9. Gravitational and Electric Fields Flashcards
What is a force field?
A region in which a body experiences a non-contact force
What is a gravitational field?
An area where an object with mass experiences a force of attraction
What do field lines tell you?
The direction of the force acting on a mass at that point in the field and they indicate the shape of the field
What does the density of fields lines indicate?
The strength of the field
What is a radial field?
A field where all the field lines meet at the centre of mass, implying the strength of the field weakens the further you are from the object
What type of field does the Earth have?
A radial feild
What is a uniform field?
A field where all the field lines are parallel and equally spaced, implying the strength of the field is constant
What is the Earth’s field like close to the surface?
Uniform (almost)
What does the inverse square law tell us?
The strength of the field is inversely proportional to the square of the distance from the centre of mass
What is Newton’s law of gravitation?
- Gravitational force between two objects is proportional to the product of the two masses
- It is inversely proportional to the distance between their centres squared
- It is always attractive
What is gravitational field strength?
The force on an object per unit mass (vector)
What is gravitational potential?
The energy required (work done) to move a 1kg mass from infinity to that point in the field (scalar)
What are equipotential lines/surfaces?
Where the gravitational potential is the same value
Where is gravitational potential 0?
At infinity
Why is gravitational potential negative?
Infinity is where potential is zero so work must be done against the gravitational field to get there, hence the need to start at a negative value
What is the gradient of gravitational potential against distance graph at a point?
The gravitational field strength at that point
What needs to be done to move between equipotential lines?
Work
Does a satellite in orbit do work?
No work is done because it is travelling along an equipotential line, so the gravitational potential difference will be zero
Which direction does the force act on a satellite?
The force acts perpendicular to the direction of travel due to circular motion
What is the work being done to move between equipotential lines equal to?
The change in potential energy
How can gravitational potential difference be found from a graph of g against r?
Area under the graph
State the altitude, time period and uses of a Low Earth Orbit?
- Between 200 - 2000 km altitude
- Approximately 2 hour time period
- Used by the space station, weather satellites and spy satellites
State the altitude, time period and use of Geostationary Orbits?
- Only 35,000 km altitude
- Exactly 24 hour time period
- In line with the equator
- Used by communication satellites
Why is an LEO ideal for satellites that take pictures (spy and weather)?
The orbit is close to the Earth’s surface, allowing for detailed pictures
Why can LEO satellites be bigger?
It requires the least energy to get to so satellites can be bigger
What inclination to the equator can an LEO be?
Any inclination from 0˚ directly above the equator (equatorial orbit) up to 90˚ (polar orbit)
What is a polar orbit? What type of satellites is it ideal for?
- The Earth continues to rotate under the satellite meaning that every time the satellite orbits round, it passes over a new part of the Earth’s surface
- Ideal for weather satellites
What is the main difficulty with satellites in LEO?
You need multiple tracking stations, each with a moveable dish to track the satellite as it passes over head
What is a geostationary orbit? What type of satellite is it ideal for?
- The satellite has the same time period as the Earth, meaning the satellite remains about the same point on the Earth’s surface
- Communication satellites
What is the main advantage and disadvantage of a geostationary orbit?
- Advantage: tracking dish doesn’t need to move
- Disadvantage: satellite must be directly above the equator
What is a synchronous orbit?
Where an orbiting object has an orbital period equal to the rotational period of the body it is orbiting
What is a geosynchronous orbit?
An orbit with the same time period as the period of the Earth’s rotation (24 hours)
What is a geostationary orbit in relation to a geosynchronous orbit?
A geosynchronous orbit which is also directly above the equator
How does an object horizontal velocity change as a satellite is moved closer to a planet?
The closer a satellite is to the planet, the faster it must go because the gravitational force is much stronger so you need greater horizontal velocity
Does kinetic and gravitational potential energy change for a satellite in circular orbit? Why?
Speed is constant and altitude is constant, meaning the kinetic energy and the gravitational potential energy of the satellite remains constant
Does kinetic and gravitational potential energy change for a satellite in elliptical orbit? Why?
- Total energy of the satellite is still the sum of the kinetic and gravitational potential energies and remains constant
- At closest approach GPE will be lowest, therefore, to conserve energy, KE must increase
- The opposite happens at the greatest distance
What is used to derive Kepler’s 3rd law?
Centripetal force (using angular velocity) = gravitational force
What is escape velocity?
The speed required to escape the gravitational pull of a mass
What is used to derive the escape velocity equation?
Kinetic energy = gravitational potential energy
What is used to derive the orbital velocity equation?
Centripetal force (using linear velocity) = gravitational force
How is the equation for kinetic energy of an orbit derived?
From substituting the orbital velocity equations into the standard kinetic energy equation
What is an electric field?
An area where an object with charge experiences a force of attraction or repulsion
What is the field and electric field strength around a point charge like?
- Radial field
- All the field lines meet at the middle
- Electric field strength varies according to the inverse square law
What is the field and electric field strength between two charged plates like?
- Uniform field
- All field lines are parallel
- Electric field strength is uniform everywhere in the field
What do electric field lines tell you?
The direction of the force acting on a positive charge at that point in the field
Which way do electric field lines point?
Away from a positive charge and towards a negative charge
Which way do field lines point in a uniform field?
From the more ‘positive’ plate to the more ‘negative’ plate
Which way do field lines point in a uniform field if both plates are positive?
The field lines go towards the least positive plate
What is Coulomb’s law?
- Electrical force between two objects is proportional to the product of the charges
- It is inversely proportional to the distance between their centres squared
- It can be attractive or repulsive
What is electrical field strength?
The force on an object per unit of positive charge (vector)
What path does a charged particle follow when entering a uniform electric field? Why?
- Parabolic path
- The charged particle will experience a constant force in one direction
Why will a charged particle in a vertical electric field never go completely vertical?
The path will never go completely vertical as there is no horizontal force acting so the horizontal component of velocity does not change
What is thermionic emission?
When a wire gets hot it releases free electrons
What is electric potential?
The energy required (work done) to move a positive 1C charge from infinity to that point in the field (scalar)
What are equipotential lines/surfaces?
Where the electric potential is the same value
What direction are equipotential lines compared to field lines?
Equipotential lines are at right angles to the field lines
What is the gradient of a graph of electric potential against distance at a point?
Electrical field strength at that point
What needs to be done to move between equipotential lines?
Work
What is the work done to move between equipotential lines equal to?
The change in electrical potential energy
What is the area under a graph of E against r?
Electrical potential difference
