My document is an A4 poster on Topic 7 A-Level Physics, Electric and Magnetic fields. It covers the key points and summarises them all briefly in short paragraphs with diagrams and equations to help.
Static Electricity Electric fields and charge Capacitors
Matter is either positive or negative A charge comes in discrete Capacitors store electrical energy, it
(or neutral). Static electricity is quantities of q (q=1.6x10^-19). is like an electrical reservoir, they
caused by friction, the rubbing of consist of two metal plates
two materials makes the electrons An electric field is a region of space
separated by a thin insulator.
move. where a charged particle
experiences a force.
Equipotential lines are a line where If you were going to do a practical
Permittivity
all the electric field strength is the where you were trying to calculate
Permittivity is how easy an electric same. how much charge could be stored
field can pass through an object or against the voltage supplied, you
Electric field lines are used to show
region. would see it is directly proportional.
the direction a charged particle
ε 0 is the permittivity of a vacuum would move in. V=CQ
−12 −1 −1
and ε 0=8.85 ×10 CV m Where C is the capacitance, its
ability to store charge, in Farads.
Electric field strength from a point 1 Farad is the 1 coulomb of charge
source: stored for 1 volt.
Q The area under a Q-V graph is the
E= 2 energy stored by a capacitor, 1/2 QV
ε0 4 π r
= ½ CV^2.
Electric field strength = 1/d^2
Coulombs law:
Uses of capacitors are in camera
k Q1 Q 2 flashes, defibrillators and converting
F= AC-DC.
r2 Electric field strength, E
Force is proportional to the product E = F/Q
of the two charges but inversely Discharging capacitors
From a sphere:
proportional to the separation of the
two charges.
When a capacitor is discharged it
AC Supplies:
falls exponentially,
I0 −t
I RMS =
√2 Q=Q 0 e RC
Peak power = IV −t
RC
V =V 0 e
−t
I =I 0 e RC
Time Constant:
Between two plates: τ =RC
E = V/d You can use it to find the half-life of
a capacitor by doing RCln(2) or τ
Pith Plates
ln(2).
A ping-pong ball is covered in tin-foil
and placed between two oppositely
charged plates, it will then
experience a force (E=F/Q) and
move towards one of the plates, as
soon as it touches it, the ball will
discharge and repel.
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