Presentation on Physics Prepared By :- Heena (Applied science Deptt. Gpes Meham)

Electrostatics ELECTROSTATICS-Branch of physics which deals with study of effect of a stationary charges is called electrostatics .

Some materials attract electrons Electric Charge The Transfer of Charge Glass Rod SILK Some materials attract electrons more than others.

Electric Charge The Transfer of Charge - + SILK Glass Rod SILK - + As the glass rod is rubbed against silk, electrons are pulled off the glass onto the silk.

Electric Charge The Transfer of Charge - + + - SILK Glass Rod SILK - + + - Usually matter is charge neutral, because the number of electrons and protons are equal. But here the silk has an excess of electrons and the rod a deficit.

Electric Charge The Transfer of Charge + + - - - + - + - + SILK Glass Rod SILK + - - - + - + - + Glass and silk are insulators: charges stuck on them stay put. 1 3 1

Two positively charged rods Electric Charge + + Two positively charged rods repel each other.

q = multiple of an elementary charge e: e = 1.6 x 10-19 Coulombs Charge is Quantized q = multiple of an elementary charge e: e = 1.6 x 10-19 Coulombs Charge Mass Diameter electron - e 1 0 proton +e 1836 ~10-15m neutron 0 1839 ~10-15m positron +e 1 0 (Protons and neutrons are made up of quarks, whose charge is quantized in multiples of e/3. Quarks can’t be isolated.) 3 3 7

Coulomb’s Law k = (4pe0)-1 = 9.0 x 109 Nm2/C2 q1 q2 F12 r12 r12 Force on 2 due to 1 k = (4pe0)-1 = 9.0 x 109 Nm2/C2 e0 = permitivity of free space = 8.86 x 10-12 C2/Nm2 Coulomb’s law describes the interaction between bodies due to their charges 4 8 4

What is electricity? Electrons have a negative charge (Q) measured in coulombs (C). Electrons move round a circuit from negative to positive (remember like charges repel, opposites attract) giving rise to an electric current.

Electric Circuit

Diagram of Electric Circuit

Ammeter Measures electric current. Must be placed in series.

Example: What charge flows through a cross sectional area of a wire in 10min, if the ammeter measures a current of 5mA? Answer: 3C

Resistance Resistance of an object to the flow of electrical current. R= V / I Resistance equals the ratio of voltage to current. Unit: Ohm (Ω)

Ohm’s Law (Georg Ohm, 1787-1854) V = IR The voltage , V, across a resistor is proportional to the current, I, that flows through it. In general, resistance does not depend on the voltage.

Ohmic Resistor A device that obeys Ohm’s Law, who’s resistance does not depend on the voltage.

Resistor An object that has a given resistance.

A Battery Provides Energy The battery “pumps” positive charges from low (-) to high (+) potential. Electric Circuit

Resistors use up Energy A resistor uses up energy. When the current goes through the resistor it goes to a lower potential. Electric Circuit

Part III Factors that affect resistance. Potentiometer Voltmeter

Resistance Depends on type of material, size and shape, temperature. R=ρ L A L: length of the wire A: cross-sectional area ρ: resistivity (inherent to material)

Temperature Dependence of Resistance For metals: as temperature increases the resistance increases. At very low temperatures resistance can become zero: superconductivity. For semiconductors: the opposite occurs.

Voltmeter Measures the voltage between two points in an electric circuit. Must be connected in parallel.

A voltmeter is connected in parallel.

SEMICONDUCTOR Semiconductors are materials whose electrical properties lie between Conductors and Insulators. Ex : Silicon and Germanium

What are P-type and N-type ? Semiconductors are classified in to P-type and N-type semiconductor P-type: A P-type material is one in which holes are majority carriers i.e. they are positively charged materials (++++) N-type: A N-type material is one in which electrons are majority charge carriers i.e. they are negatively charged materials (-----)

Diodes Electronic devices created by bringing together a p-type and n-type region within the same semiconductor lattice. Used for rectifiers, LED etc

Diodes It is represented by the following symbol, where the arrow indicates the direction of positive current flow.

Forward Bias and Reverse Bias Forward Bias : Connect positive of the Diode to positive of supply…negative of Diode to negative of supply Reverse Bias: Connect positive of the Diode to negative of supply…negative of diode to positive of supply.

Characteristics of Diode Diode always conducts in one direction. Diodes always conduct current when “Forward Biased” ( Zero resistance) Diodes do not conduct when Reverse Biased (Infinite resistance)

I-V characteristics of Ideal diode

I-V Characteristics of Practical Diode

Rectification Converting ac to dc is accomplished by the process of rectification. Two processes are used: Half-wave rectification; Full-wave rectification.

Half-wave Rectification Simplest process used to convert ac to dc. A diode is used to clip the input signal excursions of one polarity to zero.

Full-Wave Rectification – circuit with center-tapped transformer

A full-wave center-tapped rectifier circuit is shown in Fig. 3. 1 A full-wave center-tapped rectifier circuit is shown in Fig. 3.1. Assume that for each diode, the cut-in voltage, V = 0.6V and the diode forward resistance, rf is 15. The load resistor, R = 95 . Determine: peak output voltage, Vo across the load, R Sketch the output voltage, Vo and label its peak value.   ( sine wave )

3.8V Vo t

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