Electricity in Physics

Electron has a negative charge equal to -1.6 x 10-19 coulomb while the charge on a proton is exactly equal and opposite to that on electron and is equal to +1.6 x 10-19 coulomb.Charge can neither be created nor destroyed. This is known as law of charge conservation; charge conservation is followed in nuclear reactors, electricity, electrification by friction, electrification by induction, etc.

For the electrification of a body only electrons are responsible.

If a body is negatively charged then it has an excess of electrons, while if a body is positively charged then it has a deficit of electrons.

Good conductors are the substances which allow an easy flow of charge through them.

They have a large number of free e- in them. Best conductor of electricity is Ag. All metals are good conductors.

If a small charged conductor is placed inside another big and hollow charged conductor and the two are joined by a wire then the charge flows from smaller conductor to bigger conductor because the potential of smaller conductor is more than that of bigger conductor.

Fuse Wire

Fuse Wire is used in a circuit to control the maximum current flowing in a circuit. It is a thin wire having high resistance and is made up of a material with low melting point.

House Wiring circuits are in parallel therefore the voltage across each bulb is constant.

The filament of 60 W bulb is thinner than the filament of 100 W bulb.

If two bulbs, one of 60 W and the other of 100 W are connected in parallel, then 100 W bulb will glow more.

If two bulbs, one of 60 W and other of 100 W are connected in series, then the 60 W bulb will glow more, because in series combination current through both the bulbs is the same and the Power consumed P = I²R or P ∞ R.

Now, since the resistance of 60 W bulb is more than the resistance of 100 bulb, the 60 W bulb will glow more.

In Series Resistance Circuit, it should be remembered that :

• The current is same in every part of the series circuit.
• The total resistance in the circuit is the sum of the individual resistances including internal resistance of the cell (if any).
• Voltage across any part of a circuit is proportional to the resistance of that part
• Current in the circuit is independent of the relative positions of the various resistances in the series.

In Parallel Combination of Resistances, it should be remembered that :

• Total current through the combination is the sum of individual currents through the various branches
• The potential difference across all the resistances is the same.
• The current through each branch is inversely proportional to die resistance of that branch.
• The reciprocal of the total resistance of the combination is equal to the sum of the reciprocals of the individual resistances.

• Resistivity of a conductor changes with impurity. The impurity increases the resistivity.
• Resistivity of an alloy is greater than the resistivity of its constituents.
• Resistivity increases with temperature.

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