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CNMS ICSE - 7 Std
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Answers to textbook exercises

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Chapter: 08. Magnetism

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CONNECT (Page 164)

Given below is a bar magnet. Draw the magnetic lines of force.

Answer / Drawing Description: To draw the magnetic lines of force for the bar magnet:
Draw smooth, continuous curved loops starting from the North Pole (N) and ending at the South Pole (S) outside the magnet.
Add arrows pointing outwards from the North Pole (N) and inwards towards the South Pole (S) along these lines.
Ensure the lines of force are crowded closely together near the poles (N and S) to represent the maximum magnetic strength, and spread out wider as they move further away from the magnet.
Make sure that no two magnetic lines of force cross or intersect each other.
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CHECK YOUR PROGRESS (Page 169)

Choose the correct option.

Which of the following is not a magnetic material?
Answer: a. plastic
Which of the following is the surest test of magnetism?
Answer: b. repulsion
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REASON CORNER (Page 171)

Based on the above activity, which statement is correct?

Answer: c. It was not a fair test because we did not test whether the nail alone would pick up the paperclips.
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SCIENCE TALK (Page 172)

Is the human body magnetic? If so, what produces magnetism in the human body?
Answer: Yes, the human body exhibits weak magnetic properties (known as biomagnetism). This magnetism is primarily produced by the tiny electric currents generated by moving ions during chemical reactions and electrical impulses in active organs, particularly the heart, brain, and nervous system.
What minerals in the body are magnetic?
Answer: Magnetite (
math
⁠
), a highly magnetic mineral of iron oxide, is present in trace amounts within the human brain and other bodily tissues.
Is blood magnetic because of iron?
Answer: No, blood is not ferromagnetic despite containing iron. The iron atoms in hemoglobin are isolated from one another and cannot align collectively. As a result, oxygenated blood is weakly repelled by magnetic fields (diamagnetic), while deoxygenated blood is weakly attracted (paramagnetic).
What can magnetic fields do to your brain and body?
Answer: Low-intensity magnetic fields have no observable effect. However, extremely high-intensity magnetic fields (such as those in MRI machines) can interact with electrical impulses in the brain and heart, occasionally causing temporary sensations of dizziness or metallic tastes, and can powerfully pull or heat any metallic implants within the body.
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REASON CORNER - ELECTRIC BELL (Page 173)

When you keep the push button of an electric bell pressed, the hammer keeps striking the gong repeatedly. What makes the hammer continue striking?

Answer:
When the button is pressed, current flows and magnetizes the electromagnet, which attracts the soft iron armature. This causes the hammer to strike the gong.
As the armature moves forward, it loses contact with the contact screw. This breaks the electrical circuit, stopping the current.
The electromagnet immediately loses its magnetism, and the spring pulls the armature back to its original position.
Once the armature returns, it touches the contact screw again, completing the circuit once more. The current flows, and the entire cycle repeats continuously as long as the button remains pressed.
⁠

CHECK YOUR PROGRESS (Page 175)

Choose the correct option.

When the magnet is completely inside the coil but not moving, the galvanometer reading is
Answer: a. zero.
On increasing the number of turns of the coil, the strength of the electromagnet
Answer: b. increases.
Which of the following is NOT a permanent magnet?
Answer: c. electromagnet
The magnitude of the induced current can be decreased by
Answer: a. decreasing the number of turns in the coil.
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LIFE SKILLS (Page 175)

Fill a tray with water and float an aluminium can on it. Tie a thread around a magnet so that it is suspended freely. Spin the magnet by twisting the thread and bring the magnet close to the can. What happens to the can? What do you think is causing the can to spin?

Answer:
Observation: The floating aluminium can starts to spin in the same direction as the spinning magnet.
Reason: The changing magnetic field of the spinning magnet induces electrical currents (called eddy currents) in the conducting aluminium can. These induced currents generate their own magnetic fields which interact with the rotating magnet’s field. Under Lenz’s Law, this interaction produces a rotational magnetic force (torque) that drags the aluminium can, causing it to spin.
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PREPPING FOR PISA (Page 175)

Assertion (A): MRI Scan uses electromagnetism. ​Reason ®: There is no exposure to radiation during an MRI scan.
Answer: b. Both A and R are true; R is not a correct explanation for A.
Which of the following statements are true about CT scan and MRI?
Answer: The true statements are:
a. MRI produces more detailed images of organs and soft tissues.
c. CTs are preferred for people with metallic implants in their body.
d. CT Scans are preferred in case of an emergency when results are needed quickly.
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EVALUATION (Page 177-179)

A. Choose the correct option.

Which of the following instruments is used to detect electric current in an electric circuit?
Answer: d. galvanometer
Which of the following is not a part of an electric bell?
Answer: a. transformer
The magnitude of the induced current in a coil cannot be increased by:
Answer: b. increasing the distance between the coil and magnet.
The branch of physics which deals with the magnetic effect of electric current is called
Answer: c. electromagnetism.
Four magnets P, Q, R and S are immersed in a pile of iron filings. What can you infer about the strengths of the magnets?
Answer: b. R > S > P > Q (The strength of a magnet is determined by the quantity of iron filings attracted to its poles; magnet R attracts the most, followed by S, P, and Q with the least).
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B. Fill in the blanks.

Current is induced in the coil due to the relative motion between the coil and the magnet.
A magnet shows maximum attraction at its poles.
The current produced by the method of electromagnetic induction is called induced current.
The soft iron piece in an electromagnet is called the core.
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C. Name the following:

Device used to measure small electric currents: Galvanometer
A type of magnet in which a magnetic field is produced with electric current: Electromagnet
Phenomenon of producing a current in a conductor due to change in the magnetic lines of force associated with the conductor: Electromagnetic induction
The surest test for magnetism: Repulsion
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D. Write true (T) or false (F) against the following statements.

A current is induced in the coil when the number of magnetic lines of force associated with a coil changes. [ T ]
In Faraday’s experiment, if the movement of the magnet is continuous, the needle does not show any deflection. [ F ]
An electromagnet becomes weaker when the amount of current through the coil increases. [ F ]
The force exerted by a magnet on the magnetic material is called magnetic force. [ T ]
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E. Short-answer-type questions.

Write the name of two magnetic and two non-magnetic materials.
Magnetic materials: Iron, Steel (or Cobalt, Nickel)
Non-magnetic materials: Plastic, Wood (or Glass, Copper)
What is the difference between a bar magnet and a U-shaped electromagnet?
Bar magnet: It is a permanent magnet that retains its magnetic properties indefinitely without any external electric supply. It has a straight rectangular shape.
U-shaped electromagnet: It is a temporary magnet made of a U-shaped soft iron core wound with a copper coil. It behaves as a magnet only when electric current passes through the coil and loses its magnetism when the current is turned off.
Write four uses of an electromagnet.
In electrical appliances like telephones, electric motors, and electric bells.
In industrial cranes to lift and move heavy loads of scrap iron and steel.
In modern high-speed maglev (bullet) trains.
By doctors to remove steel splinters from a patient’s eyes or wounds.
State the two laws of electromagnetic induction.
First law: An electric current is induced in a coil whenever the number of magnetic lines of force associated with the coil changes.
Second law: The magnitude of the induced current is directly proportional to the rate at which the magnetic lines of force associated with the coil change.
In Faraday’s experiment, what happens when the magnet is completely inside the coil?
When the magnet is completely inside the coil but remains stationary, there is no relative motion, meaning the magnetic lines of force do not change. Consequently, the galvanometer needle returns to zero, showing that no induced current is flowing.
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F. Long-answer-type questions.

Define an electromagnet. What are its two types? Define them with the help of a diagram.
Definition: An electromagnet is a temporary magnet consisting of a core of magnetic material (usually soft iron) surrounded by a coil of insulated wire, which becomes magnetic only when an electric current passes through the wire.
Types of Electromagnets:
Bar-shaped electromagnet: This consists of insulated copper wire wrapped around a straight bar-shaped soft iron core. One end acts as the north pole and the other as the south pole when current flows.
U-shaped electromagnet: This consists of insulated copper wire wrapped around a U-shaped (horseshoe) soft iron core. This design brings both poles closer together to provide a much stronger magnetic pull in the gap.
(Diagrams should be drawn replicating Fig. 8.9 for the Bar-shaped electromagnet and Fig. 8.10 for the U-shaped electromagnet, showing the coil windings connected to a battery).
Demonstrate the working and construction of an electric bell with the help of a diagram.
Construction: It consists of a U-shaped electromagnet, an iron armature, a contact spring connected to a contact screw, a hammer, a gong, and terminals connected to a battery and a push-button switch.
Working:
When the push-button switch is closed, current passes through the coils of the electromagnet, magnetizing it.
The electromagnet pulls the iron armature toward itself, causing the hammer attached to it to strike the gong and produce a ringing sound.
The movement of the armature pulls the contact spring away from the contact screw, breaking the circuit.
With the circuit broken, the current stops, the electromagnet loses its magnetism, and the spring pulls the armature back to its original position.
This re-establishes contact with the screw, completing the circuit again. The hammer strikes the gong once more, repeating the sequence rapidly.
(A circuit diagram should be drawn replicating Fig. 8.13 to show the layout of the armature, gong, electromagnet, and screw connection).
Write the observations of Faraday’s experiment.
Observations:
When the magnet is kept stationary outside the coil, the galvanometer needle shows zero deflection (no current).
When the magnet is moved quickly towards the coil, the galvanometer needle deflects to one side, indicating the presence of induced current.
When the magnet is completely inside the coil but not moving, the galvanometer needle immediately returns to zero (no current).
When the magnet is moved away from the coil, the galvanometer needle deflects in the opposite direction, indicating that current is induced in the reverse direction.
If the relative movement between the magnet and coil is continuous, the galvanometer needle deflects continuously.
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HIGHER ORDER THINKING SKILLS (HOTS) (Page 179)

Which of the following images illustrate what happens to an electromagnet when the electrical current in the circuit is turned off?
Answer: Option a (The switch is open/off, meaning no current is flowing, the electromagnet loses its magnetism, and the paper clips fall down).
In which of the following cases will the highest current be produced?
Answer: Option c (Moving the magnet and coil towards each other).
Reason: Moving both objects towards each other simultaneously results in the highest relative speed of motion. Since the magnitude of the induced current is directly proportional to the rate of change of magnetic lines of force, this maximum speed yields the highest current.
Why is it advised to keep smartphones away from high intensity magnets?
Answer: Smartphones contain highly sensitive electronic components, internal magnets (in speakers and microphones), and sensors like magnetometers (digital compasses). High-intensity external magnets can permanently magnetize or corrupt these internal parts, leading to screen distortion, compass navigation failure, or hardware damage.
Shaurya made an electromagnet using one battery. Shagun also made an electromagnet using two batteries. Which electromagnet will attract more iron filings? Give reason.
Answer: Shagun’s electromagnet will attract more iron filings.
Reason: The strength of an electromagnet is directly proportional to the amount of electric current flowing through its coil. Since Shagun used two batteries, a larger voltage was applied, generating a greater electric current, which made her electromagnet much stronger than Shaurya’s.
⁠

INTEGRATE (Page 179)

You have been given a dry cell, two wires and a pencil. Will it be possible to make the pencil into an electromagnet? Why?/Why not? List a few metals that are non-magnetic but can be made into magnets.
Pencil Electromagnet: No, it is not possible. The core of a pencil is made of graphite and clay, while the outer casing is made of wood. Neither of these materials is ferromagnetic, meaning they cannot align magnetic domains to act as a magnetic core.
Non-magnetic metals that can be made into magnets: Aluminium and Copper are non-magnetic on their own but can be combined with other elements to form strong permanent magnetic alloys (such as Alnico, made of aluminium, nickel, cobalt, and copper).
What is magnetic declination? List the places where this phenomenon is visible on the Earth.
Definition: Magnetic declination is the angle on the horizontal plane between magnetic north (the direction a compass needle points) and true geographic north (the direction along a geographic meridian toward the North Pole).
Places where visible: This phenomenon is observable everywhere on Earth using a compass and a map. However, the value of declination changes depending on your exact geographical coordinates. It is most extreme and highly variable at high latitudes near the geographic and magnetic poles (such as in northern Canada, Greenland, and Antarctica), where the compass needle may deviate drastically from true north.
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