MCAT Physics: Electrostatics and Magnetism

Objectives

• Discuss charges
• Compare and contrast insulators and conductors
• Discuss Coulomb's Law; define electric field
• Discuss electrical potential energy
• Discuss electrical potential
• Consider special cases of electrostatics: equipotential lines and electric dipoles
• Discuss magnetism: define magnetic fields and forces

Introduction to Electrostatics

• Electrostatics: Study of stationary charges and the forces created by and acting upon these charges.
• Electricity is fundamental in forces between atoms/molecules, metabolic processes.

Charges

• Types of Charge: Positive and negative charges.
  • Like charges repel; unlike charges attract.
• Conservation of Electric Charge: Net electric charge in any process is zero.
• Atomic Structure: Atoms have a positive nucleus (protons) and negative electrons.
• Ions: Atoms with net charge due to electron gain/loss.
• Charging Objects: Transfer of electrons (e.g., by rubbing).
• SI Unit of Charge: Coulomb (1.6 x 10^-19 C).

Insulators and Conductors

• Conductors: Allow free electron flow (e.g., metals).
• Insulators: Resist electron flow (e.g., rubber, glass).

Coulomb's Law

• Describes force between two charges at rest.
• Coulomb's Law Equation: [ F = k \frac{q_1 q_2}{r^2} ]
  • ( k ): Coulomb's constant ( (8.99 \times 10^9 \text{Nm}^2/\text{C}^2) )
  • Force is proportional to charge product and inversely proportional to square of distance.
• Direction of force determined by charge interaction (attraction/repulsion).

Electric Field

• Definition: Space around a charge where it exerts force on other charges.
• Electric Field Equations: [ E = \frac{F}{q} \quad \text{or} \quad E = \frac{kQ}{r^2} ]
• Field lines represent direction and strength of electric field.
  • Lines radiate outward from positive, inward to negative.

Electrical Potential Energy

• Definition: Potential energy due to position in electric field.
• Equation: [ U = k \frac{Qq}{r} ]
• Opposite charges: Negative potential energy.
• Like charges: Positive potential energy.
• Units: Joules (J).

Electrical Potential

• Definition: Electric potential energy per unit charge.
• Equation: [ V = \frac{U}{q} \quad \text{or} \quad V = \frac{kQ}{r} ]
• Difference in potential across space points is called voltage.
• Varies depending on source charge and distance.

Recap

• Coulomb's Law: Magnitude of electrostatic force between charges.
• Electric Field: Ratio of force exerted on test charge to magnitude.
• Electric Potential Energy: Work to bring charge from infinity.
• Electric Potential: Potential energy per unit charge.
• Next session to cover special cases and magnetism.