1. Electric Field Measurement
Integrated Science II

2. 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 = × m3 kg-1 s-2

3. 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

4. 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

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

6. 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

7. 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

8. -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

9. 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

10. 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

11. 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