Electrostatics & Magnetism

Question 1

Charges

Answer 1

• Opposite charges exert attractive forces.
• Like charges exert repulsive forces.
• Static Electricity (Static Charge Buildup) is more significant in drier air; lower humidity makes it easier for charge to become and remain separated.

• SI unit of charge is Coulomb (C).
• e = 1.60 × 10⁻¹⁹ C.
• Proton (q = +e); Electron (q = -e).

Question 2

Insulators and Conductors

Answer 2

• Insulator: Will not easily distribute a charge over its surface and will not transfer that charge to another neutral object, like another insulator.
• Typically nonmetal; used as dielectric materials in capacitors or to isolate electrostatic experiments from the environment to prevent grounding.

• Conductor: Will distribute charges evenly on its surface and are able to transfer and transport those charges.
• Typically metal or electrolyte (ionic) solutions; used in circuits or electrochemical cells.

Question 3

Coulomb’s Law

Answer 3

• Quantifies the magnitude of the electrostatic force (Fₑ) between two charges (q) using the distance (r) between them and Coulomb’s constant (k).

• Fₑ = (kq₁q₂)/r²

Question 4

Electric Field

Answer 4

• Electric fields are produced by source charges (Q); when a test charge (q) is placed in an electric field (E), it will experience electrostatic force Fₑ = qE.

• E = Fₑ/q = kQ/r²

• Units are newtons per coulomb.
• The direction of the electric field is given as the direction that a positive test charge would move in the presence of the source charge.
• Positive source charges have electric field vectors that radiate outward; negative source charges have electric field vectors that radiate inward.
• If the test charge within a field is positive, then the electrostatic force and the electric field vector will point in the same direction; if the test charge is negative, then the electrostatic force and the electric field vector will point in opposite directions.
• The denser the field lines around a charge, the stronger the electric field.

Question 5

Electric Potential Energy

Answer 5

• Electric Potential Energy: The amount of work necessary to move a test charge from infinitely far away to a point within an electric field surrounding a source charge; obtained from W = F × d = Fₑ × r.

• U = kQq/r

• For opposite charges experiencing attractive forces, EPE will become increasingly negative (and thus become smaller in magnitude) as the distance between is reduced, implying an increase in stability.
• For like charges experiencing repulsive forces, EPE will increase as the distance between is reduced, implying a decrease in stability.

Question 6

Electric Potential

Answer 6

• Electric Potential: Ratio of a charge’s electrical potential energy to the charge itself; obtained from V = U/q.

• V = kQ/r

• Units are Volts (V).
• 1 V = 1 J/C.

• Potential Difference (Voltage): Between point 1 and point 2, the potential difference is as follows.
• ΔV = V₂ - V₁ = W₂₁/q
• W₂₁ is the work needed to move a test charge q through an electric field from point 1 to point 2.