Electric Field Measurement Integrated Science II

Fundamental Forces: Gravity Electromagnetism Weak Strong (Nuclear) Gravity -An attractive force that acts between masses -SI unit of mass: [m] = kg (kilogram) -Universal gravitational constant: G = 6.67384 × 10-11 m3 kg-1 s-2

Electric Charge Two types: positive and negative Like charges repel Opposite charges attract SI unit of charge: [q] = C (Coulomb) Analogy: Mass is to the gravitation force as electric charge is to the electric force

Electric Force (Coulomb Force) -Acts between electric charges (q) Electromagnetism can be broken into two parts: Electric (We’ll study in today’s lab) Magnetic (We’ll study in a later lab) Fundamental Forces: Gravity Electromagnetism Weak Strong (Nuclear) Electric Force (Coulomb Force) -Acts between electric charges (q) -Charges can be positive or negative -Can be attractive (opposite charges) or repulsive (same type charges) -Universal constant: k  = 8.99 ×109 Nm2/C2

Electric field lines for isolated positive and negative (point) charges: Point charges placed together: Parallel-plate capacitor:

Connection between charge and electric force For charges present in an electric field E: -Positive charges experience a force along the same direction as E -Negative charges experience a force opposite to the direction of E

U = electric potential energy V = electric potential (voltage) Electric potential (voltage) – Difference in electric potential energy per unit charge between two points U = electric potential energy V = electric potential (voltage) q = charge SI unit of voltage: V (volt) Equipotential lines (lines of the same voltage) around positive and negative point charges

-Contour lines show points at same altitude Equipotential map Topographical map -Contour lines show points at same altitude Equipotential map -Contour lines show points at same voltage Place a ball anywhere on this map. In which direction will it move? Equipotential lines show us the direction of the force on a positive charge -The electric field: F=qE

Relationship between electric field and voltage Mathematics: Translation: Electric fields always point in the direction of steepest decrease in V Two important rules: 1) E-lines always perpendicular to equipotential lines 2) E-lines always point from higher potential to lower potential

Analogy: Top of ramp: -All of the ball’s energy is stored as gravitational potential energy -Kinetic energy is zero Bottom of ramp: -All initial energy is converted to kinetic energy (motion) -Gravitational potential energy is zero Voltage difference in battery – Height difference of ramp Force of gravity moves ball – Electric force moves charges

Today’s lab: -We will place two conductors on a sheet of conductive paper at different voltages (3 cases) -The voltage difference will generate an electric field in the region between the conductors -You will measure equipotential lines to determine the electric field map in the region surrounding the conductors Two important rules: 1) E-lines always perpendicular to equipotential lines 2) E-lines always point from higher potential to lower potential