Gravitational and electric fields Flashcards
What do gravitational field lines represent?
The direction of the force that would affect a mass placed at that point in a gravitational field
Newtons law of gravitation
-The force is directly proportional to the product of the masses
-The force is inversely proportional to the distance between the centres squared
-The force is always attractive
Gravitational force equation
F=GMm/r^2
Define gravitational field strength + units
The gravitational force per unit of mass.
Nkg^-1 or ms^-2
Is gravitational field strength scaled or vector?
Vector
Gravitational field strength equation
g=GM/r^2
g=F/m
Define gravitational potential + units
The work done to move a 1kg mass from infinity to that point in the field.
Jkg^-1
Why is gravitational potential negative?
Infinity is where potential is 0, but work has to be done to get there starting from negative.
What is an equipotential surface?
Where gravitational potential is all the same
Gravitational potential equation
V=-GM/r
Remember negative
Why is no work done when a mass moves along an equipotential in a gravitational field?
The mass is moving perpendicular
W=Fd and d=0 so no work done
What is Keplers third law?
r^3=GMT^2/4π^2
What is the escape velocity?
The minimum speed an unpowered object needs to leave a gravitational field
Low Earth Orbit examples
Space station
Weather satellites
Spy satellites
Because detail pictures can be taken
Also bigger satellites available as less energy required
Low earth orbit time period
2 hours
Low Earth Orbit altitude
200-2000km
Geostationary orbit example
Communication satellites
Geostationary orbit time period
24 hours same as earth
Geostationary orbit altitude
35000km
In line with equator
Escape velocity equation and how to derive
1/2mv^2=GMm/r
V=sqrt(2GM/r)
Velocity of orbit equation
V=sqrt(GM/r)
Polar orbit
Earth rotates under satellite
So every time the satellite orbits round it passes over a new part of earths surface
This allows the satellite to scan the whole earths surface
Ideal for weather satellites available
What is the main difficulty with a low Earth orbit?
Multiple tracking stations needed
Each with a moveable dish
To track the satellite as it passes over
What do electric fields lines represent?
The direction and magnitude of a force acting on a positive charge in an electric field
State Coulomb’s law
For a force between 2 point charges
The force is proportional to the product of the charges
The force is inversely proportional to the distance between the centres squared
Electric force equation
F=1/4πϵ x Qq/r^2
What direction to electric field lines go in?
From positive to negative
Positive to less positive
Etc
Define electric field strength + units
The electrostatic force per unit positive charge
NC^-1 or Vm^-1
Electric field strength equation
General -> E=F/Q
Radial field -> 1/4πϵ x Q/r^2
Uniform field -> E=V/d
Define electric potential and state its units
The work done per unit charge in moving the positive charge from infinity to that point in the field
JC^-1
Why is escape velocity higher in practice?
Air resistance means extra energy is required
How to derive Keplers law?
GMm/R^2 = mw^2R
W=2pi/T
Why might gravitational field lines converge?
Due to an area of greater density
Which makes the field stronger
