Topic D - Fields

Kepler's First law

Each planet orbits the Sun along an elliptical path. The Sun is located at a focus of the elliptical orbit.

Kepler's Second law

The imaginary line joining the Sun and a planet sweeps equal areas of space in equal time intervals as the planet follows its orbit.

Orbital radius

The distance between the centre of the source of the gravitational field and the orbiting object.

Orbital period

The time it takes an object to complete one full orbit.

Kepler's Third Law

The square of the orbital period of a planet is directly proportional to the cube of the average distance of the planet to the Sun.

Newton's universal law of gravitation

Every object in the Universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

Gravitational Field

An area of space where a mass experiences a gravitational force.

Gravitational field lines

Lines that show the direction of the gravitational force acting on a mass at that point in a gravitational field.

Gravitational field strength

Gravitational force per unit mass experienced at a point by a small test mass in a gravitational field.

Gravitational potential energy

Energy stored in a mass due to its position within a gravitational field.

Why is gravitational potential energy negative?

Because work must be done to separate two masses against gravity. This negative value reflects the fact that gravity is an attractive force, pulling objects together and lowering their energy compared to when they are infinitely apart.

Gravitational potential

Gravitational potential energy per unit mass at a point in a gravitational field, which equates to the work done per unit mass to move a mass from infinity to a point in a gravitational field.

Equipotential lines/surfaces

Lines/surfaces that connect all points where the potential experienced is the same. No energy is required to move a charge to different parts of an equipotential line.

Rule of equipotential lines and field lines

They need to be perpendicular to each other.

Gravitational potential gradient

The steepness of the change in gravitational potential as a function of distance.

Orbital speed

The speed with which an object moves along its orbit.

Orbital energy

Sum of the kinetic energy and potential energy of an object moving along an orbit.

Escape Speed

The minimum speed needed to escape the gravitational field of a mass.

Transfer of electric charge by friction

The transfer of negative charges from one object to another due to friction which makes one object negatively charged and one object positively charged.

Transfer of electric charge by contact

When a positively charged rod touches a neutral sphere (or any object) the negative charges are transferred from the sphere to the rod (rod to sphere if rod is negatively charged).

Grounding (earthing)

Connecting an object to earth so that any excess charge can flow to the earth and neutralise the object in question.

Electrostatic induction

Charging a neutral object by bringing another, already charged object close to it, then removing the earth connection from the second object.

Coulomb's Law

The size of the force between two charged particles is proportional to the product of their charges and inversely proportional to the square of the distance between them.

Permittivity

The ease at which an electric field can pass through a substance. The higher the permittivity of the substance, the higher the resistance to the electric field.

Electric field

A region of space where an electric charge experiences a force.

Electric field strength

The force per unit charge experienced by a positive test charge.

Electric field lines

An aid for visualising electric fields.

Electric Field

The arrow on each field line shows direction as they point towards the negative charge and the density of the field lines indicates the field strength.

Radial field

Electric Field around a spherical charge.

Millikan oil drop experiment

An experiment that provided evidence for the quantisation of electric charge.

Terminal velocity

The constant speed that a freely falling object eventually reaches when the resistance of the medium prevents further acceleration.

Magnetic field lines

An aid for visualising magnetic fields.

Magnetic field

A region of space where a magnetic material experiences a force.

Solenoid

A coil of wire with many loops.

Electric potential energy

Energy associated with a charge due to its position in an electric field.

Electric potential

Work done per unit charge to bring a small, positive test charge from infinity to that point.

Motion of charged particles in uniform electric field

They have uniform acceleration and have a parabolic path when they enter at right angles to the electric field.

Motion of charged particle in uniform magnetic field

When it moves perpendicularly to the magnetic field, it experiences a force that is perpendicular to the velocity.

Work done on charged particles by uniform magnetic field

The field just changes the direction of the charged particle but there is no change in the kinetic energy as the speed and the radius of the particle path is constant.

Magnetic field strength

A measurement of the force on a north pole of a magnet at a particular point in a magnetic field.

Direction of the force on a straight current-carrying conductor in magnetic field

Perpendicular to the direction of current flow and the external magnetic field.

Magnitude of the force on a straight current-carrying conductor in a magnetic field

Proportional to the strength of the magnetic field, the magnitude of the current, and the length of the conductor.

Magnetic force between parallel current-carrying wires

Attractive if currents are flowing in the same direction; repulsive if currents are flowing in opposite directions.

Magnetic flux

The total number of magnetic field lines.

Magnetic flux density

A measure of the strength of a magnetic field.

Induction

A measurement of the force on a north pole of a magnet at a particular point in a magnetic field.

Ways to change magnetic flux

Moving magnet or coil, changing area and changing angle.

Lenz's Law

States that the direction of an induced current is such that it creates a magnetic field to oppose the change that created it.

Self-induction

A phenomenon where changing the current flowing through a coil induces an emf in the coil itself.

How does moving magnet or coil change magnetic flux and produce emf output?

There will be an increase in magnetic flux as the conductor crosses more magnetic field lines.

How does changing the area of the loop affect magnetic flux and produce emf output?

Increase in area of the loop increases magnetic flux.

How does changing the frequency of rotations (angular frequency) affect magnetic flux and produce emf output?

The change in magnetic flux follows a cosine graph over time but the maximum magnetic flux does not change if you increase the frequency of rotations.