Electricity 1 Flashcards
when is an object neutral
when it has the same number of protons as electrons
positive charge
more protons than electrons
loss of electrons
negative charge
more electrons than protons
gain of electrons
like charges
repel
opposite charges
attract
charge resides…
… on the outer surface of the object
measurement of surface charge density
charge per m squared
surface charge density is greatest at
sharp corners of the object
3 uses of outer surface charge distribution
- coaxial cable
- electrostatic shielding
- faraday cage
what happens to air molecules near a sharp charged object
they are ionised, split into positive and negative ions
definition of electric field
the region around a charged object in which its electric forces act
electric field lines radiating from the charge
radial field
electric field lines travel from positively charged plate to negatively charged plate
uniform field
electric field lines travel from positive charge to negative charge
opposite charges
2 properties of electric field lines
- there are no electric field lines inside an electric charged conductor
- a metal like aluminium can be used to alter an electric field
what is the basis for electrostatic shielding
a metal like aluminium can be used to alter an electric field
basis of the faraday cage
there is no electric field inside a hollow charged conductor
electric field in an airplane
an aircraft in flight may become charged due to friction between the air and the body of the aircraft. This build up of charge could lead to sparks when the aircraft lands and is being refuelled. Tyres made of conducting materials are used to remove the charge from the body of the aircraft upon landing
electrostatic precipitator
used to remove dust particles from the chimney of a power station. The dust particles become charged as they pass through a charged wire grid, they are then attracted to the oppositely charged metal plates.
Coulomb’s law
the force between two point charges is proportional to the square of the distance between them
ℨ
permitivity value
Definition of electric field strength
the electric field strength at a point in an electric field is the force acting on a test charge q, placed at a point divided by the magnitude of the charge
using coulomb’s law
E=
Q
___________
4 pi e dsqared
E
electric field strength
Q
magnitude of charge causing electric field
d
distance
e
permitivity
how many decimal places should you put down for e
4 decimal places
charge of electron
pg 47 log tables
charge of proton
same as electron
alpha particle charge
half an electron
quark charges
the one in table is wrong
negative charge is not a negative when?
in calculations
definition of potential difference
the potential difference between points a and b is the work done per unit charge to move the charge from b to a
2 units for potential differents
J/C
Volt
other word for potential difference
voltage
simple explanation for voltage
ability to attract or repel
gain in kinetic energy formula
1/2 m v squared
work done to accelerate formula
Vq
formula for gain in kinetic energy and work done to accelerate
v sqared = 2Vq
——–
m
V
potential difference (volt)
W
work done (joule)
q
charge (cuolomb)
m
mass (kg)
v
speed (m/s)
s
displacement (m/s/s)
definition of capacitance
the ratio of charge q on the object to the voltage of the object
C
capacitance
unit of capacitance
f = farad
1 farad =
1 coulomb / 1 volt
how big is capacitance usually?
very small around x X 10 -6 or x X 10 -12
what do capacitors consist of?
2 metal plates separated by an insulator
name of the insulator between capacitor plates
dielectric
what’s the use of a capacitor
to store electrical charge
A in capacitors
common area
4 uses of capacitors
tuning a radio
flash on a camera
block dc flow and allow ac flow
smoothing capacitor makes voltage constant
the build up of charge in a capacitor is…
non-linear
output of energy in a capacitor
non-linear
to charge a gold leaf electroscope by induction:
first step
bring a positively charged rod near the cap of the electroscope, the gold leaf rises, the leaf is positively charged and the cap i negatively charged
to charge a gold leaf electroscope by induction:
once the leaf is positively charged and the cap i negatively charged
touch the cap with your finger (earth it). the leaf falls as the charge in that region is neutralised, electrons come from the earth up to neutralise the positive charges
to charge a gold leaf electroscope by induction:
once it has been earthed
remove finger (earth connection) from the cap,then remove the positively charged rod, leaf rises again as negative charge from the cap spreads over the leaf region
to charge a gold leaf electroscope by induction:
conclusion
the gold leaf electroscope has been charged by induction
if you have 15 plates acting as capacitors, how many capacitors do you have
14
what is relative permitivity 1.5
Eo x 1.5
TO SHOW THE FACTORS THAT A CAPACITOR DEPENDS ON DISTANCE
what do you do
move the plates closer together and keep A and E constant
TO SHOW THE FACTORS THAT A CAPACITOR DEPENDS ON DISTANCE
what happens
smaller distance, larger capacitance
inversely proportional
TO SHOW THE FACTORS THAT A CAPACITOR DEPENDS ON COMMON AREA
slide them apart (less common area same distance)
keep E and d constant
smaller common area, smaller capacitance
directly proportional
TO SHOW THE FACTORS THAT A CAPACITOR DEPENDS ON PERMITTIVITY
wax between plates
A and d constant
larger permittivity, larger capacitance
directly proportional
charging a capacitor
positive curve
non linear
when the capacitor is fully charged
the current stops flowing and the voltage across the plates is the same as the voltage of a battery
DISCHARGING A CAPACITOR
discharge time very short
negative curve
output non-linear
DISCHARGING A CAPACITOR
what would happen to the electric field of the charged capacitor
would adversely affect surrounding electrical components
what type of current do capacitors allow
ac but not dc
how to charge a gold leaf electroscope by induction (3 steps)
- bring a positively charged rod near the cap, leaf rises (positively charged), cap is negatively charged
- touch the cap with your finger (earth it), leaf falls as is neutralised by electrons coming from the earth
- remove finger, then remove positively charged rod, leaf rises again as electrons from cap move down to it
Charging a capacitor:
graph of charge over time
non linear, rises quickly and starts to level off
Charging a capacitor:
when a capacitor is fully charged
the current stops flowing and the voltage across the plates is the same as the voltage of the battery
Discharging a capacitor:
graph of charge over time
output is non-linear, discharge time very short - a few seconds
what would the electric field of a charged capacitor do?
it would adversely affect surrounding electrical components
circuit of a capacitor, a.c source and a bulb:
the larger the capacity of capacitor
the brighter the light from the bulb
circuit of a capacitor, a.c source and a bulb:
the higher the frequency of a.c supply
the brighter the bulb
