Gravitational and electric fields Flashcards
What is a force field?
A region in space in which a body experiences a non-contact force
How do force fields arise?
From the interaction of mass, of static (stationary) charge and between moving charges
What is gravity?
An attractive force that acts between all masses and has an infinite range
What does Newton’s law of gravitation show?
The attractive force due to gravity acting between two masses
What is the relationship between the force due to gravity acting between two masses and their separation?
They follow an inverse square law so the force is inversely proportional to the separation squared
What is a gravitational field?
A region in space in which objects (masses) experience a force due to gravity
What do gravitational field lines show?
The direction of the force acting on a mass in a gravitational field (the direction a stationary mass would move if placed in the field) and the closeness of the lines show the strength of the field at that point
What is gravitational field strength?
It is a measure of the intensity of a gravitational field at a particular point, in other words it is the force experienced per unit mass by an object at that point
How do the gravitational field lines around the entire Earth differ to those on the surface of the Earth?
The field lines around the Earth as a whole are radial but as you get closer to the field lines near the surface can be considered uniform
What is gravitational potential?
The work done per unit mass moving against the field to bring a small test mass from infinity to a specific point in a gravitational field, without changing its kinetic energy
Why is gravitational potential always negative?
Because you’re moving against the direction of the field (hence moving in thenegative direction relative to the field)
What is the gravitational potential at an infinite distance away from the mass that created the gravitational field?
Zero
What is gravitational potential difference?
The change in gravitational potential energy per unit mass between two points in a gravitational field, it is the work being done when moving a unit mass between those two points
How can gravitational potential difference be calculated from a gravitational field strength graph?
By working out the area underneath the curve
How can gravitational field strength be calculated from a gravitational potential graph?
By calculating the negative gradient of the tangent to the curve at any point
What is meant by an equipotential surface (in relation to a gravitational field)?
A line or surface where all of the points along it have the same gravitational potential, meaning there is no gravitational potential difference when a mass is moved along it (hence no work is done)
What is the connection between gravitational field lines and equipotential surfaces?
They are perpendicular to eachother at each point and the closer together field lines are the closer togehter the equipotential surfaces are
Derive the equation that links the orbital speed of a planet to its radius
Look in booklet for derivation
What is a synchronous orbit?
When an orbiting object has an orbital period equal to the rotational speed of the object it is orbiting
What is a geostationary orbit?
A specific type of synchronous orbit where a satellite orbits the Earth at the same rate at which the Earth rotates (meaning it completes one orbit in 24 hours), this means the satellite is always directly above the equator and always remains stationary relative to the Earth
What is a low polar orbit satellite?
A satellite that orbits close to the Earth
Compare the heights of the orbits for a low polar orbit satellite and a geostationary orbit satellite
The low polar orbit satellite orbits much closer to Earth whereas a geostationary orbit satellite orbits further away from the Earth to ensure it orbits at the Earth’s rotational speed
Compare the orbital periods of low polar orbit satellites and geostationary orbit satellies
A low polar orbit satellite is closer to Earth so it experiences a greater gravitational pull compared to the geostationary orbit satellite and so the low polar orbit satellite orbits faster and has a shorter orbital period
What are 2 common uses of geostationary orbits?
Communication (TV, phone, internet) and weather monitoring
Why are geostationary orbits suitable for communication purposes?
It provides a constant signal because the satellite is stationary relative to the Earth so you don’t have to angle your reciever to get a signal
Why are geostationary orbits suitable for weather monitoring purposes?
You get a continuos view of the same region so it allows real-time weather tracking
What are 2 common uses of low polar orbit satellites?
Earth observation (mapping and spying) and weather monitoring
Why are low polar orbit satellites suitable for Earth observation?
The satellite is close to the Earth so you can get higher resolution images
Why are low polar orbit satellites suitable for weather monitoring?
The satellite has a short orbital period (passes past the same point more frequently) so the weather can be updated every few hours
How do you work out the total energy of a satellite?
Add the kinetic and potential energy together as its total energy is always constant
Describe the kinetic and potential energy of a satellite in a circular orbit
The satellites speed and distance above the mass it is orbiting are constant so both kinetic and potential energy are also both constant
Describe the kinetic and potential energy of a satellite in an elliptical orbit
The satellite speeds up as its orbital radius decreases so the kinetic energy increases and potential energy decreases (and vice versa) , so that total energy remains constant
What is meant by escape velocity?
The minimum speed an unpowered object needs in order to leave the gravitational field of a planet without falling back towards it because of gravitational attraction
Explain how it is possible for rockets to leave the surface of the Earth travelling at a speed much lower than the escape velocity
The escape velocity tells you the speed needed to escape with a single, unpowered launch whereas rockets use a continuous thrust to overcome gravity, meaning they accelerate gradually and gain enough kinetic energy over time
What are the laws of electrostatic attraction and repulsion?
Like charges repel eachother (e.g. 2 negative charges or 2 positive charges) and opposite charges attract (e.g. a negative and a positive charge)
What does Coulomb’s law show?
The electrostatic force acting between two charges
What is the relationship between the electrostatic force between two charges and their separation?
They follow in inverse square law so the force is inversely proportional to the separation squared
What is the permittivity of free space?
A measure of how easy/ how difficult it is for an electric field to pass through air or a vacuum
What is an electric field?
A region in space in which a charged object experiences an electrostatic force due to the presence of other charges
What do electric field lines show?
The direction that a positive test charge would move if placed in the field (hence the direction of the force that would act on the positive charge)
What is the definition of electric field strength?
The force per unit positive charge experienced by a charged object placed within an electric field
Combine the two equations for electric field strength in a uniform field to obtain an equation for work done
Look in booklet for derivation
Describe what happens as a charged particle enters an electric field
It enters the field at right angles to the field and feels a constant force acting parallel to the electric field lines, if the particle is positively charged it will move towards the negative plate and if it is negatively charged it will move towards the positive plate causing the particle to follow a curved path (parabola)
What is electric potential?
The work done per unit positive charge moving against the field to bring a positive test charge from infinity to a specific point in an electric field
How does the value of electric potential change for attractive and repulsive forces?
Electric potential is positive for repulsive forces and negative for attractive forces
What is the electric potential at an infinite distance away from the charge that created the electric field?
Zero
What is meant by an equipotential surface (in relation to an electric field)?
A line or surface where all of the points along it have the same electric potential meaninh there is no electric potential difference when a charge is moved along it (hence no work is done)
What is the connection between electric field lines and equipotential surfaces?
They are perpendicular to eachother at each point and the closer together the field lines the closer together the equipotential surfaces are
How do you calculate the electric potential from an electric field strength graph?
By working out the area underneath the curve
How do you calculate the electric field strength from an electric potential graph?
By calculating the gradient of the tangent at any point on the curve
Give 4 examples of contact forces
Friction, air resistance, tension and the normal contact force
Give 3 examples of non-contact forces
Gravitational, electrostatic and magnetic forces
What are the similarities between gravitational and electric fields?
- Non-contact forces acting at a distance
- Follow inverse square laws
- Have radial field patters around point sources
- Use field lines to show strength and directions
- Work is done when moving a mass or charge in the field
What are the differences between gravitational and electric fields?
- Gravitational fields are caused by masses and electric fields are caused by charges
- Gravitational fields are always attractive and electric fields can be attractive or repulsive
- Gravitational potential is always negative but electric potential can be positive or negative
- Different constants (G and the permittivity of free space)
