A2 Physics Unit 4

define capacitor

two metal plates with a dielectric inbetween that store electrical energy and charge

what is conventional current

direction of positive charge, plus to minus

how do things behave in series

current is equal, voltage is split, resistance is sum, capacitance is reciprocal

how do things behave in parallel

current is split, voltage is equal, resistance is reciprocal, capacitance is sum

energy in a capacitor equation and graph

v against q, area underneath line

capacitance force equation

f = eq derived from e=v/d and v = w/q

voltage in terms of work done

work done per unit charge w/q

discharging equation

the one without 1

what % for discharging and charging

37

electric fields in a capacitor

if gap between plates is small compared to length a uniform electric field is produced, f = eq, can equate electric potential energy to work done and divide by q to get V = -Ex

B =

magnetic field strength or magnetic flux density

what happens to a current carrying wire placed into a magnetic field

experiences a force

what happens around a current carrying wire

circular magnetic field generated

right hand grip rule for a wire

thumb is current fingers is field

right hand grip rule for a solenoid

thumb is field fingers is current in wire coil

what happens if current in two wires is in the same direction

force attracts

what happens if current in two wires is in opposite directions

opposite forces

circular paths in magnetic field

equate magnetic force to centripetal force, no work done as magnetic force perpendicular

LINACs

proton accelerates toward a cathode, LINAC switches polarity of drift tube to keep acceleration, frequency is constant so tube lengths increase, electric field used so work is done (acceleration)

cyclotrons

electric field accelerates charged particles, a uniform magnetic field at right angles causes circular motion, alternating pd keeps charges moving, polarity switches and radius increases, angular velocity is constant, equate magnetic force to centripetal force

equation from cyclotrons

f = bq / 2 pi m

synchrotrons

particles travel in a circle but radius is constant so B field increases in proportion with velocity, charges accelerated 4x per cycle

the hall effect

if an electric field flows through a conductor inside a magnetic field, the field will exert a perpendicular force on the moving charge carriers pushing them to one side, which will produce a potential difference

hall effect equations

equate magnetic force to electric force (bqv = eq) and use i=nave and e = v/d

magnetic flux / magnetic flux linkage

phi, measured in webers or tm squared

emf

electromotive force, basically a voltage acting as a force pushing charge carriers but no potential difference

electromagnetic induction occurs when

an emf is induced in a conducting wire when the magnetic flux linkage changes with respect to time

faradays law

the magnitude of induced emf is equal to the rate of change of magnetic flux linkage

what happens when a magnet falls through a coil of wire

magnet induces a current in the wire, this creates its own magnetic field, this induced emf must oppose the motion of the magnet otherwise conservation of energy disobeyed

lenz’s law

an induced emf or current will generate a b field that opposes the change of flux that produced it, in the opposite direction

why does the current dampen

opposing magnetic field and energy dissipated as heat from resistance in the wire

define electric field strength

force per unit charge of a test charge placed into an electric field

V = (electric fields)

work done per unit charge, W/Q

applications of electrostatic fields

photocopiers and electrostatic dust collectors

potential energy of a test charge

work done by the field on the charge from the point to infinity

electric fields go from

high to low potential

potential of a charge

work done by the field per unit charge

potential energy (electric fields graph)

the area under an er graph x q

1 u =

931 MeV

electric field strength = (graph)

gradient of vr graph

what do you do when theres more than one charge

field strength as a vector, potential as a scalar

newtons law of gravitation

every body of mass attracts every other body with mass

g =

force / mass

GPE = (graph)

m x area under gr graph

g = (graph)

negative gradient of vx graph

what happens when you add a dialectric

capacitance increases

what happens when you move a capacitor further apart (force)

work done transfers to electric potential energy so more energy is stored

uncertainty things

use absolute when adding, percentage when multiplying or dividing