7. Fields and their consequences Flashcards
What are the similarities and differences of gravitational and electrostatic forces?
Similarities: Both have inverse-square force laws that have many characteristics in common, eg use of field lines, use of potential concept, equipotential surfaces etc
Differences: gravitational forces act on masses and are always attract, but electrostatic forces act on charges and may attract or repel.
What is gravity?
A universal attractive force acting between all matter.
What is g?
Force per unit mass (g=F/m or in a radial field GM/r^2)
What is gravitational potential (V)?
The work done to bring a unit mass from infinity to that point.
Potential is 0 at infinity and energy is released moving it to that point so it’s always negative.
What is a field?
An area which an object experiences a non-contact force.
What is the gravitational potential difference?
The energy needed to move a unit mass between two points (where work done = mΔV)
What is an equipotential surface?
A line across which a constant potential exists.
No work is done when moving along an equipotential surface.
What about a graph of potential, V, against distance, r,?
V is inversely proportional to r in the fourth quadrant.
Gravitational field strength, g, at a certain distance is equal to minus the gradient at that point on the graph.
And the gravitational field strength, g, against distance, r, graph?
The gravitational potential difference is the are between two points under the curve.
And how would you derive Kepler’s third law?
T^2 is proportional to r^3:
centripetal force (where v is given in terms of circumference divided by time period) = gravitational force.
What is the energy system of orbiting satellites?
Total energy = kinetic energy + potential energy
(if height decreases, gravitational potential energy will decrease, and the satellite will travel at a higher speed, v=sqrt{GM/r})
What is the escape velocity?
The minimum velocity an object must travel at in order to escape the gravitational field at the surface of a mass.
When the object kinetic energy is equal to it’s gravitational potential energy.
Who, where, what, why and how are geostationary satellites?
Geostationary satellites have synchronous orbits (they have orbital periods equal to the rotational period of what they orbit, the Earth).
They always stay above the same point on the Earth because they orbit directly above the equator.
Because of this they are used to send TV and telephone signals so that aerials and transmitters don’t have to be moved.
And low orbit satellites?
These satellites have much lower orbits and therefore travel much faster so their orbital periods are much smaller.
Therefore, they need less powerful transmitters and could orbit across the entire Earth’s surface.
This makes them good for monitoring the weather, making observations, military purposes, (and communication because they travel so quickly).
How might the magnitude of gravitational and electrostatic forces in subatomic particles compare?
The magnitude of electrostatic forces is much grater than that of gravitational forces. This is because their masses are much smaller when compared to their charges.
What is the electric field strength, E,?
The force per unit charge experienced by an object in an electric field.
E=F/Q, therefore in uniform fields E=V/d and in radial fields E=Q/4πεr^2.
What direction does an electric field act?
Away from the positive charge and towards the negative charge.
What is the derivation for work done by moving a charged particle in a uniform field?
work done = Fd
E=F/Q => F=EQ
E=ΔV/d => d=ΔV/E
=> work done = QΔV
How can an electric field be used to find out which properties of a particle?
When a particle is fired at right angles to an electric field, if it is charged it will experience a constant electric force and therefore it will accelerate and follow a parabolic shape.
It’s charge will determine the direction it travels with respect to the field (positive charge = same direction as field and vice versa).
What is absolute electric potential, V,?
The potential energy per unit charge of a positive point charge at that point in a field.
It is greatest at the surface of a charge. As the distance increases, potential decreases so at infinity the electric potential is zero.
How does the charge, Q, affect the absolute electric potential, V, and how is this shown graphically?
If the charge is positive, potential is positive and force is repulsive (a V-r graph in the first quadrant showing a curve with asymptotes with both axises).
If the charge is negative, potential is negative and force is attractive (a V-r graph in the fourth quadrant showing a curve with asymptotes with both axises).
The gradient at a tangent to a V-r graph gives the electric field at that point.
What is the electric potential difference, ΔV,?
The energy needed to move a unit charge between two points.
Work done = QΔV
