Define gravitational potential, V.
The gravitational potential energy that a unit mass would have at a specific point in a gravitational field.
It indicates the potential energy per unit mass due to gravity.
What is gravitational potential difference?
The energy needed to move a unit mass between two points.
How is the orbital speed of an object related to the radius of its orbit?
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The greater the orbital radius, the slower the orbital speed.
In a circular orbit, a satellite’s orbital speed and distance above the mass it’s orbiting are constant.
In an elliptical orbit, a satellite will speed up as its orbital radius decreases (and slow down when its orbital radius increases).
This relationship is described by Kepler’s laws of planetary motion.
In an elliptical orbit, where is the planet in relation to the satellite?
The object the satellite is orbiting isn’t at the centre of the ellipse, it’s over to one side. This always happens in an elliptical orbit.
What energy(-ies) remains constant for an orbiting object?
The combined potential and kinetic energy
This principle is essential for understanding orbital mechanics.
What is escape velocity?
The minimum speed an unpowered object needs to escape a gravitational field.
This speed varies depending on the mass and radius of the celestial body.
How do you derive the formula for escape velocity?
Equate the objkect’s kinetic energy (lost) with the gravitational potential energy (gained) and rearrange for the velocity.
The r in the formula will be the distance between the centre of the planet and the object.
Name some applications of geostationary and low orbiting satellites.
- Communication
- Satellite connections for e.g. TV/telephone signals
- Weather monitoring
- Earth observation (e.g. imaging for mapping and spying)
Synchronous orbit + example
When an orbiting object has an orbital period equal to the rotational period of the object it is orbiting e.g. geostationary satellites
When using Coulomb’s law, how should air be treated? Why?
As a vacuum.
Air has a relatively low permittivity, which is very close to that of a vacuum - this means the electrostatic force between charges in air is very close to what it would be in a vacuum.
This simplifies calculations involving electric forces.
What does a graph of E against r for a radial field look like?
A curve with a negative gradient (first quadrant) that decreases with increasing distance and tends to zero; E is inversely proportional to r/d.
Define absolute electric potential, V.
The potential energy that a positive unit charge would have at a specific point in the electric field.
This is a measure of the work done in bringing a charge from infinity to that point.
What does a graph of (electric) V against r look like?
A curve that tends towards zero (x-axis) with increasing distance.
The curve can have a positive or negative gradient (be in the first or fourth quadrants) depending on the type of charge / force.
What does a graph of (electric) V against r look like for a positive charge?
A curve that tends towards zero (x-axis) with increasing distance.
The curve has a negative gradient (is in the first quadrant) as the force is repulsive so the electric potential is positive.
Remember electric potential is defined for a positive unit charge.
What does a graph of (electric) V against r look like for a negative charge?
A curve that tends towards zero (x-axis) with increasing distance.
The curve has a positive gradient (is in the fourth quadrant) as the force is attractive so the electric potential is negative.
Remember electric potential is defined for a positive unit charge.
What does a graph of (electric) V against r look like for an attractive force?
A curve that tends towards zero (x-axis) with increasing distance.
The curve has a positive gradient (is in the fourth quadrant) as the point charge is negative so the electric potential is negative.
Remember electric potential is defined for a positive unit charge.
How do you derive the equation for the work done in moving a charge between charged plates?
E = ΔV / d = F / Q
which rearranges to QΔV = Fd = ΔW
What are the similarities and differences between gravitational and electric fields?
- Both have field lines
- Both exert non-contact forces
- Gravitational fields are always attractive
- Electric fields can be attractive or repulsive
- A uniform spherical mass/point charge produces spherical equipotentials
- Newton’s & Coulomb’s Laws both have force inversely proportional to 1/r^2
These concepts are fundamental in physics, especially in understanding forces at subatomic levels.
