electric and magnetic fields Flashcards
force field
a force field is an area in which an object experiences a non-contact
electric field
a force field in which charged particles experience a non-contact force
electric field strength
force per unit charge experienced by an object in a field
E=F/Q
coulombs law
the magnitude of force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them
electric field strength in a radial field equation
E=KQ/r^2
electric potential
potential energy per unit charge of a positive point charge at that point in the field
where is the magnitude of electric potential maximum
at the surface of the point charge
where is the magnitude of electric potential smallest
it is 0 at infinity
electric potential in a radial field equation
V=KQ/r
electric potential difference
energy needed to move a point charge between two points
what does the distance between field lines represent
the strength of the field at that point
capacitance
charge per unit potential difference stored in a capacitor
how can you find the energy stored by a capacitor in a charge-potential difference graph
energy will be the are underneath the graph
how do capacitors charge once a switch is closed
-current starts flowing and negative charge builds up on the negative terminal plate.
-on the opposite plate, electrons are repelled by the build up of negative charge on the plate, therefore these electrons move to the positive terminal and an equal but opposite charge is formed on each plate, creating a potential difference.
-as the charge increases, so does the potential difference, but electron flow decreases due to the repulsion, so current eventually becomes zero.
capacitor discharge
the current flows in the opposite direction. electrons transferred to the positively charged plate through the resistor. the current, charge and potential difference fall exponentially
time constant
value of time taken to:
-discharge a capacitor to 37%
-charge a capacitor to 63%
t=RC
magnetic flux density (B)
strength of the magnetic field, measured in Teslas
magnetic flux (Φ)
magnetic field lines passing through a given area
Φ=BA
Magnetic flux linkgage
magnetic flux multiplied by the number of turns in the coil, N.
NΦ=NBA
charged particles moving in a magnetic field
they experience a force, F=BqV. the direction if the force can be found by using lemmings left hand rule, the force is alway perpendicular to the motion of travel, which causes charged particles to follow a circular path as it acts as a centripetal force.
current carrying conductor in a magnetic field
when a current flows through a wire, a magnetic field is produced. therefore when this wire is in a field, it experiences a force, F=BIL
induction of emf in a coil
when a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force, and build up on one side of the rod , causing an emf to be induced. known as electromagnetic induction.
if the coil forms a complete circuit, a current is also induced
two laws which govern electromagnetic induction
-faradays law:the magnitude of induced emf is equal to the rate if change of flux linkage
-lenzs law: the direction of induced current is such that it oppose the motion causing it
factors affecting the emf induced
-number of turns in the coil
-magnetic flux density
-area of the cross section
-time taken for the motion
induction of emf in a coil through the change in current of another coil
magnetic field is induced in a current carrying coil. if the current through the wire changes (alternating current), the magnetic field will also change, which will induce an emf in the other coil if it is within the magnetic field. this is called mutual inductance. the emf in the second coil is proportional to the change in current in the first coil.
example of mutual inductance
transformer
factors influencing the emf induced in the second coil
-magnetic flux density (B) of the field created by the initial coil
determined by the number of turns in the coil
-the distance between the coils
further apart means less magnetic flux passed on
-number of turns (N)
-area of cross section (A)
-time taken for change in current
how can lenzs law be shown
1) magnet falling approaches coil, causes a change in flux and so an emf is induced
2) due to lenzs law, the current opposes the motion, so the same pole as the pole of the magnet approaching will be induced which repels the magnet and slows the magnet down
3) as the magnet passes through the coil there is no change in flux
4) is it leaves, emf is induced and the current opposes the motion therefore it attracts the magnet and so it is slowed again
why must lenzs law slow the magnet down
as it is a consequence of the conservation of energy. of the magnet sped up then energy would have Been created.
split ring usage
swaps the positive and negative connections every half turn. direction of the current therefore swaps every half turn causing the forces on the coil to always act in the same direction. the coil can continuously rotate in the same direction thanks to this.
