Fields and Their Consequences P2 Flashcards
What does the electrostatic force tell us if the sign is negative?
The force is attractive
What is the magintude of the electric field strength at a given point?
The force per unit charge that a positive test charge would experience without effecting the field strength.
What occurs when a positive test charge is moved closer to a negative point charge?
Work is done by the electrostatic force therefore the elelctric potential of the particle decreases
What is the electric potential energy of a postive test charge in an electric field defined as?
The work done required to move the test charge from infinity to the position it is in
What happens to the electric potential energy of a positive test charge if you move it closer to a positive source charge?
It becomes more positive (Increases)
What has done the work if the electric potential energy of a small positively charge particle at a given position is -3J?
3J of work is done by the electrostatic force to move the particle from infinity to that point
What is the potential gradient at a point on a potential graph?
The change in potential per unit distance at that point
What is the magnitude of the electric potential gradient at a point equal to?
The magnitude of the electric field strength at that point
What graph can we find the potential difference from by calculating the area underneath it?
Electric field strength against distance graph
What is the difference between a good conductor and a good insulator?
A good conductor has lots of free electrons whereas a good insulator doesn’t have many (e.g. Silver and Diamond)
What is a capacitor made of?
Two conducting components, seperated by an insulated material
What is the potential difference across a battery?
The difference in potential energy per coulomb between one terminal and the other
What is electric current?
The coulombs of charge moving past a given point per second
When does a capacitor stop charging?
When the potential difference across the capacitor is the same as the potential difference across the battery
What is the capacitance of a capacitor?
The charge per unit volt between the capacitor plates
How can we find the capacitance from a graph?
Plot a graph of charge against potential difference and find the gradient
What are dielectrics?
Insulating materials that polarise in the presence of an electric field
What happens as we increase the permittivity of a dielectric?
It becomes easier to polarise therefore the weaker the electric field is that passes through it
What is the relative permittivity of a material? (Dielectric Constant)
The ratio of the permittivity of the material to the permittivity of free space
What relationship does capacitance have with permittivity and the area of each plate?
They are directly proportional
Why does increasing the area of each plate increase the capacitance?
More charge can be stored on each plate
What are the three factors which affect capacitance?
The permittivity of the insulator, the area of the plates and the distance between each plate?
Why is the distance between the plates inversely proportional to the capacitance?
Since the electric field strength remains constant, the potential difference increase which means the capacitance decreases.
How can a constant current be achieved when discharging a capacitor?
Use a variable resistor and decrease the resistance as the capacitor discharges
What is the relationship between charge and time when a capacitor discharges through a fixed resistor?
Exponential
What is the time constant?
The time it takes the charge stored in the capacitor to fall by 37% of the charge stored in the fully-charged capacitor.
How do we find the time taken for the charge on a capacitor to halve?
Multiply the time constant (RC) by 0.69
What does the area represent underneath a current/time graph?
The initial charge of a capacitor
When we start charging an uncharged capacitor through a fixed resistor, what happens after a time of one time constant?
The charge stored increases by 63% and the current decreases by 37%
What does the magnitude of the Force of a current carrying wire depend on?
The length of wire in the field, the current, the magnetic field strength and the angle between the current and the magnetic field lines
What is the magnetic flux density defined as?
The force per unit current per length of wire when the wire is perpendicular to the magnetic field.
How do you find the current required to levitate a wire in a magnetic field?
Set F as the weight of the wire in F = BIL
What conditions must be met for a particle to experience a force in a magentic field?
It has a charge and is not moving parallel with the field
What must be changed if we use Fleming’s left hand rule for negatively charged particles?
The velocity must be in the opposite direction
What is the formula for the radius of a charged particle’s circular path in a uniform magnetic field?
r = mv/BQ
What is within and between the D’s of a cylcotron?
There is a uniform magnetic field within the D’s and an alternating electric field between them.
What is the purpose of the alternating electric field and uniform magnetic field in the cyclotron?
The electric field increases the proton’s speed and the magnetic field forces the protons round a circular path.
What must the time period of the alternating electric field equal to in a cyclotron?
The time taken for a proton to complete one full cycle around the cyclotron
What is the formula for the time period for a proton to complete a full cycle around the cyclotron?
T = 2πm/BQ
What happens when a wire cuts through magnetic field lines?
An EMF is induced in the wire (Electromagnetic Induction)
What is Lenz’s law?
The direction of the induced emf opposes the change that caused it
What pole does the thumb point towards in the right hand grip rule?
The north pole
What happens if we move a magnet towards a coil?
The coil develops a magnetic field which repels the magnet therefore, if the end of the magnet is the north pole, the part of the coil closest to the magnet will also become a north pole.
If a coil is repelling a magnet, which direction is the initial velocity of the magnet?
Towards the coil
What is Faraday’s law?
The magnitude of the emf induced in a coil is equal to the rate of change of flux through the coil
What is the magnitude of the emf in a wire dependent on?
The rate at which it cuts through magnetic field lines
What are the four ways to induce an EMF in a coil?
Change the flux density.
Change the area of the coil.
Rotate the coil.
Move a coil into or out a magnetic field.
In what graph is the gradient the EMF?
Flux linkage against time
In what graph is the area the change in flux linkage?
EMF against time
At what point is the EMF of a coil at its maximum when moving a magnet through it?
Just as it enters and leaves
What happens when a coil is rotated within a magnetic field?
An alternating current is created
How do you find the mean induced EMF when generating AC?
Divide the flux linkage by the time taken for a 90 degrees turn (BAN/t1/4)
How do you find the maximum EMF?
BANω
How do you reduce the uncertainty when measuring voltage from an oscilloscope?
Increase the Y-Gain so it just fits on the screen
How do you find the mean power?
Half the peak power
How can a coil provide an AC to another coil without being connected to it?
An AC is passed through the the first coil which creates a magnetic field which is constantly changing, this then means the 2ns coils oppose to change is also constantly changing which means so does the current across it
What is the purpose of the iron core in a transformer?
To maximise the flux through the secondary coil
What happens as you increase the number of coils on a coil in a transformer?
The alternating voltage across the coil increases
How can you reduce eddy currents in an iron core?
The core is made from alternating layers of iron and insulation (Lamination)
What are the four main sources of inefficiency in a transformer?
Eddy currents in the core.
Heat loss through coils.
Not all flux from the primary coil going through the secondary coil.
Magnetising and demagnetising the core.
What are the four solutions to stop the previous inefficiencies?
Laminate the core to stop eddy currents.
Use thick copper wires with high resistance to stop heat loss.
Wrap both coils around the same part of the core.
Use a soft, magnetic core like iron.
How are transformers used in the national grid?
Step-up transformers are used between power stations and transmission lines which reduces current and power lost as heat.
Step-downs are then used to reduce the voltage to a safe level for domestic use.
What is the magnitude of the gravitational force of two objects proportional to?
The mass of both objects
What is the magnitude of the gravitational force of two objects inversely proportional to?
The distance from the centre of both objects squared (Inverse square law)
What is a point mass?
Where the mass is concentrated at a single point
What is gravitational field strength?
The gravitational force per unit mass of a test mass
What is a test mass?
A particle with a very small mass used to test a gravitiational field without affecting it
What is gravitational potential energy?
The amount of energy/work done required to move a mass from infinity to a given point in a gravitational field
What is gravitational potential energy at infinity?
Zero
What happens to the GPE as it gets closer to the source mass?
The more negative the GPE becomes
What is escape velocity?
The minimum velocity required for an object to move from a point in a planet’s gravitational field to a point where the gravitational field is negligible.
What is gravitational potential?
The work done to move a 1kg mass from infinity to a point in the field
What are equipotentials?
A set of points which have the same gravitational potential
What is potential gradient?
The change in potential per unit distance at that point.
What does the area between two points below a gravitational field strength graph tell us?
The gravitational potential difference
At what seperations are equipotentials when the gravitational field strength is high?
Large seperations
What do satellites do in low Earth orbits?
They move over lots of different points on the surface and orbit relatively close to the surface
What do satellites do in geostationary orbits?
They appear to stay still in the sky over a single point on the surface and orbit at distances relatively far from the surface
How do you derive the escape velocity?
Set the equations of kinetic energy and gravitational force equal to each other
What is the formula for the total energy of a satellite?
-GMm/2r
Why is gravitational potential difference a negative value?
Because work needs to be done to move an object from the inside of the field to the outside. Since infinity is zero, below infinity must be negative
How is the orbital period realted to the radius of a circular orbit?
T^2 = k x r^3
How could you find the speed of an orbiting satellite?
Set the centripetal force to the gravitational force and sub in v=2πr/T
What is an eddy current?
As the primary coil induces emf in the secondary coil, it induces mini currents within the core.
