PHYS 2120 Chapter 18: Electric Forces & Electric Fields

C18-13. State 5 rules for electric field lines.

1) Electric field lines BEGIN only on positive charges or infinity. 2) Electric field lines END only on negative charges or infinity. 3) Electric field lines never INTERSECT. 4) Electric fields are STRONGEST where field lines are closest together. 5) The NUMBER of electric field lines leaving or entering a charge is proportional to the magnitude of the charge. Lines begin and end on equal amounts of charge.

C18-2. State the charge acquired by an object from which electrons have been added or removed.

Adding N electrons to an object gives it a negative charge of: q = -Ne Removing N electrons from an object gives it a positive charge of: q = +Ne

C18-14. Describe the electrostatic charge distribution and electric field (E) inside and outside of a conductor.

All charges are confined to the surface(s) of the conductor. The electric field is zero at any point within a conductor. Just outside of the conductor, the electric field is perpendicular to the surface, and its magnitude depends on the local surface charge density σ (charge (C)/area (m²)) according to: E = |σ|/ε₀

C18-1. The electrical nature of matter is inherent in ______.

Atomic structure

Electric field lines are always directed _____ from positive charges and _____ negative charges.

Away; Toward

It is possible to transfer electric _____ from one object to another. The body that gains electrons has an excess of ______ charge, and the body that loses electrons has an excess of ______ charge.

Charge; Negative; Positive

C18-10. The surrounding ______ create the electric field (E) at a given point.

Charges (q)

C18-6 Describe the process of charging a metal object by contact.

Charging by contact is the process of giving a metal object a net electric charge by placing it in contact with a charged object. After this process is complete, the charges on the objects have the same sign.

C18-7 Describe the process of charging a metal object by induction.

Charging by induction is the process of giving a metal object a net electric charge by placing it near a charged object, and by temporarily grounding the opposite side of the metal object. After this process is complete, the charges on the objects have opposite signs.

C18-3. State the law of conservation of electric charge.

During any process, the net electric charge of an isolated system remains constant (is conserved).

C18-12. Apply Coulomb's law to derive the magnitude of the electric field a distance r away from a point charge q, and state the direction of this field.

E = F/|q₀| = k(|q||q₀|/r²)⋅1/|q₀| = k|q|/r² The electric field (E) points away from positive charges (+) and toward negative charges (-).

C18-15. Write the electric field (E) of a parallel plate capacitor using its area (A) and charge (q).

E = |σ|/ε₀ = |q|/ε₀A (σ = charge/area)

______ or lines of force provide a map of the electric field in the space surrounding electric charges.

Electric field lines

C18-5. Define "electrical conduction."

Electrical conduction is the movement of electric charge through a material.

C18-2. In nature, atoms are normally found with ______ numbers of protons and electrons, so they are electrically _____.

Equal; Neutral

C18-8. State the magnitude of the electrostatic force (F) between point charges or Coulomb's Law.

F = k(|q₁||q₂|/r²) k = Coulomb force constant = 8.99×10⁹ N⋅m²/C² |q₁| = charge 1 (C) |q₂| = charge 2 (C) r = distance between the charges (m)

C18-11. State the force (F) on a charge placed (q) in an electric field (E).

F = qE The direction of the force (F) is in the direction of the electric field (E) if the charge is positive (+), and is opposite the direction of the electric field (E) if the charge is negative (-).

C18-1. State the charges on the proton (p), the electron (e), and the neutron (n).

The charges on the proton, the electron, and the neutron are, respectively, +e, -e, and 0, where e = 1.60×10⁻¹⁹ C C is "Coulombs"

C18-10. Define the electric field (E).

The electric field at a point is the electrostatic force on a test charge (q₀) located at that point divided by the charge itself, and is measured in N/C. Its direction is the direction of the force on a positive test charge. E = F/q₀ Magnitude = E =F/|q|

C18-5. Not only can electric charge exist on an object, but it can also move ______ an object.

Through

Electric fields (E) from different sources add as ______.

Vectors

C18-9. Write the permittivity of free space (ε₀) in terms of the Coulomb force constant.

ε₀ = 1/4πk

C18-4. State the direction of the force between charges.

Like charges repel and unlike charges attract.

C18-5. Define "electrical insulator."

Materials that conduct electric charge poorly are called electrical insulators.

C18-5. Define "electrical conductor."

Materials that readily conduct electric charge are called electrical conductors.