Topic 7 - Electric an Magnetic Fields Flashcards
Magnetic Fields
A region surrounding a magnet or current carrying wire which acts upon any other magnet or current carrying wire placed within the field
Electric Field
A region of space in which a (small, positively) charged particle feels a force
Electric Field Strength
force per unit charge, E = F/Q
what do field lines show
Strength and direction
closer together lines shows a stronger field
Uniform fields
Same strength at any point in the field
- evenly spaced lines, arrows from + to -
Radial fields
strength decreases as distance from the point charge increases
- all equal angles, arrows from + to -
field line drawing rules
lines can never cross
Electric neutral/ null point
where there are no fields lines: the forces from the field is balanced.
electric field strength of a uniform field
E = v/d
charging
electrons are transferred from one material to another
inducing a charge in an object without touching the object
- A negatively charged strip is bought close to 2 touching metal spheres.
- Electrons from sphere A are repelled to sphere B.
- The spheres are separated with the strip nearby
- the strip is removed and the charged spreads out so it is distributed evenly
- B is now negatively charged and A is positively charged
coulomb meter
measures charge
force between two charges
- directly proportional to each of the charges Q1 and Q2
- inversely proportional to the square of their separation
Coulombs law
F = kQ1Q2 / r^2
F can be repulsive or attractive
electric potential
the potential energy that each coulomb of positive charge would have if placed at that point in the field.
potential difference
the energy transferred when one coulomb of charge passes from one point to another point
W = VQ
Equipotentials
positions within a field with zero potential difference between them.
They are perpendicular to field lines
Equipotentials between parallel plates
even spaced perpendicular to field lines
Equipotentials for a point charge
concentric circles, closer together near the point charge
charge moving along equipotential
no work is done because the potential energy does not change
Capacitor
an electrical component that stores and releases charge (and therefore energy)
Capacitor uses
Defibrillators & Camera Flashes
charging up a capacitor
- when the switch is closed, electrons flow from the negative terminal of the battery onto the first plate of the capacitor, this becomes negatively charged
- electrons are repelled from the second plate around the circuit
- there is a force of attraction between the plates
- this charging process continues until the pd. across the capacitor is equal to the pd. of the supply
discharging a capacitor
- disconnect the battery
- Initially there is a large current due to the large potential difference across the plates. The current drops as pd drops
- current flows opposite way round the circuit when discharging
- the charge drops quickly at first then more slowly
Capacitance
the ability to store charge on the plates of a capacitor. The charge stored per unit of pd. across it.
Q = VC
Capacitance Unit
Farads
energy stored in a charged capacitor
1/2 QV
(proof is VQ graph, area under graph = area of triangle)
(also can sub in Q = CV, V = C/Q to m=have a different form)
Capacitors in paralell
C = C1 + C2 ….
Capacitors in paralell proof
Q = Q1 + Q1 ... Q = CV CV = C1V2 + C2V2 ... V is constant in parallel C = C1 + C2...
capacitors in series
1/C = 1/C1 + 1/C2 ….
Capacitors in series proof
V = V1 + V2 V = Q/C Q/C = Q1/C1 + Q2/C2 Q is constant in series 1/C = 1/C1 + 1/C2
The greater the capacitance
The greater the charge stored
As the pd. falls during the discharge of a capacitor the time…
for the pd to drop takes longer and longer
time constant
the time it takes for the charge to drop to 37% (1/e) of the original value (=RC)
RC / time constant unit proof
RxC = V/I*Q/V = Q/I = Q * t/Q = t
capacitance discharging graph (Qt, It, Vt)
exponential decay
capacitance charging graph (Qt, It, Vt)
current asymptotically reaches 0 (gradient decreases as it goes up)
exponential
same fractional change in y for each interval change in x
to plot a linear graph from discharge equations
take logs from both sides and follow log rules
magnetic field lines
lines of flux (always from N pole to S pole)
Magnetic Flux Density, B
the flux per unit area ( an indication of the strength of the magnetic field).
Magnetic Flux Density Unit
Tesla, T
Magnetic Neutral point
the point between two north poles where the magnetic field cancels and the resultant field strength is zero.
Flux Φ
the flux passing through an area A perpendicular to the magnetic field B is defined as Φ=BA
Flux Linkage
the product of magnetic flux and the number of turns on the coil NΦ = BAN
Flux/Flux Linkage Unit
Weber, W (sometimes Flux linkage is Weber Turns)
right hand rule
always a magnetic field around a current carrying wire
Flemings left hand rule fingers
Thumb - Force
Index (First Finger) - Field
Middle (Second Finger) - Current
Flemings Left hand rule
the conductive wire cuts lines of flux
A force acts perpendicular to both current and magnetic field
If the current is parallel to the lines of flux
no force acts
The force due to flemings left hand rule is greatest when
the wire and field are perpendicular otherwise only a component of the field strength is acting
The force due flemings left hand rule depends on
- size of current
- magnetic field strength
- length of wire
- angle of wire to magnetic field
Force on current carrying wire equation
F = BILsinθ
Force on single moving charged particle in a magnetic field
+equations needed for proof
F = BQvsinθ
from F = BIL L=vt and I = Q/t
shape of the path of charged particle in a magnetic field
Circular
Why is the path of a charged particle in a magnetic field circular?
by fleming left hand rule the force on amoving charge is always perpendicular to its direction of travel, this the condition for circular motion
EMF Induction
- movement of a conductor in a B-Field
- cuts lines of flux/ change in flux linkage
- an emf is induced
(- if part of a complete circuit an induced current flows)
Faraday’s Law
the induced emf is directly proportional to the rate of change on flux linkage
Ɛ = d(NΦ)/dt
Lenz’s Law
The induced emf is always in such a direction to oppose the change that caused it
Ɛ = - d(NΦ)/dt
Why is Lenz’s Law good
It agrees with the principle of conservation of energy
Factors effecting the emf induced in a coil
- Angle between the coil and the field
- Number of turns on the coil
- Area of coil
- Magnetic Field Strength
- Angular Speed of Coil
- magnitude/ frequency current in the wire
- add an iron core
Factor: angle between coil and field reasoning (emf induction)
smaller angle = less lines of flux cuts = lower emf
Factor: Number of turns on the coil reasoning (emf induction)
more turns = more points which cut each flux line = higher emf
Factor: Area of the coil reasoning (emf induction)
larger area = more lines of flux pass through = higher emf
Factor: Magnetic Field Strength reasoning (emf induction)
higher flux density = more lines of flux per unit area = coil cuts more lines of flux = higher emf
Factor: Angular Speed of Coil reasoning (emf induction)
increased angular speed = increased number lines of flux cut in a certain time = higher emf
Using Lenz’s law to predict direction of induced emf
fleming left hand rule… force is opposite direction to motion, B field as told then current shows direction of induced emf
EMF induction using two coils
- one coil has an AC current passed through producing a changing magnetic field
- second stationary coil then cuts lines of flux
- induces an emf in the second coil
Eddy Currents
if the conductor is large enough small rings of induced currents form. These will grow depending on the size of the conductor. These lose energy as heat.
if a conductor is moving through a B-field and the induced eddy currents are big enough it will stop the motion as all the KE s transferred to Heat.
conductor with cracks moving through B-field
the motion will not be stopped / slowed as much because the cracks stop large Eddy currents from forming.
relation between electric field and electric potential
electric potential is a property of electric fields
Where is there always a magnetic field?
around a current carrying wire
x
into the page
.
out of the page
solenoid
a long coil with many turns
magnetic field of a solenoid
lines through the centre looping around to the opposite end of the solenoid
flux density in the centre of a solenoid
constant
Why is the magnetic field of a solenoid greatly increases when a magnetic material is placed inside it
The atomic magnets of the core line up along the lines of flux inside the solenoid so the core is magnetised
what factors effect the force of a charged particle in a B field?
- the magnetic flux density
- charge on the particle
- the velocity of the particle
time period
is the time for one complete cycle
frequency
the number of cycles in one second
peak values
are the largest voltages or current an AC supply produces
root-mean-square value
when you square all the values, take the mean then square root the values.
other method of finding the rms value
divide by √2
what type of current does a transformer change?
AC
how does a transformer work?
an alternating current flows in the primary coil. This produces an alternating magnetic field in the soft iron core. This means the flux linkage of the second coil is constantly changing so an alternating voltage is induced across it.
transformer equation
Vs/Vp = Ns/Np
step-up transformer
increases the ac voltage (more turns on secondary coil)
step-down transformer
decreases the ac voltage (less turns on secondary coil)
where is energy lost of transformers?
eddy currents are induced in the soft iron core
how i energy los reduced in transformers?
the core is made of laminated iron sheets
is the time taken for a capacitor to loose half its energy greater or less than the time taken to lose half its charge?
W = QV/2
Q and V both increase over time
W will decrease faster so it takes LESS time to half in value
advantage of data loggers which isn’t about reaction time
- take multiple readings at the same time
- more readings can be taken in a shorter time / higher sample rate
how does pd vary with charge for a capacitor
pd is directly proportional to charge
exponential decay curve
x decreases by proportional fractions in equal time intervals. decreases very rapidly then more slowly.
when asked to work out the resistance of a capacitor circuit
RC = time constant
time constant is the time it takes for I/V/C to drop to 37% of the original value
frequency in capacitor circuits
f = 1/t t = RC
arrows on a uniform electric field
positive to negative
electric field stength
force per unit charge acting on a small positive charge
faradays law of em induction
The induced e m.f. is proportional to the rate of change of magnetic flux linkage
what do to a DC current carrying wire to induce an emf in a coil
- make it AC
- move either coil
- turn the power on/off
what does an iron core do in emf induction
it becomes magnetised and increases the magnetic field
why is there a minus sign
- lenzs law/conservation of energy
- induced emf opposes the change that caused it
capacitance
the ability to store charge
AC root mean square
square root of the arithmetic mean of the squares
what energy transfers occur in the motor effect?
electrical to kinetic
electric potential , V =
V = kQ/r
difference between EM induction and motor effect
EM is when a conductor moves in a B field
Motor effect is when a current carrying wire in a B field feels a force
Motor effect
If two magnetic fields combine (i.e. a current carrying wire in a B-field) then a force is exerted (depending on fleming left hand rule.
size of force in motor effect is effected by
- increasing the magnetic field strength
- increasing he length of the wire in the field
- increasing the current in the wire
how to show an electric field in the lab
set up two parallel plates with a pd. across them. Put charged oil drop in the region between the two plates to show the force acting on the droplet.
what happens to a capacitor circuit when the switch changed to the power supply
Capacitor charges up so that the pd across the capacitor equals the pd of the supply. It has opposite charge on both plates.
As a capacitor charges…
current drops to zero
what happens to a capacitor circuit when the switch changed to the resistor circuit
Discharges over a period of time
why may a diode be used when inducing an emf
so the current is not discharged by alternating emf from AC supply
discharging a capacitor
- Electrons/charge transferred from negatively charged plate to positively charge plate through the resistor
- Hence the charge on capacitor decreases
- Until the charge on the capacitor equals 0
