Exam practice

A substance that is attracted by a magnet is called a magnetic substance.

Dilute sulphuric acid is used in a simple cell.

The region around a magnet in which it attracts or repels is called its magnetic field.
The word “electromagnet” is not needed.

False. It is called the magnetic field. The poles are the two ends of the magnet, where its pull is strongest.

True.

True.

True.

When too much current flows, a fuse wire melts, and it must be taken out and a new wire of the right size fitted, which is slow and can be unsafe. An MCB simply switches itself off; once the fault is put right, it is pushed back up and works again. Nothing has to be replaced. So an MCB is easier to use than a fuse.

A fuse wire melts at a low temperature and breaks the circuit as soon as more current flows than the circuit can safely carry. This stops the current before the house wiring and the appliances get dangerously hot, so it protects them from damage and the house from fire. That is why a fuse is called a protective (safety) wire.

The region round a magnet in which its effect (its pull or push on magnetic substances) can be felt is called its magnetic field. A strong magnet’s field reaches farther than a weak magnet’s.

A magnet made by passing an electric current through a coil of wire wound round a piece of iron is called an electromagnet. It is a magnet only while the current flows.
To make one:

1. Take an iron nail and about 1 m of insulated copper wire.
2. Wind the wire round the nail, 80 to 100 turns, always in the same direction.
3. Scrape the plastic off the wire’s two ends, and join them to the two terminals of a dry cell (through a switch).
4. While the current flows, the nail picks up pins and iron dust: it is an electromagnet. When the current is stopped, the pins drop.

The continuous path made by joining a source of electricity, conducting wires and a load, so that electricity can flow through it, is called an electric circuit.
Its basic parts are:

1. a source of electricity, such as a cell;
2. conducting wires, such as copper wires;
3. a load, such as a bulb.

A switch is usually added to open and close the circuit.

The metal coil (usually of nichrome) that gets very hot when an electric current flows through it, changing electrical energy into heat, is called a heating element.
Examples: the coil of a room heater, and the element inside an electric kettle (also: an electric iron, an immersion rod, a rice cooker).

A cell made of two different metal plates, such as copper and zinc, dipped in an acid (dilute sulphuric acid), which changes chemical energy into electrical energy, is called a simple cell.
A potato cell:

1. Take a potato, a small zinc plate (or a galvanized nail) and a small copper plate (or a thick copper wire).
2. Push the two plates into the potato, a few centimeters apart, without touching each other.
3. Join the two plates with copper wires to a galvanometer.
4. The galvanometer’s needle moves: a small current flows. The juice of the potato acts like the acid, and the two different metals make the electricity.

Join the cell, the switch and the bulb in one loop, with the wires drawn as straight lines.

1. I would not try to change the fuse myself, because touching the fuse box can give a dangerous electric shock.
2. I would switch off or unplug the appliances that were on, especially big ones like a heater, an iron or a kettle, because too many of them may have caused the fuse to blow.
3. I would stay away from the fuse box and any wires that smell of burning, and never touch them with wet hands.
4. I would tell my parents or a neighboring adult, or call an electrician, so that a grown-up who knows how can find the fault and fit a new fuse wire of the right size.

It is a dangerous idea, and Ronish’s father should not do it.

1. A fuse wire is made thin, to melt at a low temperature. A thick copper wire doesn’t melt easily, so it would not break the circuit when too much current flows.
2. Then the house wiring and the appliances would get very hot, and could be damaged or catch fire.
3. A fuse that blows often is a warning that something is wrong: too many appliances on at once, or a fault in the wiring or in an appliance.

Instead, he should find the cause: use fewer heavy appliances at the same time, have an electrician check the wiring and the appliances, and replace the fuse with a fuse wire of the right rating (or fit an MCB).

If the MCB is held up with a stick, it can’t switch off when too much current flows. The house is then as unprotected as if it had no MCB. The wires would get very hot, appliances could be damaged, and the house could catch fire; someone touching a faulty appliance could get a severe shock.
Suggestions:

1. Remove the stick at once, and never block an MCB.
2. Don’t run many heavy appliances (heater, iron, kettle, water pump) at the same time.
3. Call an electrician to find and repair the fault in the wiring or the appliance that makes the MCB trip.

1. At home: we use electricity for light, and to run fans, refrigerators, rice cookers, irons, water pumps and washing machines.
2. Communication: phones, televisions, radios, computers and the internet all need electricity.
3. Education and health: schools use computers and projectors; hospitals run X-ray machines, life-saving machines and fridges for medicines on electricity.
4. Industry and farming: factories, mills and irrigation pumps run on electricity.
5. Transport: electric buses, cars and cable cars run on it, and they don’t pollute the air of our cities as petrol and diesel do.
6. Clean energy: Nepal makes most of its electricity from its own rivers, so using it saves the money spent buying petrol, diesel and gas from abroad.

So almost no work in modern life can be done without electricity: it has become an essential source of energy.