fields Flashcards
what is a force field
area in which an object experiences a non-contact force
what two ways can force fields be represented as?
vectors or field lines
what does a vector in force fields show
direction of the force that would be exerted on the object
what does the distance between field lines represent
strength of the force exerted by the field in that region
how are force fields formed
interaction of masses, static charge or moving charges
what type of field is formed during the interaction of masses
gravitational fields
what type of field is formed during the interaction of charges
electric fields
what are the differences between gravitational fields and electric fields
- in gravitational fields, the force is always attractive whereas in electric fields the force can be repulsive or attractive
- electric force act on charge whereas gravitational force acts on mass
what are the similarities between gravitational fields and electric fields
- both follow inverse square law
- use field lines to be represented
- have equipotential surfaces
what does gravity act on?
any object with mass
what type of force is gravity
attractive
what does newtons law of gravitation show
the magnitude of the gravitational force between two objects is directly proportional to the product of the masses and is inversely proportional to the square of the distance between them
newtons law of gravitation equation
F=Gm₁m₂/r²
what are the two types of gravitational fields
uniform field or radial field
how does a radial field exert gravitational force?
the force exerted depends on the position of the object in the field
how does a uniform field exert gravitational force?
the same gravitational force is exerted on a mass everywhere in the field
what Is gravitational field strength
force per unit mass exerted by a gravitational field on an object
what is gravitational potential?
work done per unit mass when moving an object from infinity to that point
what is gravitational potential at infinity
0
is gravitational potential always negative or positive
negative
what is the gravitational potential difference
energy needed to move a unit mass between two points
what is potential on an equipotential?
it is constant on the equipotential surface
what are equipotential surfaces
surfaces that are created through joining points of equal potential together
how much work is done when moving along an equipotential surface
none
what is gravitational potential and distance between centres of objects relationships
inversely proportional
how to calculate gravitational field strength from he graph of gravitational potential and distance
the gradient multiplied by -1
how do you calculate gravitational potential difference from the graph of gravitational field strength against distance
area under graph
what is Keplers third law
the square of the orbital period is directly proportional to the cube of the radius
what is the total energy of an orbiting satellite
its kinetic and potential energy
what is the escape velocity
the minimum velocity an object must travel at to escape the gravitational field at the surface of a mass
at escape velocity what is kinetic energy equal to
gravitational potential energy
what is a synchronous orbit
orbital period of the satellite is equal to the rotational period of the object it is orbiting
what is a geostationary satellite
their orbital period is 24hrs and they always stay above the same point as they orbit directly above the equator
uses of geostationary satellites
tv and telephone signals
what is a low orbit satellite
they travel much faster and their orbital periods are much smaller
if charges have the same sign they will be….
repulsive
what is coulombs law
magnitude of forces between two point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of the distance between charges
if charges have different signs the force will be….
attractive
uses of low-orbit satellites
monitoring weather, making scientific observations, military
what is electric field strength in uniform field Vs radial
uniform field is constant and varies in radial field
why is the magnitude of electrostatic forces between subatomic particles greater than the magnitude of gravitational forces
the masses of subatomic particles are incredibly small whereas their charges are much larger
what is electric field strength
force per unit charge experienced by an object in an electric field
What is absolute electric potential
potential energy per unit charge of a positive point charge at that point
what do electric field lines show
direction of force acting on a positive charge
when is the magnitude of electric potential greatest
at the surface of a charge
for uniform electric fields, what shape does a particle fired at a right angle make?
parabola
when is the absolute magnitude of electric potential at its greatest
when its at the surface of the charge
why does a particle fired at a right angle follow a parabolic shape through a uniform electric fiend
it experiences a constant force which causes particle to accelerate
what is the absolute electric potential
potential energy per unit charge of a positive point charge at that point in the field
what is electric potential at infinity
zero
when electric potential is positive, what is the charge
positive and repulsive
when electric potential is negative, what is the charge
negative and attractive
what happens when a charge moves along an equipotential surface
no work is done
what is electric potential difference
energy needed to move a unit charge between two points
what is the gradient of a tangent on potential against distance graph
electric field strength
what is work done in moving a charge across a potential difference equal to
product of potential difference and charge
when a charge moves along an equipotential surface, what work is done
none
why is no work done when a charge moves along an equipotential surface
potential on an equipotential surface is the same everywhere
what is the area under the graph on the graph of electric field strength against distance
electric potential difference
what is capacitance
charge stored by a capacitor per unit PD
what is a capacitor
an electrical component that stores charge
what is a capacitor made of
two conducting parallel plates with a gap between them which may be separated by an insulating material
what is the insulating material between the two plates of a capacitor called
dielectric
what happens when a capacitor Is connected to power
opposite charges build up on the plates causing a uniform electric field to be formed
what is permittivity
a measure of the ability to store an electric field in a material
what is a dielectric formed of
polar molecules that align themselves with the field when an electric field is present, each molecule has its own electric field which the strength of which depends on the dielectrics permittivity
what does the field caused by the dielectric do
opposes field formed by the capacitor reducing the field so the pd required to charge the capacitor decreases causing capacitance to increase
what is represented by the area under a graph of charge against pd
energy stored by a capacitor
how do you charge a capacitor
by connecting it in a circuit with a power supply and a resistor
how does a capacitor charge
negative charge builds up on the plate connected to the negative terminal
electrons on the opposite plate are repelled by the negative charge so they move to the positive terminal
an equal but opposite charge is formed on each plate causing PD
as charge across plates increase, pd increases
electron flow decreases due to the force of electrostatic repulsion also increasing so current eventually reaches zero
how do you discharge a capacitor
connect it to a closed circuit with a resistor
how can you investigate a capacitor charging, discharging
use a data logger to measure values of pd and current to plot graphs of voltage and current against time which you can then draw a graph of charge against time
what is time constant
the product of resistance and capacitance that is the value of the time taken to discharge a capacitor to 0.37 of initial value or charge to 0.63 of initial value
what happens in terms of fields when a current passes through a white
a magnetic field is induced
what do the field lines of an induced magnetic field when a current passes through a wire form
concentric rings
what is magnetic flux density (B)
measure of the strength of a magnetic field
what is the unit for magnetic flux density
Tesla
what is the definition for one Tesla
force of 1N on 1m of a white carrying 1A of current perp to magnetic field
what happens when a current carrying wire is placed in a magnetic field
a force is exerted on the wire
if a current carrying wire is placed in a magnetic field and the current is parallel to the magnetic field what is the force
0N
what formula is used for magnetic field if the field is perpendicular to current
F=BIL
what do you use to find the direction of force exerted on a wire when field is perpendicular to the current
Flemings left hand rule
what fingers are what for lemmings left hand rule
thumb - direction of force
first finger - field
second finger - direction of conventional current
why is a force exerted on a current carrying wire
cause a force acts on charged particles moving in a magnetic field and the wire contains negatively charged moving electrons
is the force exerted on a current carrying wire parallel or perpendicular to motion of travel
perpendicular
why do charged particles follow a circular path when a current carrying wire is in a magnetic field
the force induced by the magnetic field acts as a centripetal force
what is an application of the circular deflection of charged particles in a magnetic field
cyclotron
what is a cyclotron and its uses
a particle accelerator which can produce ion beams for radiotherapy and radioactive tracers
what is a cyclotron formed off
two semi circular electrodes called dees with a uniform magnetic field acting perpendicular to the electrodes
high frequency alternating voltage applied between the electrodes
what happens to the charged particles in the cyclotron
they move from the centre of one of the electrodes and are deflected in a circular path, the speed will not increase due to the magnetic field
what happens when particles reach the edge of the electrode in a cyclotron
they move across to the other electrode and are accelerated by the electric field so the radius of their circular path will increase. when they move across again the alternating electric field changes direction allowing particles to be accelerated again. repeats until required speed is reached and they exit cyclotron
How do you work out the escape velocity
Ek = Ep therefore
1/2(m)(v^2)=(GMm)/r
What orbit do geostationary satellites follow
Geosynchronous
What is magnetic flux
A value which describes the magnetic field passing through a given area when the field is perpendicular to the area
What is magnetic flux linkage
Magnetic flux multiplied by the number of turns of a coil
What happens with calculating magnetic flux if the field is not perpendicular to a coil of wire
Use trigonometry to resolve the components
What is the value of magnetic flux when the field is parallel to the coil
0
What is electromagnetic induction
A conducting rod moves relative to a magnetic field, electrons in the rod experience a force and build up on one side of the force causing emf to be induced
What are the two laws for electromagnetic induction
Faraday and lenz’s law
What is faradays law
The magnitude of induced emf is equal to the rate of change of flux linkage
What is lenz’s law
Direction of induced current is such as to oppose the motion causing it
How do you demonstrate lenz’s law
By measuring the speed of a magnet falling through a coil of wire and its speed when falling from the same height without falling through the coil
How does lenz’s law work
As magnet approaches coil, there is a change of flux through the coil
so an emf and a current is induced.
The direction of induced current is such as to oppose the motion of the magnet which causes magnet to slow down
As magnet passes through the centre of the coil there is no change in flux so no emf is induced.
As magnet leaves coil there is a change in flux so a current is induced that opposes the motion of magnet
What happens when a coil rotates at a constant frequency
The emf induced can be calculated using a formula derived from the formula for magnetic flux linkage with respect to time
How does flux linkage and angular speed of a coil relate
Flux linkage varies depending on the angular speed of the coil
What type of current is the emf induced by a coil rotating in a magnetic field
Alternating
What does an oscilloscope show for a direct current
A straight line parallel to the axis
For an alternating current what does an oscilloscope shoe
Sinusoidal waveform or when time base is turned off it’ll be a vertical line
How to measure the peak voltage from an oscilloscope
Distance from the equilibrium to the highest/lowest point
How to measure peak-to-peak voltage from an oscilloscope
Distance from minimum point to maximum point
How to measure the root mean square voltage from an oscilloscope
Average of al the squares of the possible voltages
How to measure the time period from an oscilloscope
Distance from one point on a curve to the point where the curve repeated
What is electricity supplied to homes in the uk voltage
230v
What are transformers used for
To change the voltage of alternating currents
What are transformers made up of
A primary coil attached to input voltage and a secondary coil connected to the output voltage and an iron core
What is the relation between the voltage in the primary coil and secondary coil to the number of turns on primary coil to the secondary coil
They are the same
What is a step-up transformer
Increases the input by having more turns on the secondary coil than primary
What is a step-down transformer
Decreases input by having less turns on the secondary coil
What are the causes of energy loss in a transformer
Eddy currents, resistance, if the core is not easily magnetised
What are eddy currents
Currents induced by alternating magnetic fields in the primary coil
due to lenz’s laws they oppose the field that produced them
reducing flux density and generating heat causing energy to be lost
How can eddy currents effects be reduced
By using a laminated iron core because eddy currents cannot pass through an insulator so their amplitude is reduced.
High resistivity material also works
How can energy lost in transformers from resistance reduced
By using a thick wire with low resistance
When transferring electrical power, what is the power lost due to resistance equal to
I^2R
As voltage is stepped up what happens to current
It decreases
