Electricity1 Flashcards
To charge a gold leaf electroscope by induction
(a) bring a positively charged rod near the cap of the electroscope. The gold leaf rises. The leaf region is positively charged. The cap has a negative charge.
(b) touch the cap with your finger I.e. earth theThe leaf falls as the charge in that region is neutralised. Electrons come from the earth to the leaf region to neutralise the positive charges
(
To charge a metal sphere by induction
(a) take a conducting sphere (metal) sitting on an insulated stand.
(b) bring a negatively charged rod near the sphere. Charge separation happens on the sphere i.e. Some electrons move to the far side of the sphere leaving behind atoms lacking some of their electrons ( I.e. Positive charge)
(c) when the right side of the sphere is earthed the electrons that moved to the right now run to earth.
(d) remove the earth connection and THEN remove the negative rod. Some electrons from atoms on the right size of the sphere move over to the positive atoms on the left side. The result is now that some atoms all over the sphere lack electrons, i.e. the positive charge has been distributed all,over the sphere.
Note: - charging by induction only works for conductor type materials
- charging by contact (friction) works for conductor and insulator
type materials
Describe the experiment to show that total charge resides on the outside of a conductor
- a metal can is placed on the top of a Van de Graaff generator as it is being charged up. Charge accumulates on the sphere of the generator and on the outside of the can
- let a proof plane touch the inside of the metal can. Transfer the proof plane to the cap of the gold leaf electroscope. The gold leaf does not diverge as there is no charge on the inside of the metal can
- let a proof plane touch the outside of the metal can. Transfer the proof plane to the cap of a gold leaf electroscope. The gold leaf will diverge as charge reside on the outside of the metal can.
Point discharge(point action):
- Air molecules near the sharp point of a charged object are ionised. The air molecules split into positive and negative ions.
- the negative ions are attracted to the large concentration of positive ions at the sharp point. The negative ions combine with some of the charge on the sharp point.
- the positive ions are repelled by the very large concentration of positive ions at the sharp point. This movement. Of positive ions away from the sharp point gives rise to an electrostatic wind.
Demonstrate experiment to show that charge concentrates at the sharp corner of an object.
- the pear shaped conductor shown above is charged by induction
- two similar proof planes touch the conductor, A at the sharp end and B at the blunt end
- the proof planes are now placed on the caps of identical gold leaf electroscopes.
- proof plane A causes a larger deflection than proof plane B, showing that there was a greater concentration of charge at the sharp end.
Give the definition of electric field
An electric field is the region around a charged object in which its electric forces act.
What are the properties of electric field lines?
- electric field lines start on a positive charge and end on a negative charge
- electric field lines do not intersect each other
- electric field lines are perpendicular to the surface of a charged conductor
- there are no electric field lines inside an empty hollow charged conductor
- a metal like aluminium can be used to alter an electric field. This is the basis of electrostatic shielding.
Demonstration of an electric field pattern in the laboratory.
- Place two aluminium electrodes in a beaker of olive oil. The voltage across the electrodes would be of the order of 2 000 V.
- Sprinkle semolina grains onto the oil.
- The grains line up in such a way that they show the electric field pattern.
Describe a co-axial cable
- the central wire carries the electric signal
- a copper mesh surrounds the central wire. The mesh prevents any interference from external electric fields. The mesh acts like a Faraday cage.
Give two applications of Electric Fields in everyday life.
- 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.
- An electrostatic precipitator is 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. The charged dust particles are then attracted to oppositely charged metal plates further up the chimney.
What is Coulomb’s Law?
Coulomb’s Law:
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.
F= 1/4piE q1q2/ d2
What is Relative Permittivity?
Relative Permittivity is the number of times the permittivity of a medium is greater than the permittivity of free space.
What is 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 the point divided by the magnitude of the charge.
E= F/q
E= electric field strength; F= force on test charge; q=magnitude of charge
What is Definition of Potential Difference?
The Potential Difference between the points A and B is the work done per unit charge to move the charge from B to A.
V= W/q
What is Definition of Capacitance?
The Capacitance of an object is the ratio of the charge q on the object to the voltage of the object.
C= q/ V C= Capacitance; q= charge; V= voltage
Note: the unit for Capacitance is coulomb/ volt = farad
