Physics SAT2 - Relativity and Electricity

Reference Frame

A set of coordinates used to define events

Inertial Reference Frames

A reference frame where Newton’s Laws of Motion apply - either stationary or constant velocity

Relativistic Velocity

When an object travels close to the speed of light

Rest Frame

The frame of reference of the observer at rest

First Postulate of Einstein’s Theory of Relativity

The speed of light in a vacuum is an absolute constant (i.e. same for all inertial frames of reference)

Second Postulate of Einstein’s Theory of Relativity

The laws of physics are the same in all inertial reference frames

Proper Time

Time interval between two events when measured in the stationary reference frame (shortest time).

Dilated Time

Larger time, lorentz factor proportional to proper time

Simultaneity

Events that are simultaneous in one frame of reference may not be in another.

Proper Length

Length of an object or distance measured in its rest frame (longest length).

Contracted Length

Shorter length, lorentz factor inversely proportional to proper lengt

Direction of Length Contraction

Same direction as motion of object

Rest Mass

Mass of an object measured in its rest frame (lightest mass).

Relativistic Mass

Larger mass, lorentz factor proportional to rest mass

Relativistic Momentum

Momentum calculated with relativistic mass, always greater than ‘proper’ momentum

Lightspeed Travel Limitations

Time dilated until infinitely long, lengths contract to 0, and mass approaches infinity

Charge

A scalar property that causes objects to experience force when exposed to electromagnetic fields

Coulomb’s Law

Magnitude of force experienced is proportional to the magnitude of both charges and inversely proportional to the square of the distance between them

Electric Field

The space surrounding a charged particle where other charged objects experience a force

E Field Conventions

Arrows represent direction of E Field for a positive test charge, concentration of arrows represents field strength

Positive Hollow Conductor E Field

Outside has evenly distributed E field going outwards, inside has no E field

Curved Surface E Field

Charge redistributes to form equilibrium, causing build-up around sharper points and stronger E field

Corona Discharge

Strong E field at sharp points makes air particles form dipoles and attract, as air particle touches it exchanges charge, charged air particle is repelled

Electrostatic Shielding

Charged electric rod wishes to discharge, charge jumps to hollow conductor to travel to Earth’s surface, cage becomes charged but cage inhabitants are safe due to no internal charge.

Lightning

Charge accumulates in clouds during storms strong enough to cause charges in Earth to separate. Charge jumps to surface in the form of lightning

Lightning Rods

Reduces charge build-up in clouds and acts as path of least resistance for lightning due to corona discharge - ions neutralise clouds

Work

When energy is transferred or transformed

Electrical Potential Difference

The work done by/on the E field per unit charge in moving a charged particle

Electronvolt, eV

The work done on an electron as it moves through a potential difference of 1V

Parallel Plates

Produce a uniform electric field where charges experience constant force

Movement of Positive Charges

With or parallel to the field

Movement of Negative Charges

Against or anti-parallel to the field