Fields, Capacitance, Nuclear, Option Flashcards
Motor Effect
motor effect: when a current-carrying wire placed in a magnetic field experiences a force.
the force on a wire is:
- greatest when the wire is perpendicular to the magnetic field ( T = BIlndcos (0) )
- zero when the wire is parallel to the magnetic field ( T = BIlndcos (90) )
direction of force,current and field are related which we can find via fleming’s left-hand rule ( ie. if current is reversed or field is reversed then the direction of force is reversed)
Electron beams undergoing a magnetic field
- beam is deflected downwards when a magnetic field is directed towards the vacuum tube
- each electron in the beam experiences a force due to the magnetic field
- the beam follows a circular path
- ## this is because the direction of the force on each electron is perpendicular to the direction of motionof the electron (and to the field direction)
Why do current-carrying wires in a magnetic field experience a force?
The electronics moving along the wire are pushed to one side by the force of the field.
How does the direction of motion of a charged particle in a magnetic field affect the force on the particle?
- force on a charged particle in a magnetic field = BQv
-The direction of motion of a charged particle in a magnetic field is at angle θ to the lines of the field
- the component of the field perpendicular to the direction of motion of the charged particle is given by Bsin θ
- if the velocity of the charged particle is perpendicular to the direction of the magnetic field, θ = 90°, so the equation is F=BQv
- if the velocity of a charged particle is parallel to the direction of the magnetic field, θ = 0, F= 0 so no force is experienced.
Hall Probes
- hall probes are used to measure magnetic flux density
- they contain a slice of semiconducting material
how it works:
- a constant current passes through
- the charge carriers are deflected by the magnetic field
- a potential difference (hall voltage) is created between the top and bottom edges of the slice ( this is the hall effect )
- once the hall effect occurs charge carriers passing through the probe no longer are deflected because the forced caused by the magnetic field is opposed by the force of the electric field.
- the voltage produced is proportional to the magnetic flux density (provided a constant current)
Force of the magnetic fields on a MOVING charged particle is…
The force of the magnetic field on a moving charged particle is at
right angles to the direction of motion of the particle
Why is the kinetic energy of a charged particle unaffected by the magnetic field?
- NO work is done by the magnetic field on the particle as the force ALWAYS acts at RIGHT ANGLES to the velocity of the particle.
- The direction of motion is changed by the force but not its speed
- The kinetic energy of the particle is unchanged by the magnetic field
Circular Path of a particle in a magnetic field
- the magnetic force is always perpendicular to the velocity at and point along its path
- the particle thus moves on a circular path with the force ALWAYS acting towards the CENTRE of curvature of the circular path
- the force causes a centripetal acceleration
- the path is a complete circle due to the magnetic field being uniform and the particle remaining in the field
- the radius r of the circular orbit is dependent on the speed and magnetic flux density
- r = mv/BQ
The cyclotron (incomplete)
The mass spectrometer (incomplete)
Present the Universal Law of Gravitation in a sketch graph
Also known as the inverse square Law
*insert
test
test
Carbon Dating
- plants and trees contain a small percentage of radioactive isotope, carbon-14
- due to carbon being taken in by living plants due to photosynthesis a small percentage of the carbon content of any plant is carbon-14.
- the isotope has a half-life of 5570 years ( thus there is negligible decay during the lifetime of a plant)
- after a tree is dead the proportion of carbon-14 decreases due to nuclei decay.
- by measuring the activity of the dead sample its age can be calculated
( carbon-14 is formed in the atmosphere as a result of comics rays knocking out neutrons from nuclei )
Argon dating FINISH THIS
- ancient rocks have argon gas trapped within them as a result of the radioactive decay
What is a Capacitor?
- Capacitors are electrical components which can be used to store electrical charge in a circuit
Battery vs Capacitor
Capacitor -
- Stores potential energy in the electric field
- Stores relatively little charge
- Charges and Discharges quickly
Battery -
- Stored potential energy as a chemical store
- Stores relatively large amounts of charge
- Charges and discharges slowly
What is Capacitance?
The Capacitance of an object is a measure of the amount of charge stored in the object per potential difference used to store it
Assumptions made with capacitors
- the capacitance of a capacitor is constant throughout it’s use
( for alevel only the minimum capacitance of a capacitor, which occurs when the potential difference is a maximum, is considered)
Remember when using the capacitance equation
- the charge calculated for a capacitor will also be the charge of the circuit as the capacitor is discharging or charging because charge must always be conserved
- the potential difference calculated for a capacitor is ONLY for the capacitor so if there are other components in a circuit that will not be the emf/total V of the circuit.
Dangers of Capacitors
Due to capacitors being able to discharge and thus deposit charge very quickly, they can be lethal
How does a Capacitor work?
- A capacitor consists of two conducting parallel plates separated by a gap.
- capacitor is placed in a direct current source, charge builds up on its plates
- the plate connected to the negative terminal of the power supply gains electrons, making it negatively charged
- the plate connected to the positive terminal of the power supply loses electrons, making it positively charged
- this is due to the repulsion caused by the negatively charged electrons on the opposite plate and their attraction to the positive terminal
- A potential difference forms between the plates
- electrons can not travel between plates as the air gap is a poor conductor
- the better the insulator between the plates, the greater the charge imbalance that can form
- thus the gap creates a charge imbalance over the two plates
Dielectrics
- A solid insulating material ( typically a poorer conductor than air) placed between the two plates of a capacitor in order to increase the amount of charge it can store.
- the insulated gap of capacitor creates a charge imbalance, the better the insulator between the plates the greater the charge stored.
Are Capacitors charged?
- no extra charge is stored in a capacitor when it is charged compared to when it is uncharged
- when a capacitor is charged the positive plate has lost electrons and the negative plate as gained them
- therefore the no. of electrons leaving the plate is the same as the no. of electrons gained by the negative plate.
- thus overall the capacitors when charged will not gain charge, rather they remain electrically neutral
What happens to the potential difference and current of a capacitor when a capacitor is discharged?
- when a capacitor is discharged the current in the circuit goes up as there are more electrons flowing ( with greater energy)
- but the potential difference across the capacitor decreases as there is less potential energy stored in the dielectric material between the capacitor.
Relative premativity
The ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not present.
( this relates to The increase in capacitance of a capacitor after the addition of a dielectric material or the increase in charge after the addition of a dielectric material.)
Polar Dielectrics
- a dielectric being polarised increases the effect of the dielectric
- ∴ a polarised material can increase the dielectric constant
- when a capacitor is uncharged the polar molecules of the dielectric are distributed randomly
- when a capacitor is charged the polar molecules rotate according to the electric field
-the delta-negative end goes to the positively charged plate, the delta-positive end goes to the negatively charged plate - the molecules themselves also produce an electric field as charged particles which are opposite in direction to the electric field produced by the capacitor
- this reduces the overall electric field
- at each plates the polar molecules force further electron movement during the charging process, due to the further repulsion from one end and further attraction from another on the electrons at each plate by the molecules (see diagram)
When a capacitor is charged will all the electrons at the positive terminal be attracted off the plate?
- the positive terminal attracts the electrons off the plate initially
- this occurs quickly due to the number of electrons occupying the plate being a lot
- over time positive charge builds up on the plate
- this build up of positive charge makes it more difficult to remove more charge off the plate as the electrons are now attracted to the now more positive plate.
How does too high of a potential difference impact a capacitor?
- risk of breaking capacitor
- too much charge is forced onto the plates causing charge to leak across them and destroy the dielectric between them
When Discharging a capacitor in a circuit why does the change in current replicate exponential decay?
- there is initially a large current as the electrons leave the negative plate
- the current flows in the opposite direction from the charging current
- as the number of electrons on the negative plate falls so does the size of the repulsive force making current fall at a slower rate
- when no more electrons move in the circuit the current drops to zero
When discharging a capacitor why does the change in voltage over time form exponential decay?
- the discharging process happens the fastest with the greatest potential difference across the plates
- as the capacitor is discharged the rate of change of the voltage decreases with time
What is the charging process of a capacitor dependent on?
- the capacitance of the capacitor
- the resistance of the circuit
What is the time constant?
- The time taken for the value measured to fall by 63% when a capacitor is charged or discharged
(x0.37)
after one time constant a value should have fallen to be only 37% of what it was originally
this means that Q=Q(0)0.37 - the gradient of a linearised graph of a measure value (ie voltage, charge etc) will = 1/-RC
- T1/2 = 0.69RC
How is power effected by the rate of discharge of slow discharging capacitor?
- if a capacitor is discharged slowly through a high resistance circuit, it produces a low current over a longer time
- the energy is therefore released slowly, (the rate at which energy is released js dependent on the time constant of a discharged circuit)
- thus less power
- ( this can be useful for temporary power supplies)
How is power effected by the rate of discharge of a fast discharge capacitor?
- If a capacitor is discharged quickly through a low resistance circuit, it produces a short burst of large current
- the energy is thus released quickly from the capacitor
- thus greater power
(e.g how a camera flash is powered)
Energy stored in a capacitor
- when a charge builds up on the plates of a capacitor, electrical energy is stored by the capacitor, it becomes a store of potential electrical energy
The electrical potential energy stored is the work done to move the extra charge onto the plates
- this is work done against the potential difference across the plates
- on a graph the total energy stored in a capacitor is E = 1/2QV, the energy supplied is E=QV
( other iterations: E=1/2CV^2, E=1/2Q^2/C)
Electrostatic forces vs gravitational forces
Gravitational forces
- ALWAYS attractive
ELECTROSTATIC
- attractive and repulsive
what is a field?
A force field is a region where an object within it can experience a non-contact force due to it’s position in the field
( an object must contain a property of that field to experience the non-contact force, e.g gravitation field - mass)
Assumptions made when suing Newton’s Law of Gravitation
- the masses are point masses
- the objects have a uniform density
- there is a vacuum between objects
Gravitational Field Strength
The force per unit mass experienced by an object inside a gravitational field (Nkg^-1)
- before radius R of earth where the distance between an object and the earth’s core decreases, the gravitational field strength would decrease linearly, until it reaches zero at the centre
Properties of a Field
- Force law
- Field strength
- Potential
- Potential difference
Why are there areas of denser gravitational field lines on a planet’s surface?
- A variation in gravitational field strength on a surface of a planet is caused by the material varying in density
- the greater the density of material (for the same volume), the greater the mass, the greater the gravitational field produced.
Describe two causes of the energy losses in a transformer and discuss how these energy losses may be reduced by choice of design and materials
- AC currents in the primary and secondary coils.
- Coils will have residence and the currents cause heating of the coils, causing some energy to be lost
- loss reduced by using low resistance wire/ low resitivity material for coils
- the magnetic flux passing through the core is changing continuously.
- the metallic core is being cut by this flux and the continuous change in flux induces emfs in the core itself.
- A continuous core with induced emfs will cause eddy currents
- these heat the core causing energy to be wasted.
- to reduce eddy currents layer and laminate the core because currents cannot flow in a conductor which is discontinuous.
