IB HL Physics (2023) - Fields

Point Mass

a body with all its mass concentrated at an infinitesimally small point called the centre of gravity

Newtons Law of Gravitation

The force of attraction between two masses is proportional to each mass and inversely proportional to the distance between their centres squared

Gravitational Field Strength

The gravitational force per unit mass experienced by a small point mass placed within the field

Types of Gravitational Fields

• Radial - lines coming out of a spherical object from the surface

• Uniform - equidistant lines meeting perpendicularly with a flat horizontal surface

Kepler’s First Law

Planets move on elipses with the sun at one of the foci of the elipse

Kepler’s Second Law

The line joining a planet and the sun sweeps equal areas in equal times

Kepler’s Third Law

The square of the orbital period is proportional to the cube of the semi major axis

Gravitational Potential Energy

The work done to move mass from infinity to that point in the field

Total Orbital Energy

The total energy of an orbit is equal to the sum of kinetic and potential energy

Atmospheric Drag

1. Satellite in low orbit experiences drag

2. E_T and r decrease

3. V increases because V ∝ 1/r

4. drag increases because F_d ∝ V²

Gravitational Potential

The work done required per unit mass to move a test mass from infinity to a point in the field

Equipotential surfaces (Gravity)

A set of points perpendicular to the field lines which have the same gravitational potential

Potential Gradient (G)

the gradient of a potential vs position graph will always be equal to the gravitational field strength

Conservation of Charge

(net) charge cannot be created or destroyed and can only be transferred

Coulomb’s law

the force of attraction or repulsion between two charges is proportional to each charge and inversely proportional to the distance between their centres squared

Electric field

A region where a charge experiences a force

(Arrows on electric field diagrams show the motion of a positive test charge)

Electric Field Strength

The force per unit charge acting on a positive test charge placed at that point

Electric Field inside inside a sphere

The electric field inside a hollow or conducting field is 0

Fields in Parallel Plates

• Field lines equally spaced and parallel in the middle

• ‘edge effects‘ cause the lines to bulge around the sides of the plate

Milikan’s Oil drop experiment

1. drops of oil fired from an atomiser in between two parallel plated at a specific distance with a known adjustable voltage

2. the drops are reviewed with microscope to measure their velocity

3. when no voltage is applied drops fall at terminal velocity

4. the voltage is adjusted to cause the drop to stay suspended in air

5. charge can be worked out from the voltage mass and velocity

Electric Potential Energy

• The work done to move a charge from infinity to a point in a field

  • EPE is negative with an attractive force

  • EPE is posotive with a repulsive force

Electric Potential

The work done per unit charge in moving a positive test charge from infinity to the point in the field

Equipotential surfaces (Electricity)

A set of points perpendicular to the field lines which have the same Electric potential

Electric Potential Gradients

the gradient of a potential vs distance graph will always be equal to the electric field strength