7.0 Fields And Their Consequences Flashcards
What is a force field
A region in which an object feels a non contact force
Representation of a force field
By vectors in the direction of the field
Direction of field lines
From north to south or positive to negative
Similarities between gravitational and electrostatic forces
Both have inverse square laws, same field line characteristics, have potentials, and equipotential surfaces
Differences between gravitational and electrostatic forces
Masses always attract whereas charges may attract or repel
What is the inverse square law in fields
Where field strength and force is inversely proportional to the square of the distance
What fields do the inverse square law effect
gravitational and electric
What are field lines
A visual representation of a fields direction and strength
What does the width of field lines represent
The closer the field lines, the stronger the field
What is a uniform field
Field strength is constant, and the field lines are parallel and evenly spaced
What is a radial field
The field radiates outward or inward from a point source, and the field strength decreases with distance
What is potential energy in a field
The potential energy is the mass or charge has due to its position in a field
What is gravity
A universal force of attraction between all mass
What is gravitational potential
The work done per unit mass to move a small test mass from infinity to a point in a gravitational field
Why is gravitational potential zero at infinity
At infinite, the grav field becomes negligible, so the work done to move the test charge is zero
What is gravitational potential difference
The difference in gravitational potential between two points in the grav field
What is an equipotential surface in gravitational fields
Surfaces in a field where all points have the same gravitational potential
Properties of equipotential surfaces
No work is done to move along them and they are always perpendicular to field lines
What is significant about the negative in the grav potential equation
shows that the potential is always negative, as gravitational forces always attract
relation of orbital period and speed to radius
the square of the period if directly proportional to the cube of the radius
What is the total energy of an orbiting body
it is always negative. the more negative, the lower the orbit
What is escape velocity
The minimum speed an object must have to leave a grav field without requiring additional energy
What must occur for escape in grav fields
the objects total energy must reach zero
Letter for central body in grav fields
M
Letter for object in grav fields
m
What is a synchronous orbit
An orbit of period of that of the rotational period of the body it orbits
Characteristics of synchronous orbits
Object remains fixed relative to a point on the central mass
Conditions for a synchronous orbit
planet must have a fixed rotational period, orbital period = rotational period, must be circular
What is LEO
Low Earth Orbit. Between 200 - 2000 km
Uses of LEO
Observation, scientific research, communication, surveillance
Use of geostationary orbits
communication and weather monitoring
Define coulomb’s law
The magnitude of the force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between
What is the permittivity of free space
the value of the absolute dielectric permittivity of a vacuum
When calculating force between charges, what can we do
Treat air as a vacuum
Where is charge concentrated in a charged sphere
Radiating out from the centre
Compare the magnitude of gravitational and electrostatic
forces between subatomic particles
The electrostatic force is much greater than the gravitational force. Grav can be considered negligable in electrostatic calculation
Define electric field strength
the intensity of an electric field at a particular point
Define absolute electric potential
The work done per unit charge to move a small test charge from infinity to a point in an electric field
Why is electric potential zero at infinity
At infinite, the electric field becomes negligible, so the work done to move the test charge is zero
What is electric potential difference
The difference in electric potential between two points in the electric field
What is an equipotential surface in electric fields
Surfaces in a field where all points have the same electric potential
Define capacitance
The charge stored per unit potential difference
What is a dielectric
an electrical insulator that can be polarised by an applied electric field
How do dielectrics work
When in a field, the polarised molecules in the material rotating slightly to the opposite orientation to the field, creating another field that opposes the original one
What is the dielectric constant
How many times more efficient the dielectric is at storing charge than a vacuum
What is relative permittivity
the factor with which the electric field is decreased relative to vacuum
What is the area below a charge - pd graph
The energy stored by the capacitor
What is the time constant
RC
What is the time to halve of a charged capacitor
0.69RC
What is the force on a current carrying wire in a mag field
F = BIL when perpendicular to current
Fleming’s left hand rule
Thumb: Motion, Index finger: Field, Middle finger: Current
What is magnetic flux density
The strength of a magnetic field defined by the density of field lines
Definition of the tesla
The flux density that causes a force of 1 N on a 1 m wire carrying a current of 1 A at right angles to the field
What is the force on a charged particle moving in a mag field
F = BQv when field if perpendicular to velocity
Direction of positively charged particle in mag field
With field lines
Direction of negatively charged particle in mag field
Against field lines
Uses of mag fields
Circular path in cyclotron
Define magnetic flux
The number of field lines passing through a region of space
Define magnetic flux linkage
The magnetic flux for a multiple turn coil
What is electromagnetic induction
The process where an emf is induced from a change in flux on the material
Define Faraday’s law
The magnitude of the induced e.m.f is directly proportional to the rate of change in magnetic flux linkage
Define Lenz’s law
The induced e.m.f acts in such a direction to produce effects that oppose the change causing it
Applications of electromagnetic induction
Electric generators, Induction motors, Induction cooking
What are sinusoidal voltages and currents
The voltage and current of AC. Where they oscillate periodically
What is root mean square of the current and voltage
The average current and voltage of the AC graph
What is the UK mains voltage
230V
What is a transformer used for
To change the voltage of an AC
What is a step up transformer
Increases voltage by having more secondary coils
What is a step down transformer
decreases voltage by having less secondary coils
Production of eddy currents in a transformer
due to changing magnetic flux, free electrons in the iron move due to the induced emf, creating a current
Causes of inefficiencies in a transformer
Resistance in wires, induced eddy currents, poor insulation
Reducing inefficiencies in a transformer
Laminating core, use soft iron, use thick wires
Relationship between kinetic energy and gravitational potential
Ke = 0.5Vg
