Chapter 10 — Electric Current and its Effects

The longer line in the symbol for an electric cell represents its positive terminal. The shorter but thicker line represents the negative terminal.

A combination of two or more electric cells is called a battery. In a battery, the positive terminal of one cell must be connected to the negative terminal of the next cell.

When the switch is in the ON position, the circuit from the positive terminal of the battery to the negative terminal is complete (closed). Current flows throughout the circuit instantly. When the switch is OFF the circuit is open and no current flows in any part of the circuit.

The filament is a thin wire inside the electric bulb that glows when electric current passes through it. When the bulb gets fused, the filament is broken, making the circuit incomplete.

When electric current passes through a wire, the wire gets hot. This is called the heating effect of electric current. It is used in appliances such as electric room heaters, irons, immersion heaters, hotplates, geysers, electric kettles, and hair dryers, all of which contain a coil of wire called an element.

The amount of heat produced in a wire depends on its material, length, and thickness. For different requirements, wires of different materials, lengths, and thicknesses are used.

A fuse is a safety device made from a special material that melts quickly and breaks when a large electric current passes through it. This breaks the circuit and prevents the wires from becoming overheated, which could otherwise cause fires or damage electrical appliances. There is a maximum safe limit on the current that can flow through a circuit; if current exceeds this limit, the fuse wire melts and breaks.

A short circuit occurs when wires touch each other directly, which can happen if insulation wears off due to wear and tear. An overload occurs when many electrical devices are connected to a single socket, causing excessive current in the circuit. Both can cause fires.

A miniature circuit breaker (MCB) is a switch that automatically turns off when the current in a circuit exceeds the safe limit. Unlike a fuse (which must be replaced after it melts), an MCB can be turned back on to restore the circuit. ISI mark should be looked for on MCBs.

Hans Christian Oersted (1777–1851) was the first person to notice that the needle of a compass gets deflected every time an electric current is passed through a nearby wire. This led to the discovery of the magnetic effect of electric current.

When electric current passes through a wire, the wire behaves like a magnet. This is called the magnetic effect of electric current. A current-carrying coil can be used to make magnets.

An electromagnet is made by winding a coil of insulated wire around a piece of iron and passing electric current through it. The coil acts as a magnet when current flows. When the current is switched off, the coil generally loses its magnetism.

Electromagnets can be made very strong and are used in cranes to lift very heavy loads, to separate magnetic material from junk, by doctors to remove small pieces of magnetic material that have accidentally fallen in the eye, and in many toys. Electric bells also use electromagnets.

An electric bell has a coil of wire wound on an iron piece that acts as an electromagnet, and an iron strip with a hammer at one end placed near it. When current flows, the coil becomes an electromagnet and pulls the iron strip, causing the hammer to strike the gong. This also breaks the circuit, so the coil loses its magnetism and the iron strip springs back to touch the contact screw, completing the circuit again. The process repeats in quick succession, making the bell ring continuously.

Incandescent bulbs give out heat along with light, which wastes electricity. For producing a given intensity of light, LED bulbs consume less electricity than incandescent bulbs, fluorescent tubes, or CFLs, making them much more electricity efficient. Note: fluorescent tubes and CFLs contain mercury vapour, which is toxic, so damaged ones must be disposed of safely.

Yes — the NCERT Class 7 Science Chapter 10 PDF is available free with no sign-up on cbseprepmaster.com.