In-text — Lifting Electromagnets and What Comes Next — "Electricity: Magnetic and Heating Effects"

1. 13 marksCuriosity Grade 8, Chapter 4, page 52, section 4.1.2

   What is a lifting electromagnet? How does the crane operator control it, and where are such magnets used?

   Hint. The operator's only control is a switch.

   What it is. Lifting electromagnets are strong electromagnets, that may be hung to the cranes. They are the same device as your nail-and-coil, built large and strong.

   How the operator controls it. The crane operator can control the magnet by switching the current ON and OFF.

   Switch	What happens
   ON	The electromagnet lifts the iron/steel objects
   OFF	The magnetic field disappears, and the objects are released

   Where they are used. Widely used in factories and scrap yards, to move, lift, and sort heavy metal items efficiently.

   ✦ This is the model Sumana displayed at the science exhibition in the chapter's opening story — instead of a hook like a typical crane, there was an iron nail wrapped with a wire, which was connected to a battery. Closing the circuit picked up the paper clips; opening it dropped them. A scrap-yard crane is that model scaled up, and nothing about the principle has changed.

   The word 'sort' in the chapter's sentence is worth pausing on. Because the magnet attracts only magnetic materials such as iron and steel, running it over mixed scrap picks out the iron and steel and leaves aluminium, copper, plastic and glass behind. The crane is separating materials, not just moving them.

2. 23 marksCuriosity Grade 8, Chapter 4, page 52

   Why is an electromagnet used for a scrap-yard crane rather than a very strong permanent magnet?

   Hint. Think about the part of the job that happens after the load has been lifted.

   Because the load has to be let go. A permanent magnet is always a magnet — it can pick the scrap up, but it has no way of releasing it at the other end of the yard. You would have to prise every piece off by hand, which defeats the purpose.

   An electromagnet is a magnet only while current flows. The operator switches off, the magnetic field disappears, and the whole load drops where it is wanted.

   The other advantages that follow from the same fact:

   	Permanent magnet	Electromagnet
   Can be switched off	No	Yes — that is the point
   Strength	Fixed	Adjustable, by current and number of turns
   Poles	Fixed	Reversible, by reversing the current
   Very high strength	Hard to achieve	Achieved with many turns and a large current

   ✦ A caution that follows from the same design. Everything the crane is holding depends on the current continuing to flow. A power failure while a load is in the air means the load falls. Real lifting magnets are therefore used with strict rules about never carrying a load over people, and often with a backup supply.

   So the switchability that makes the electromagnet right for the job is also the thing that has to be engineered around.

3. 33 marksCuriosity Grade 8, Chapter 4, page 52, A step further

   The chapter says that in higher grades you will learn that 'just as electricity can produce magnetism, a moving magnet can also lead to an electric current'. Why is this connection described as vital to our daily lives?

   Hint. One direction of the link gives you motors. What does the other direction give you?

   Because the two directions between them cover almost everything electrical we use.

   Direction	What it makes possible
   Electricity → magnetism (this chapter)	Electromagnets, electric bells, motors, fans, loudspeakers
   Magnetism → electricity (higher grades)	Power generators — the machines that produce the electricity in the first place

   The chapter names both ends: it forms the basis of many devices, from electric motors to power generators.

   ✦ Why the second direction is the bigger deal. Everything in this chapter runs on a cell or a battery, and a cell's chemicals run out. The electricity that reaches houses is not made that way — it is made by spinning magnets and coils past each other in a power station, using the reverse effect. Without that, there would be no electric supply to plug an electric iron into.

   A fair warning about what has and has not been shown. Nothing in this chapter demonstrates the reverse effect. Your compass deflected because a current was flowing; you have not yet seen a magnet produce a current. The chapter is telling you the result, honestly labelled as something for later, not proving it. It is worth keeping that distinction — a stated fact and a demonstrated one are different things.

4. 43 marksCuriosity Grade 8, Chapter 4, pages 47-52

   Name the devices listed in the chapter that work on the magnetic effect of electric current, and say in one line what the magnetic effect does in each.

   Hint. The list is given right after Activity 4.1, and section 4.1.2 adds one more.

   The chapter's list: electromagnets, electric bells, motors, fans, loudspeakers, and more — plus lifting electromagnets on cranes from section 4.1.2.

   Device	What the magnetic effect does
   Electromagnet	The coil itself becomes a magnet that can be switched on and off
   Lifting electromagnet on a crane	Holds iron and steel scrap while the current is on; releases it when switched off
   Electric bell	An electromagnet pulls an iron piece so that a hammer strikes the gong
   Electric motor	A current-carrying coil in a magnetic field is made to turn
   Electric fan	Its motor turns the blades
   Loudspeaker	A changing current in a coil makes the cone move and produce sound

   ✦ What all six have in common. In each case a current is being used to produce a mechanical result — a pull, a turn, a movement of air — which is something no amount of heating effect could do. The magnetic effect is how electricity is converted into motion.

   The chapter itself explains only the crane and the electromagnet in any detail; the workings of a bell, a motor and a loudspeaker come in later grades. The list is here so that you know how wide the reach of one simple discovery turned out to be.

5. 54 marksCuriosity Grade 8, Chapter 4, pages 47-52

   A friend says: 'The nail picks up clips because iron is a magnetic material — the current has nothing to do with it.' Use evidence from the chapter's activities to show that this is wrong.

   Hint. Two of the activities were designed exactly to settle this.

   The friend has confused two different things: being attracted by a magnet and being a magnet. Iron is a magnetic material, which means a magnet attracts it. It does not mean a piece of iron can attract other iron.

   Three pieces of evidence from the chapter, each closing off an escape route:

   1. The plain nail does nothing. Before the wire is wound on, the iron nail lying on the table picks up no clips at all. Iron alone is not enough.

   2. The clips fall the moment the current stops (Activity 4.2). The nail is still made of iron; the only thing that changed was the current. So the current is doing the work.

   3. The coil works with no iron in it at all (Activity 4.3). With only a rolled paper cylinder inside, the coil still deflected both compass needles. Paper is not a magnetic material, so there is nothing magnetic in the arrangement except what the current produced.

   ✦ Where the friend is partly right. The iron core is not doing nothing — Activity 4.3 also showed that inserting the nail makes the deflection much more and lets the coil pick up clips it could not lift before. Iron strengthens the magnet; it does not cause it.

   The one-line correction: the current makes the magnet, and the iron makes it stronger. Exercise question 11, with its four coils of iron, copper, aluminium and nichrome, tests precisely this point.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 4 'Electricity: Magnetic and Heating Effects', book pages 46-61 (hecu104.pdf, 16 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Figures read at high resolution: Fig. 4.4a (compass, coil ends A and B, and cell polarity - the cell's - terminal is on the left and + on the right, and the red north tip of the needle has swung towards end A, so end A is the south pole for the connections shown); Fig. 4.12 (a: iron nail and copper strip in lemon juice, b: same electrodes in pure water); Fig. 4.13 (four coils of iron, copper, aluminium and nichrome, each with a cell and a switch). Three deliberate restraints on what is claimed. (1) The chapter gives the heating effect entirely qualitatively - 'the heat generated depends on the magnitude of the electric current' and on 'the material, thickness, length of the wire, and the duration' - and gives no formula. No formula is supplied here either; Joule's law, Ohm's law, power and kWh belong to a later grade and appear nowhere in this book. (2) The two wire-heating projects are answered as experiments to be performed, not as results to be reproduced: the thickness comparison is explained, and the length comparison is explicitly left open because two effects work against each other and the chapter offers no prediction. (3) Solid-state batteries are reported in the chapter's own tense - under development, advantages expected - and the lithium and cobalt supply question is described without naming countries or quantities, since the chapter names none.. Questions are referenced from the NCERT textbook for identification.