Class 7 Science Electric Current and its Circuits Notes

4.6 Ohm’ Law :

Ohm’s law states that at constant physical condition (i.e., at constant temperature), the electrical current flowing through a conductor is directly proportional to the electrical potential difference between the two ends of the conductor.

i.e., I ∝ (V_A - V_B)(where V_A - V_B = V)

or I ∝  V

or , I = V/R

Where R is the constant of proportionality called electrical resistance of the conductor.

Electrical Resistance : Electrical resistance is the material property which obstructs the flow of current/charge through the material.

(i) It depends on the nature of the material

(ii) It also depends on the temperature.

The SI unit of resistance is Ohm. Symbol of Ohm in Ω.

Note:- Ohm’s law is defined at constnat temperature because resistance of a mateiral changes with the change in temperature and hence affects the flow of current though the conductor.

4.7 Combination of Resistances

We can combine resistances in two ways. The first combination is called series combination and second combination is called parallel combination.

Series Combination

4.8 Heating effect of current

The wire gets hot when an electric current passes through it. This is called heating effect of the electric current. You might have seen an electric room heater or an electric heater used for cooking. All these contain a coil of wire. This coil of wire is called an element.

Wires made from some special materials melt quickly and break when large electric currents are passed through them. These wires are used for making electric fuses. In all building fuses are inserted in all electrical circuits. There is a maximum limit on the current which can safely flow through a circuit. If by accident the current exceeds this safe limit, the wires may become overheated and may cause fire. If a proper fuse is there in the circuit, it will blow off and break the circuit. A fuse is thus a safety device which prevents damages to electrical circuits and possible fires.

4.9 Magnetic effect of electric current

Take the cardboard tray from inside a discarded matchbox. Wrap an electric wire a few times around the cardboard tray. Place a small compass needle inside it. Now connect the free ends of this wire to an electric cell through a switch.