A magnet on a fridge door holds up a paper all by itself. A small needle in a compass always swings round to point north. Press a switch, and in a blink a bulb across the room lights up. Neither magnetism nor electricity can be seen, yet both are hard at work in every fan, phone, motor and speaker around you.
What kind of thing is a magnet? How can you tell whether something is a magnet or not? Talk about it in class. In this unit you will find out what magnets pull, where they are strongest, and how to make one, even with electricity. Then you will make electricity from a lemon, build circuits, draw them with symbols, and find out how a thin piece of wire keeps your home safe from fire.
8.1 Magnet
A magnet is a thing that pulls (attracts) iron things towards it, like iron dust, nails and pins. The pull of a magnet is called magnetic force (some books call it magnetic power or magnetic energy). Because of it, a magnet attracts some things. The things a magnet attracts are called magnetic substances.
Not every metal is magnetic. Only iron, nickel and cobalt, and the alloys made from them, such as steel, are pulled by a magnet. Copper, aluminium, brass, gold and silver are metals, but a magnet doesn’t pull them. Wood, plastic, glass, paper and rubber are not pulled either. Things a magnet doesn’t attract are called non-magnetic substances.
Magnets are used in radios, telephones, electric motors, microphones, fans, loudspeakers, generators and many other things.
Which of these will a magnet pull?
Pick an answer to see why.
Magnetic field
Every magnet can attract magnetic substances only up to a certain distance. Beyond that, its pull is too weak to notice. The region around a magnet in which its pull (its magnetic effect) can be felt is called its magnetic field.
A strong magnet has a magnetic field that reaches far. A weak magnet’s field reaches only a short distance.
Magnetic poles
Look again at Activity 8.1. The pull of a magnet is not the same all along it. The iron dust crowds at the two ends, and very little sticks to the middle. So a magnet’s pull is strongest at its two ends. These ends are called the poles of the magnet. Every magnet has two poles: a north pole (N) and a south pole (S).
Two more things about poles:
- Hang a bar magnet from a thread so it can turn freely. It comes to rest pointing north and south. The end that points north is its north pole. This is how a compass works.
- Bring two magnets together. A north pole and a south pole attract each other; two north poles, or two south poles, push each other away (repel). Like poles repel, unlike poles attract.
Where is the pull of a bar magnet strongest?
Pick an answer to see why.
What happens when the north poles of two magnets are brought together?
Pick an answer to see why.
Methods of making magnet
There are many ways of making a magnet. Only magnetic substances can be made into magnets: a non-magnetic substance, like copper or wood, can never become a magnet. Magnets are usually made from iron and steel. A magnet made by people in this way is called an artificial magnet. (A natural magnet is one found in nature, like lodestone.) Two ways of making a magnet are:
- a.the stroking method
- b.the electrical method
Stroking method
A magnet can be made by rubbing (stroking) a magnetic substance with a magnet. There are several ways of doing this. Here is the simplest: stroking with one pole of a magnet, always in one direction.
So by stroking a magnetic substance with a permanent magnet, always in the same direction, we can make it into a magnet.
Electrical method
Activity 8.4 shows that a magnet can be made with electricity. A magnet made by passing an electric current through a coil of wire is called an electromagnet.
An electromagnet is a temporary magnet. It is a magnet only while the current flows. Switch the current on, and it becomes a magnet; switch it off, and it stops being one. That is what makes it so useful: it can pick something up and drop it again when we want.
Change in magnetic strength of an electromagnet
The strength of an electromagnet can be changed as we need:
- More turns of wire make it stronger; fewer turns make it weaker.
- More cells (a bigger current) make it stronger; fewer cells make it weaker.
How can you make an electromagnet stronger?
Pick an answer to see why.
Use of electromagnet
Electromagnets are used in electric fans, electric motors, electric bells and loudspeakers. Huge electromagnets on cranes lift heaps of scrap iron and steel, and drop them again when the current is switched off. Hospitals use very strong electromagnets in MRI machines, which take pictures of the inside of the body.
8.2 Electricity
Where do we need electricity? What electric things have you seen? Discuss in class.
We need electricity every day: to cook, to iron clothes, to wash clothes and to light our rooms. Telephones, televisions, radios, computers, heaters, irons, electric motors and air conditioners all run on electricity. Today most industries and factories run on electricity too. That is why electricity is one of the most important sources of energy in the modern age.
Sources of electricity
Electricity can be produced in many ways. The main devices that produce it are the cell, the photocell (solar cell), the dynamo and the generator. Devices that produce electricity are called sources of electricity.
- A cell makes electricity from chemicals inside it (a torch cell, a phone battery).
- A photocell or solar cell makes electricity from light (a solar calculator, solar panels on a roof).
- A dynamo makes electricity when it is turned, for example by a bicycle’s wheel, to light its lamp.
- A generator is a big dynamo, turned by an engine (a diesel generator) or by flowing water (in a hydropower station).
Cell
Have you seen the cells used in torches, radios and wall clocks? How do they work? Discuss it.
A cell changes chemical energy into electrical energy. Electric devices then change that electrical energy into light, heat, sound and other forms of energy. The cells we use in torches and clocks are dry cells: their chemicals are a paste, not a liquid, so they don’t spill.
Simple cell
Have you seen a simple cell? How is it made? How does it produce electricity? Let’s find out.
A simple cell is a cell made of a copper plate and a zinc plate dipped in dilute sulphuric acid. It produces electricity by changing chemical energy into electrical energy. It gives about 1 volt.
But a simple cell is not much use in daily life:
- it contains acid, so it is hard and dangerous to carry about;
- it can’t make much electricity;
- it stops working well after a short time.
A cell changes…
Pick an answer to see why.
Which two plates are used in a simple cell?
Pick an answer to see why.
Electric circuit
Electricity is a form of energy. It can be changed into light, heat, sound and magnetic energy. We use electrical energy to light bulbs and to run heaters, fans, radios, televisions and computers.
The path along which electricity flows is called an electric circuit. A circuit must have:
- a source of electricity (a cell);
- conducting wires, to carry the electricity;
- a load (a bulb), the device that uses it.
A switch is usually added too, to start and stop the flow.
So, 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.
Devices that produce electricity, like a dry cell, a photocell, a dynamo and a generator, are called sources of electricity. Devices that run on electricity are called loads. A bulb, a heater, a fan, a radio and a television are loads.
Closed and open circuit
Think about Activity 8.8 again. When the switch is on, the bulb glows. Such a circuit is called a closed circuit. When the switch is off, or a wire has come loose somewhere, the bulb doesn’t glow. Such a circuit is called an open circuit.
A circuit in which electricity flows and the load works is a closed circuit. A circuit with a gap in it, in which the load doesn’t work, is an open circuit.
A torch’s switch is on, but its bulb doesn’t glow because a wire inside has come loose. The circuit is…
Pick an answer to see why.
Symbols used in electric circuit
Drawing every cell, wire, switch and bulb as it really looks, as in Activity 8.8, is slow and hard. So circuits are drawn with simple symbols instead. Here are the symbols of some important devices.
In the symbol of a cell, what does the long thin line stand for?
Pick an answer to see why.
Effects of current electricity
Electrical energy can be changed into other forms of energy to do different jobs. These are called the effects of current electricity:
- the lighting effect: a bulb gives light;
- the heating effect: a heater, an iron or a kettle gets hot;
- the magnetic effect: an electromagnet, the motor of a fan;
- the chemical effect: charging a battery, coating a spoon with silver.
Here we will look at the heating effect.
Heating effect of current electricity
When electricity flows through a coil of some metals, the coil gets very hot. Producing heat in this way is called the heating effect of current electricity. It is used in room heaters, electric kettles, immersion rods (water heaters), rice cookers and electric irons. The coil in these devices that gets hot is called the heating element. It is usually made of nichrome, an alloy of nickel and chromium, which can glow red hot for years without melting or burning away.
Which of these does not use the heating effect of electricity?
Pick an answer to see why.
Fuse
Sometimes more electricity flows in a circuit than it can safely carry: when too many heaters and irons are switched on at once, or when two bare wires touch. Then the wires get very hot. The devices can be damaged, and the wires can catch fire. To prevent such accidents, a fuse is put in the circuit.
A fuse is a short piece of thin wire that melts at a low temperature. It is joined into the circuit. If too much electricity flows, the fuse wire gets hot and melts first. The circuit becomes open, and the flow of electricity stops, before the house wiring can get hot enough to catch fire.
The fuse wire is held in a holder that fits into a socket (a fuse carrier). If the fuse wire melts, the fault is put right, and a new fuse wire of the right size is fitted in the holder.
Miniature Circuit Breaker (MCB)
When a fuse wire melts, it has to be replaced, and that isn’t easy or safe for everyone. So the MCB (Miniature Circuit Breaker) was developed. An MCB is an advanced form of fuse. When more electricity flows than the circuit can safely carry, it switches itself off, and so prevents accidents. Once the fault is put right, it is simply pushed back up to switch the circuit on again.
What is a fuse wire made to do?
Pick an answer to see why.
Words to know
Conductor: a substance that lets electricity pass through it (copper, iron, aluminium).
Insulator: a substance that doesn’t let electricity pass through it (plastic, rubber, dry wood).
Constantan: an alloy of copper and nickel.
Nichrome: an alloy of nickel and chromium.
Heating element: a coil of wire that gets hot when a current flows through it.
Exercise
1. Fill in the blanks with the right words
magnetic field, electromagnet, magnetic substance, dilute sulphuric acid
A substance that is attracted by a magnet is called a ______.
Answer
A substance that is attracted by a magnet is called a magnetic substance.
______ is used in a simple cell.
Answer
Dilute sulphuric acid is used in a simple cell.
The region around a magnet in which it attracts or repels is called its ______.
Answer
The region around a magnet in which it attracts or repels is called its magnetic field.
The word “electromagnet” is not needed.
2. Choose the correct answer
a. Which one of these is not attracted by a magnet?
Pick an answer to see why.
b. Which one of these does not produce electricity?
Pick an answer to see why.
c. Which one of the drawings in Figure 8.21 is the symbol of a cell?
Pick an answer to see why.
d. What is the job of a cell?
Pick an answer to see why.
e. Why is a fuse used?
Pick an answer to see why.
3. Write true or false
The region of attraction or repulsion round a magnet is called its magnetic pole.
Answer
False. It is called the magnetic field. The poles are the two ends of the magnet, where its pull is strongest.
Dilute sulphuric acid is used in making a simple cell.
Answer
True.
Fuse wire melts at a low temperature.
Answer
True.
An MCB is an advanced form of fuse.
Answer
True.
4. Write the differences between
Closed circuit and open circuit
Answer
| Closed circuit | Open circuit |
|---|---|
| The path for electricity is complete, with no gap. | There is a gap in the path (a switch off, or a loose or broken wire). |
| Electricity flows through it. | Electricity doesn’t flow. |
| The load works (the bulb glows). | The load doesn’t work (the bulb doesn’t glow). |
Fuse and MCB
Answer
| Fuse | MCB |
|---|---|
| a short piece of thin wire that melts at a low temperature | a switch that turns itself off |
| melts when too much current flows | switches off when too much current flows |
| must be replaced with a new fuse wire after it melts | is simply pushed back up to switch on again |
| replacing it is slow and can be unsafe | easy and safe to use |
| cheap | costs more |
5. Give reasons
An MCB is easier to use than a fuse.
Answer
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 is called a protective wire.
Answer
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.
6. Answer the following questions
What is a magnetic field?
Answer
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.
Define electromagnet. How do you make an iron nail into an electromagnet? Explain with a figure.
Answer
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:
- Take an iron nail and about 1 m of insulated copper wire.
- Wind the wire round the nail, 80 to 100 turns, always in the same direction.
- Scrape the plastic off the wire’s two ends, and join them to the two terminals of a dry cell (through a switch).
- While the current flows, the nail picks up pins and iron dust: it is an electromagnet. When the current is stopped, the pins drop.
What is an electric circuit? What are the basic parts of an electric circuit?
Answer
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:
- a source of electricity, such as a cell;
- conducting wires, such as copper wires;
- a load, such as a bulb.
A switch is usually added to open and close the circuit.
What is a heating element? Give any two examples.
Answer
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).
What is a simple cell? Explain how to make a simple cell from a potato, with a suitable diagram.
Answer
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:
- Take a potato, a small zinc plate (or a galvanized nail) and a small copper plate (or a thick copper wire).
- Push the two plates into the potato, a few centimeters apart, without touching each other.
- Join the two plates with copper wires to a galvanometer.
- 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.
Draw an electric circuit using these symbols: a cell, a bulb, a wire and a switch.
Answer
Join the cell, the switch and the bulb in one loop, with the wires drawn as straight lines.
What would you do if the fuse in your home blew while you were alone? Write your opinion.
Answer
- I would not try to change the fuse myself, because touching the fuse box can give a dangerous electric shock.
- 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.
- I would stay away from the fuse box and any wires that smell of burning, and never touch them with wet hands.
- 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.
The fuse in Ronish’s home has to be changed often. His neighbors suggest that his father use a slightly thicker copper wire in place of the fuse wire, so that it need not be changed so often and electricity can be used without any trouble. What do you think of this idea? Give reasons.
Answer
It is a dangerous idea, and Ronish’s father should not do it.
- 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.
- Then the house wiring and the appliances would get very hot, and could be damaged or catch fire.
- 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).
In a house, the MCB keeps falling down and cutting off the electricity. What happens if the house owner props the MCB up with a stick so that it can’t fall, and goes on using electricity? Give your suggestions too.
Answer
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:
- Remove the stick at once, and never block an MCB.
- Don’t run many heavy appliances (heater, iron, kettle, water pump) at the same time.
- Call an electrician to find and repair the fault in the wiring or the appliance that makes the MCB trip.
Electricity has become an essential source of energy today. Justify this statement with arguments.
Answer
- At home: we use electricity for light, and to run fans, refrigerators, rice cookers, irons, water pumps and washing machines.
- Communication: phones, televisions, radios, computers and the internet all need electricity.
- Education and health: schools use computers and projectors; hospitals run X-ray machines, life-saving machines and fridges for medicines on electricity.
- Industry and farming: factories, mills and irrigation pumps run on electricity.
- 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.
- 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.
Practice every question in the book, and keep track of the ones you miss →