electricity Flashcards
static electricity
occurs when there is a build up of electric charge on the surface of a material
why its called static electricity
- bc the charges dont move
- the electricity we use everyday involves moving charges
cause of static electricity
- static electricity is caused when certain materials are rubbed against each other
- electrons can be rubbed off one material and onto another
- the material that has got extra electrons is now negatively charged. The material that has lost electrons is positively charged
dangers of static electricity
- static charges can be dangerous
eg: planes must be earthed before refuelling to allow the charge to leave, so there is not an explosion
eg: if dust gets into certain machines and work areas they can do serious damage (car painting, flour mill, food preperation)
unit of electric charge
the Coulomb (c)
-it is the amount of charge in 6.25 x 10¹⁸ electrons
induced charge
- in a metal, there are free electrons that can move
- if a negatively charged rod is brought near the metal, these free electrons will move to the opposite side of the material
- the protons will move to the side nearest the rod
- if the rod is taken away, they move back again
- the charges produced is INDUCED CHARGE
atoms
an tom as a whole is electrically neutralo ie. it has no overall electric charge
charges
negatively charged -> object has gained electrons
positively charged -> object has lost electrons
what moves when objects become charged
it is only the electrons that actually move when objects become charged
charging by induction
-if we bring a negatively charged rod near to two metal spheres that are touching each other, the free negative electrons will go into the sphere that is furthest from the rod while positive charge will go to the near one
insulator
any substance through which electric charge cannot flow is called an insulator
conductor
any substance through which electric charge can flow is called a conductor
charging a single object by induction
- a single insulated conductor can be charged by induction
- bring a negatively charged rod to a conductor, touch the conductor with your finger, the negative charge will travel through you to earth
- remove your finger, then the red, the conductor will be positively charged
- the same can be done to make a negatively charged conductor
gold leaf electroscope
-consists of:
•v thin gold leaf attached to one end of metal rod
•other end of rod has metal disc (cap) attached
•leaf + rod are in metal case w/ window
- case + window stop droughts from casing leaf to move
- rod insulated from case so charge on rod does not flow away
uses of gold leaf electroscope
- detect charge
- indicate approximate size of a charge
- test whether a charge is + or -
- test if object is an insulator or a conductor
- indicate the size of a potential difference
detect charge
- if charged object brought near, induced charges appear on electroscope
- due to force of repulsion between charges, leaf diverges
- thus, it detects electric charge
indicate approximate size of a charge
- place objects w/ diff charges the same distance from electroscope
- larger the charge on object, the greater the divergence produced
test whether a char is + or -
- give electroscope a charge of known sign
- bring object w/ unknown charge near cap
- if divergence of leaf increases, object + electroscope have charge of same sign
- if leaf collapses, opposite charges (provided that when charge removed, leaf diverges again)
test if object is an insulator or a conductor
- charge electroscope, then, holding object in hand, touch cap w/ object
- if leaf collapses, object is a conductor
- otherwise, insulator
conductors
- all charges resides on outside of a conductor eg. dome of VdG generator
- static charge tends to accumulate where it is most pointed
- can be showen using VdG generator
- these things lead to development of lightning conductor, that safely brings charge to ground to protect buildings etc
point discharge
- if there is a sharp point on an object there is a large charge
- this leads to v strong force, forming around point
- ions are attracted + repelled to and from the point
- leads to other ions being attracted + repelled and eventually the charge on point is cancelled out
- this loss of charge is known as “point discharge” or “point effect”
know diagram of how it occurs - diagram with concentration of charge at point
experiment: to show that all static charge resides on the outside of a hollow metal conductor
1) connect a cylindrical metal can (the hollow conductor) to dome of Van de Graaff generator + turn on generator
2) Touch a proof plane against inside of can + bring proof plane very near cap of an uncharged electroscope
- -leaf will not diverge
3) touch a proof plane against outside of of can + bring proof plane very near cape of an uncharged electroscope
- -leaf will diverge (showing static charge is on outside)
Van de Graaff generator
needs to be smooth so it does not lose its charge (any charge on it will stay on the dome and not leak off)
Coulumb’s Law
states that the force of attraction or repulsion between two points is directly proportional to the product of the charges and inversely proportional to the square of the distance between them
Coulomb’s Law equation
in hback
Coulumb’s Law - Inverse square law
- this law is an example of inverse square law
- if distance doubled, force is four times smaller
- if distance is made three times bigger, force is 9 times smaller
Permitivity
- The ability of a substance to store electrical energy in an electric field.
- Permitivity, ε, is different for different media
Permitivity of free space/a vacuum
If forces are in a vacuum, permitivity is known as permitivity of free space or of a vacuum ε0
Unit of permitivity
the Farad per metre
F/m
equation
ε = εr x ε0
ε = permitivity εr = relative permitivity ε0 = in a vacuum
electric field
Any region of space where a static electric charge experiences a force other than the force of gravity
electric field line
- A line drawn in an electric field showing the direction of the force on a positive charge placed in the field
- The stronger the electric field the closer the lines are together
applications
- electrostatic precipitators
- the photocopier
- effects on integrated circuit
electrostatic precipitators
a charge is transferred to dust by the point effect then attracted to plates used to clean air in chimneys
the photocopier
see earlier notes
effects on integrated circuit
static off people moving around can ruin computers, people working with ICs often need to be earthed
electric field strength, type, unit
-electric field strength E at a point in an electric field in the force per unit charge at that point
formula + notation
- type: vector quantity
- unit: newton per coulomb (N/C)
electric field strength equation
E = F/Q
Q = charge F = force E = electric field strength
to show electric field patterns
- use equipment in diagram
- connect a high voltage source to the metal plates which are in the oil
- the semolina lines up in the direction of the field, showing the electric field
potential difference (V), type, unit
potential difference between two points in an electric field is the work done in bringing a charge of +1C from one point to the other
- unit: joule per coulomb (J/C) also known as the volt (V)
- type: scalar quanitity
volts and joule per coulomb
1 Volt = 1 Joule per Coulomb
Work done equation
W = QV
work done = charge transferred x voltage
How voltage is measured
using a voltmeter, can be estimated w/ a gold leaf electroscope
volt
the potential difference between two points is 1 volt if 1 joule of work is done when 1 coulomb is brought from one point to the other
potential at a point
potential difference between a point and the Earth is called the potential of that point
Earth potential
The Earth is at zero potential
Positive charge on a conductor
if a positive charge is added to a conductor it becomes more difficult to bring a charge from Earth to it, its potential increases
capacitance, unit
capacitance of a conductor is the ratio of the charge on the conductor to its potential
C = Q/V + notation
-unit: Farad (F)
capacitance equation
C = Q/V
C = capacitance Q = charge V = voltage/p.d
farads and coulomb per volt
1 farad = 1 coulomb per volt (C/V)
parallel plate capacitor
- two parallel plates separated by an insulator (dielectric)
- a capacitor stores charges
- when, the plates carry equal but opposite charges
parallel plate capacitor - to show it stores energy:
- set up equipment
- charge capacitor by connecting the battery
- remove battery + connect bulb
- bulb will flash as capacitor discharges - capacitor stores energy
Capacitors and current
capacitors conduct alternating current but not direct
uses of capacitors
- tuning radios
- flash guns on cameras
- smoothing out variations in direct currents
- filtering unwanted frequencies in sound systems
diagrams
hardback
capacitance of parallel plate capacitor equation
C = εA/d
C = capacitance ε = permitivity of the dielectric A = overlap of plates d = distance between plates
energy stored in a charged capacitor equation
W = 1/2 CV²
W = energy stored C = capacitance V = p.d
capacitance of a parallel plate capacitor depends on:
- the distance between the plates
- the common area of the plates
- the nature of the dielectric
to show that the capacitance of a parallel plate capacitor depends on:
- the distance between the plates
- the common area of the plates
- the nature of the dielectric
Procedure:
- use equipment in diagram. Divergence of leaf is measure of the potential difference between plates
- since C = Q/V and amount of charge Q is fixed, follows that the greater the divergence, the smaller the capacitance and vice versa
- charge plates by connecting them across a high voltage source (say, 2000V)
- move plates closer together ie. decrease d. Divergence of leaf decreases => C increases. If d is increased, opposite happens
- decrease overlap area + divergence increases => C decreases. If overlap area increased, opposite happens.
- Place diff slabs of insulating material between the plates. Divergence will be seen to be less than what it is for air
- Diff materials cause capacitance to increase over its value when dielectric in air
current
a flow of electric charge
conductor
allows charge to flow eg. wire, acid
insulator
tries to stop the flow of electrical charge eg. plastic, glass
3 effects of current
- heating
- magnetic
- chemical
3 effects of current - heating
bulbs, battery + wire heat up as current flows
3 effects of current - magnetic
current causes a compass to deflect off N-S
3 effects of current - chemical
wires in sulphuric acid + wire will bubble as current flows
charge, unit
particles that exert electrostatic forces on each other are said to be charged
-unit: Coulomb (C)
current, unit
- an electric current is charged particles moving
- in a metal conductor, the electric current is a flow of electrons
-unit: Ampere (A)
ampere and coulomb per second
1 ampere = 1 coulomb per second
1 A = 1 C/S
Charge equation
C = At
charge = current x time