FIelds and their Consequences Flashcards
Coloumbs Law
The size of the force that acts between two point charges is proportional to the product of their charges and inversely proportional to the square of their separation. It is attractive for opposite charges and repulsive for like charges.
Dielectric
An insulating material placed between the two plates of a capacitor in order to increase the amount of charge it can store.
Electrical conductor
A material that contains free electrons that are able to move throughout the material and carry charge.
Cyclotron
A particle accelerator made up of two D shaped electrodes positioned opposite each other. The electric field changes direction each time a particle moves from one electrode to the other, causing the particle to accelerate
Electric field
A region surrounding a charged object which causes a force to be exerted on any charged object placed within the field.
Electric potential, V (at a point in a field)
the work done per unit charge on a positive
test charge in bringing it from infinity to that point in the field
Electric field strength, E (at a point in a field)
The force per unit positive charge exerted on a charged object placed at that point in the field.
Electromagnetic induction
When an emf is induced in a wire /conducting rod when it is moved relative to a magnetic field.
Equipotential
A surface of constant potential. No work is done by the field when an object moves along an equipotential.
Escape velocity
The minimum velocity required by an object to be able to escape a gravitational field of a mass when projected vertically from its surface.
Faraday’s law
The magnitude of the induced emf is equal to the rate of change of flux linkage through the circuit.
Force field
An area in which an object will experience a non-contact force.
Geostationary orbit
A satellite that orbits above the equator with a 24 hour period, so it will always remain above the same position on the Earth. They orbit approximately 36,000km above the surface of the Earth.
Gravitational field
A region surrounding a mass in which any other object with mass will experience an attractive force.
Gravitational potential
The work done per unit mass required to move a small test mass from infinity to that point.
Kepler’s third law
T^2 ∝ r^3
Lenz law
An induced current is always in a direction so as to oppose the change that caused it.
Magnetic field
A region surrounding a magnet or current-carrying wire that will
exert a force on any other magnet or current-carrying wire placed within it.
Magnetic flux density
The force per unit current per unit length on a current-carrying wire placed at 90º to the field lines. Sometimes also referred to as the magnetic field strength.
Motor effect
When a current-carrying wire is placed within a magnetic field (non-parallel to the field lines) and experiences a force perpendicular to both the wire and the field lines.
What is the Permittivity of free space
A measure of the ability of a vacuum to allow an electric field to pass through it.
Polarised
An atom/molecule becomes polarised when an external electric field causes the negative electron cloud to be shifted in the opposite direction to the positive nucleus – the
charges are pulled in opposite directions.
Relative permittivity
The ratio of charge stored in a capacitor with the dielectric to charge stored without the dielectric. Also sometimes referred to as the dielectric constant.
Or the permittivity of the substance/permittivity of free space
Synchronous orbit
An orbit in which the period of the orbit is equal to the rotational period of the object that it is orbiting
Uniform field
A field in which all of the field lines are parallel and equally spaced – field strength is equal in all areas of the field.
Discharging capacitor half life
T½ = (RC)ln2
What two factors effect time constant
Resistance - effects how quickly current flows therefore how quick the capacitor charges/discharges
Capicitance - the amount of charge that can be stored
Eddy currents
These are looping currents induced by changing magnetic flux in the core. They create a magnetic field that opposes the field inducing them (lenz), therefore reducing total field strength. These are reduced by using a laminated core. (Layers of the core separated by insulator, so no current can flow)
Purpose of the core in the transformer
To direct the magnetic field round to the secondary coil
Magnetic field lines direction
North to south
Dielectric impact on capacitance
It increases it, as increases €r (the ratio of charge stored with dielectric compared to when not present)
Effects on V and Q if battery remains connected or disconnected in regards to a capacitor
V is constant if connected
Q is constant if disconnected
Plot AC to DC graph
Find Rms then horizontal line through it
If charging what is RC
The time taken to reach 63%
Can you indefinitely add charge to a capacitor
No, as the dielectric between the 2 plates will eventually break and therefore allow for conducting to occur
When is a capacitor almost discharged
in 5 RC’s
Value of initial and final V and I when charging and discharging a capacitor
•Vtot = Vr + Vc
•Initial Vc =0 therefore V = Vr
•Therefore initial current = V/R
•When charged: Vc = vtot
Current flowing at each pole
North: anticlockwise
South: clockwise
Right hand screw rule
Thumb = current
Rest of finger = field direction
Discharging capacitor equation as a graph
Y = mx + c
ln(V) = -t/RC + ln(Vo)
What other factors in a circuit can you use the charge decay equations for as well?
V, I
Equipotential lines relative to field lines
Always perpendicular
When/how does capicitance change?
It remains constant unless the geometry changes as for C =Q/V, p.d will increase in linear proportion to charge
How to derive the time for a proton to travel around one Dee of a cyclotron
BQv = mv^2/r
w = v/r
BQ = mw
(2π/T)M = BQ
T= 2πm/BQ
T/2 = πm/BQ (since only one Dee)
How to derive to find Kepler’s law
m(w^2)r = GMm/r^2
(2π/T)^2 x (r^3) = GM
4π^2 x (r^3) = GM(T^2)
(T¹/T²)^2 = (r¹/r²)^3
How to derive escape velocity
1/2mv^2 = GMm/r
v = √(2GM/r)
Derive orbital speed equation
F = mv^2/r = GMm/r
v = √(GM/r)
Difference between positive and negative potential
Positive potential indicates thatwork has to be done to move the charge from one point to another whereas negative potential indicates that energy is generated when the test charge moves from one point to another
Tesla def
The magnetic flux density which would cause a wire of 1m carrying 1A to experience 1N of force.
An electron moving with a constant speed enters a uniform magnetic field in a direction at right angles to the field. What is the subsequent path of the electron?
A circular arc in a plane perpendicular to the direction of the field.
What trajectory does a charged particle which enters a uniform electric field at a right angle have, parabolic or circular?
Parabolic
For the potential of a sphere of charge
V = (1/4π€0) x Q/r what is r the radius of?
The radius of the actual sphere of charge.
Whereas for the force between two charges r would be the distance/radius between them. The radius of the 2 charges + their distance
What trajectory does an electron which enters a magnetic field at a right angle, parabolic or circular?
Circular
Capacitance of a sphere
4π€or
When you see angles mentioned in a question what should you do?
think about splitting into vectors and trigonometry
Newton’s law of gravitation
The universal force of attraction is dependent upon and proportional to the product of the 2 mass involved and inversely proportional to distance between the centre of the masses squared/radius of their orbit squared
Function of the core and the secondary coil?
1) to guide the magnetic field through the secondary coil in order to maximise the flux through it.
2) emf is induced by alternating magnetic field and since it has more turns than primary coil emf induced is higher than the v across the Pc.
Why is pd cross the national grid cables high but not too high?
To decrease current and therefore energy lost due to resistance of the cables.
If too high then isolating the current would be more difficult as would jump between cables (pylons)
What does the direction of electric field lines mean?
The direction at which a proton would move
What is a laminated core?
Insulating sheets between layers of iron, these layers are insulated from each other to prevent the flow of eddy currents within the core material and therefore reduce energy losses. The material of these sheets has a large R and therefore minimises eddy currents.
Emfs relation to flux linkage
Emf is the derivative of flux linkage and therefore when emf is at maximum, N∅ is at 0 as that’s the point of greatest change.
(90⁰ out of phase on graph (shifted left to emf as equation has negative in it € = -∆N∅/∆t))
Magnetic fields relationship to velocity of an electron
Always perpendicular therefore no change in velocitys magnitude
Vo is always the terminal pd for both the decay of charge equation and charging on the capacitor
For charging, this would be the voltage applied across the capacitor at the beginning of the charging process, while for discharging, it would be the voltage across the capacitor at the start of the discharging process.
