PHYSICS. CURRENT ELECTRICITY. INSULATORS. Insulators are materials that do not have free electrons, and thus cannot conduct electricity. CONDUCTORS.
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PHYSICS CURRENT ELECTRICITY
INSULATORS Insulators are materials that do not have free electrons, and thus cannot conduct electricity.
CONDUCTORS Conductors are materials that have free electrons and are able to conduct electricity. Other electrical conductors include electrolytes and wished gases which contain positive and negative ions that are free to move.
An electric current I is a measure of the rate of flow of electric charge Q through a given cross-section of a conductor. In symbols, I=Q/T where I=current Q=charge and t=time ELECTRIC CURRENT
Electric current cont’d We learnt that electrical insulators and conductors can be charged by friction and induction. For an isolated charged conductor, the charges on its surface do not move. In other words, they are static charges. However, if we provide a conducting path, the charges will flow. When this happen, we say an electric current is produced.
Gravitationalanalogueof a simpleelectric circuit • In the case of the source of e.m.f(the cell) doing work on the positive charge to move it from a point of lower potential (negative terminal) to a point of higherpotential(positive terminal) the person does work in lifting up the balls from the ground to the top of chute.
Electron flow and Conventional current Conventionally, the distance of the electric current is taken to be the flow of positive charge. as mentioned earlier, the current detected by the galvanometer is actually due to the negatively-charged electrons moving from plate B to plate A. This electron flow is in the opposite direction to that of the conventional current.
Relationship Q=It Worked Example If 30 C of electric charge flows past a point in a wire in 2 minutes, what is the current in the wire? Solution: Given: charge=30C time t=2*60 s Current I=Q/t 30/2*60 =0.25A
Worked Example 18.2 The current in a lamp is 0.2A.If the lamp is switched on for 2 hrs,what is the total electric charge passing through the lamp? Solution: Given: current I=0.2A time t=2x60x60s Total electric charge Q=It =(0.2)(2x60x60) =1440C
ALTERNATING CURRENT • An alternating current (AC) is an electric current whose direction reverses cyclically, as opposed to direct current, whose direction remains constant. The usual waveform of an AC power circuit is a sine wave, as this results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves. • Used generically, AC refers to the form in which electricity is delivered to businesses and residences. However, audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal.
DIRECT CURRENT • Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. In direct current, the electric charges flow in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was Galvanic current.
VOLTAGE TIME GRAPHS Voltage time graphs
PHYSICS CONT’D ELECTROSTATICS
NEGATIVE CHARGED DETECTION If a charged rod with an unknown charge is brought near to a negatively charged electroscope and there is an increase in the divergence of the gold leaf. To charge the electroscope negatively, we can slide a negatively –charged rod on the brass cap of the electroscope so that some electrons from the charged rod can be transferred to the cap causing the gold leaf to deflect or diverge.
The force between electric charges,however,acts over a distance without the charges being in contact with one another. This non-contact force exists even across vacuum. The field concept is used to explain why the electric force is a non-contact force.
Charging conductors by induction Induction is the process of charging a conductor without any contact with the charging body. A negatively charged rod used is brought near the sphere A. this causes the electrons from A to be repelled to the farthest side of sphere B. under the condition , sphere A alone will have excess positive charge due to electron loss) while B alone has excess negative charge due to gain of electrons.)
ELECTRIC FIELDS An electric field exists in a region of space where a small positive charge experiences an electric force. The direction of the field is defined as the direction of the force on a small positive charge.
REPRESENTING THE ELECTRIC FIELDS Lines of force are used to represent the direction of an electric field. The lines of force are directed outwards for a positive charge and inwards for a negative charge. The strength of the electric field is indicated by how close the field lines are to each other. The closer the field lines, the stronger is the electric field in that region.
ELECTRICFIELDS CONT’D The field pattern set up by a positive charge and a negative charge placed close together. Parallel plates The electric field between two parallel oppositely charged plates is uniform at the central region. Take note that the field lines start from positive charges on one plate and end on negative charges on the other plate.
Some hazards of electrostatics It is common to see flashes of lightning just before and also during a thunderstorm. This due to large quantity of electric charge being built up in the heavy thunderclouds. The thunder clouds are charged by friction between the water molecules in the thunder clouds and the air molecules. To prevent lightning from damaging tall buildings, lightning conductors are used.
PHYSICS ATASHA BARON END OF POWER POINT PRESENTATION