Electric field of a continuous charge distribution | →E=k∫dqˆrr |
Electric potential of a continuous charge distribution | VP=k∫dqr |
Summary
Calculating Electric Fields of Charge Distributions
- A very large number of charges can be treated as a continuous charge distribution, where the calculation of the field requires integration. Common cases are:
- one-dimensional (like a wire); uses a line charge density λ
- two-dimensional (metal plate); uses surface charge density σ
- three-dimensional (metal sphere); uses volume charge density ρ
- The “source charge” is a differential amount of charge dq. Calculating dq depends on the type of source charge distribution:
dq=λdl;dq=σdA;dq=ρdV.
- The field of continuous charge distributions may be calculated with →E=k∫dqˆrr.
- Symmetry of the charge distribution is usually key.
- Important special cases are the field of an “infinite” wire and the field of an “infinite” plane.
Electric Dipoles
- If a permanent dipole is placed in an external electric field, it results in a torque that aligns it with the external field.
- If a nonpolar atom (or molecule) is placed in an external field, it gains an induced dipole that is aligned with the external field.
- The net field is the vector sum of the external field plus the field of the dipole (physical or induced).
- The strength of the polarization is described by the dipole moment of the dipole, →p=q→d.
Calculating Electric Potential of Charge Distributions
- The potential of continuous charge distributions may be calculated with VP=k∫dqr.
Contributors and Attributions
Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).