STATIC ELECTRICITY- where charges flow from the surface of one body to the surface of another
body.
Three processes of static electricity are:
1. Friction
2. Induction
3. Contact
1. Friction
- Action of 2 bodies rubbing against each other and heat energy is generated from it.
EG. Perspex rod and cloth
Both Perspex rod and cloth are electrically neutral. Meaning they have a balance of charge on both
surfaces. When rubbed against each other, the electrons from the surface gain energy produced
from the friction force. Each material has a property called electron affinity. In this case the cloth is
more electron affinitive than the Perspex rod. Electrons will leave the rod and enter on the cloth.
This allows the Perspex rod to become positive (due to loss of electrons) and cloth gaining
electrons to become negative.
EG.2 Polythene rod and cloth
Polythene rod and cloth are electrically neutral. By rubbing by friction, the polythene rod gains
electrons transferred from the cloth. The rod gains a negative charge while the cloth upon losing
electrons becomes positively charged. This is because the polythene rod is more electron affinitive
than the cloth.
CHARGING BY INDUCTION
EG 1. Charged body near to electrically neutral body
POSITIVE ROD
The metal sphere below is placed on an insulating stand to prevent charges from flowing away from
surface. The metal sphere has and even number of positive and negative charges. When the
positively charged rod is placed near to it, the charges redistribute themselves on the surface of the
metal sphere. They are said to undergo CHARGE SEGREGATION (where oppositely negative charges
accumulate on the side of sphere closest to the positively charged rod, and the positive charges are
repelled on the other side of the sphere, the sphere is said to be earthed.)
Earth= the ground being a store house for negative charges.
An earth connection is made to the side of the sphere with the positive charges. Electrons flow from
earth as they are attracted to the positive charges, thus the metal sphere acquires a negative charge.
Charging by induction produces an opposite charge on the surface than the charged body.
NEGATIVE ROD
Consider a negatively charged rod placed near to an electrically neutral sphere. The charges of the
sphere will be repelled on the opposite side of the sphere. To reduce the number of electrons, an
earth connection is done. Electrons flow to earth as they tend to be closer to neighbouring negative
charges and so repel each other. This induces a positive charge on the sphere.
CHARGING BY CONTACT
EG.1 NEGATIVE ROD TOUCHING INSULATED METAL SPHERE
While the metal sphere is insulated so that charges cannot flow to earth, the touching of the sphere
to the metal surface allows electrons to leave the rod and become attracted to the positive charges
on the sphere. This allows the previously neutral sphere to acquire electrons and become negatively
charged.
Charging by contact produces the same charge on the sphere as the rod.
The gold-leaf electroscope demonstrates electrostatic induction
A gold-leaf electroscope consists of a vertical rod attached to a flat metal plate. A light gold
leaf is connected to the lower end of the rod.
1. When a positively charged glass rod touches the plate, both the plate and the gold leaf
become positively charged. The gold leaf is then repelled from the rod due to the like
charges.
2. Bringing a positively charged glass rod close to the plate further diverges the gold leaf
from the rod. This happens because the metal (of which the plate, rod, and gold leaf
are composed) contains electrons that can move freely inside.
3. The approach of the positively charged glass rod attracts electrons toward the plate,
increasing the excess positive charge on the gold leaf and the bottom end of the rod.
As a result, the gold leaf moves away from the rod.
4. Conversely, if a negatively charged plastic rod approaches the plate, electrons are
repelled from the plate, reducing the excess positive charge. The gold leaf then
approaches the rod.
5. By using charged rods and deftly manipulating the system, you can induce a positive
or negative charge on the electroscope.
ELECTRIC FIELD
An electric field is defined as the region around a charge where a force is experienced.
APPLICATIONS OF STATIC ELECTRICITY
1. Electrostatic painting- painting of cars. The metal of the body is earthed to make the car
negatively charged. The paint is charged by friction as it is ejected from the nozzle of the
container. The positive droplets adhere to the negative body.
2. Dust precipitator- industries to clean polluted air.
3. Xerography- process where photocopy machines produce a copied image using static
electricity.
4. Van de Graff generator- device generates static charges, producing voltage up to 14 million
volts. A pulley drives an insulating belt alongside a pointed metal comb which has a positive
charge, Electrons are removed from the belt leaving it positively charged. The charge moves
to the top of the metal sphere where it is deposited at the top.
5. Lightning- large quantity of charge being build up in heavy thunderclouds. Result of charging
by friction between polarised water molecules and surrounding air molecules. The + and –
charges separate in the cloud with – charges in the bottom of cloud. This induces a + charge
at the surface of buildings and other bodies. This facilitates the discharge of the electrons
which is a lightning strike. (Lightning conductors are used alongside tall buildings. These
metal rods have a pointed tip and they allow the flow of electrons through them to the
ground)
CURRENT ELECTRICITY
Flow of charges through a device leading to generation of electric current. The charges must have
energy to move which is provided by the battery or power source. A consequence of current is HEAT
ENERGY. As such, there must be RESISTANCE within the device and the circuit to prevent overheating.
When charges either + or – flow through a conductor, an electric current will flow. In case of metal, a
flow will be of negative charges. Electrolyte is flow of positive and negative charges.
One ampere is defined as the current produced when a charge of one coulomb flows for one second.
An electric current is measured by a device called AMMETER.
AMMETER HAS A LOW RESISTANCE AND MUST BE ALWAYS CONNECTED IN SERIES IN A CIRCUIT.
Conventional current- positive charges flow in an electrical circuit
Electron flow- movement of negative charges are considered to flow
VOLTAGE OR POTENTIAL DIFFERENCE- WORK DONE TO DRIVE A CHARGE AROUND A CIRCUIT
For charges to move around a circuit, they must have energy. For charges to become mobilised, they
must be supplied with electrical energy. (from a battery) This energy is defined according to terms of
VOLTAGE OR POTENTIAL DIFFERENCE. The voltage across the battery when there is no current
flowing is termed EMF OR ELECTROMOTIVE FORCE. When current is flowing it is the TERMINAL
POTENTIAL DIFFERENCE.
The TPD is less as charges use energy to overcome the internal resistance within a battery.
The battery has a positive terminal/potential
And negative terminal/potential. This creates a charge gradient.
Positive charges repelled by positive terminal and attracted to negative terminal. This force does the
work to drive the charges around the circuit. 2 POINTS CANNOT BE THE SAME POTENTIAL FOR
CURRENT TO FLOW IN A CIRCUIT, THERE MUST BE A POTENTIAL DIFFERENCE ACROSS THE CIRCUIT.
Volt- voltage between two points in a circuit.
VOLTMETER USED TO MEASURE VOLTAGE. LARGE RESISTANCE AND CONNECTED PARALLEL TO A
CIRCUIT
TWO CATEGORIES OF VOLTAGE SUPPLIERS
1) Primary cells
2) Secondary cells
1.
-
Primary cells
Also called a dry cell.
Cannot be reused.
Voltage of 1.5V.
Resistance of 0.5 ohms.
Zinc cathode, carbon anode.
Electrolyte ammonium chloride solution.
A depolarizer, manganese IV oxide is used to reduce the presence of H ions produced from
the electrolytic reaction.
Cheaper than secondary cell and lighter
2.
-
Secondary cells
Lead acid cell
Recharged and reused
Car battery
6 2v cells = voltage of 12V
Internal resistance of around 0.01 ohms
Greater current than primary cells
Heavier and bulkier
Cathode is lead; anode is lead iv oxide
Electrolyte= dilute h2so4
Recharged by passing a current in opposite direction
RESISTANCE- to oppose the flow of charges. Ratio of voltage across device to current flowing through
a device
Necessary because flow of charges causes heat.
If there is a negligible resistance in a circuit, a short circuit is created, can lead to burn out of wires
and start electrical fire.
Metals and electrolytes obey ohms law. If the voltage across a device increases, the charges obtain
more energy and generate a large current. The resistance of device is constant as well as temp.
Resistance is affected by the physical properties of length, cross sectional area and the resistivity (p)
of material. Also temperature, thermistors the resistance can either dec or inc with temp,