electric field

Meaning: A charge changes the space around it, and any other charge placed there feels a force. The electric field E describes that influence at each point, defined as the force a small positive test charge would feel, divided by its charge. Field lines point away from positive charges and toward negative charges. Closer lines mean a stronger field. Inside a conductor in electrostatic equilibrium, the field is zero. Between two oppositely charged parallel plates, the field is typically uniform.

Equation: E = F/q = kQ/r² (point charge); E = V/d (uniform field between plates). E is the electric field in N/C (equivalently V/m), F is the electric force in N, q is the test charge in C, k is Coulomb's constant (about 9.0 × 10⁹ N·m²/C²), Q is the source charge in C, r is the distance from the source in m, V is the potential difference in V, and d is the plate separation in m.

On the MCAT: Passages often use parallel plates or one or two point charges. You may be asked to find the direction of the force on a charge (with E for positive charges, against E for negative ones), compare field strength at two distances, or find where the net field from two charges is zero. Field vectors from several sources add by vector addition.

Common confusion: The field direction is defined by a positive test charge, so an electron accelerates opposite to E. Also, the field belongs to the source and exists whether or not a test charge is present.