Force Fields Objective: TSW understand and apply the concept of a force field by calculating the field, the force and motion of a particle in a field.

Gravitational Force Field: A property of space around a mass that causes forces on other masses. The gravitational force per unit mass exerted on a point mass (g = F g /m) F F F F g = the gravitational field The gravitational force is always in the same direction as the gravitational field The gravitational field is a vector. (Magnitude and direction)

The gravitational Force = (The mass) x (The Gravitational Field) The units for a gravitational field are N/kg

Let’s zoom in to our physics room: Constant Field FgFg

Example 1: Calculate the gravitational field 30000km from a planet with a mass of 4x10 26 kg and a radius of 20000km r 30000km

Example 2: How long will a 4kg object starting from rest take to accelerate 25meters through a constant gravitational field of 22N/kg? 4kg 25m F g = mg = (4)(22) = 88N

Electric Fields: A property of space around a charge that causes forces on other charges. The electric force per positive unit test charge (E = F E / q) _ + FEFE + FEFE + FEFE Use a positive test charge

+ + FEFE + FEFE + FEFE

The gravitational Force = (The mass) x (The Gravitational Field) The units for a gravitational field are N/kg The Electric Force = (The charge) x (The Electric Field) The units for a electric field are N/C

Things to Know Electric field lines go out of a positive charge and into a negative charge. Positive charges experience a force with the field. Negative charges experience a force against the field. The Electric Field inside a conductor is zero.

Example 3 What is the electric field at a point 25cm away from a point charge of 2µC? 2µC 25cm Away from the charge E

Example 4 A 5 gram mass with a charge of -3µC is released from rest in a constant field of 20000N/C directed in the positive x direction. Find its position and velocity after 4.0 seconds. 5g -3µC F E = qE Negative charges experience a force against the field

Example 6 What is the electric field at the origin? The electric field due to each charge must be calculated individually and then added together as vectors. E1E1 To the right (+) E2E2 E3E3 Up (+)

Now add the vectors together: E 1 =735N/C E 2 =720N/C E 3 =844N/C E 1 = E 3 = E 2 = E = 1455N/C 844N/C E θ

The electric field due to more than one point charge. + F - F

Two charged parallel plates The Electric field is constant between the two plates.

The Electric Field inside a conductor is zero E = 0N/C

An electron has a velocity of 6x10 5 m/s as it enters a constant electric field of 10000N/C as shown in the diagram below. How far will the electron travel in the x direction before striking one of the plates? 4cm _ v First the force and acceleration of the electron must be calculated. Now we have a simple projectile motion problem! The Electron will follow a parabolic path as shown 2.82 x m