Notes For All Chapters – General Science Class 7
Properties of a Magnetic Field
1. Introduction to Magnets
- A magnet is an object that can attract iron, nickel, and cobalt.
- Magnets have been used since ancient times for navigation, tools, and daily life applications.
- The Earth itself is a giant magnet, which is why compasses work.
2. Types of Magnets
✔ 1) Natural Magnets
- Found in nature (e.g., lodestone).
- Weaker than artificial magnets.
✔ 2) Artificial Magnets
- Man-made magnets that are stronger and shaped as needed.
- Examples: Bar magnet, U-shaped magnet, Ring magnet, Horseshoe magnet.
3. Properties of Magnets
1) Magnets have two poles
- North Pole (N) and South Pole (S).
- The strongest magnetic force is at the poles.
2) Like poles repel, unlike poles attract
- Same poles (N-N or S-S) = Repel (push away).
- Opposite poles (N-S) = Attract (pull together).
3) Magnetic field surrounds a magnet
- The region around a magnet where it exerts a force is called a magnetic field.
- It can be seen using iron filings arranged around a magnet.
4) Magnetic field can pass through objects
- A magnet can attract iron through paper, plastic, glass, and water.
5) A freely suspended magnet always points North-South
- This is why a compass needle always shows direction.
6) The real test of a magnet is repulsion
- Repulsion (pushing away) proves that an object is a magnet, while attraction could be due to temporary magnetism.
7) Magnets lose strength over time
- Heat, strong impacts, or exposure to other magnets can weaken a magnet.
4. Magnetic Field and Field Lines
✔ Magnetic Field:
- The invisible area around a magnet where its force can be felt.
✔ Magnetic Field Lines:
- Imaginary lines that show the strength and direction of a magnetic field.
- They travel from the North Pole to the South Pole.
✔ How to See a Magnetic Field?
- Sprinkle iron filings around a bar magnet on a paper.
- The filings will arrange along the magnetic field lines.
✔ Magnetic field is strongest near poles and weaker farther away.
5. Methods of Making Artificial Magnets
1) Single-Touch Method
- A bar magnet is rubbed repeatedly in one direction over an iron piece.
- The iron piece slowly becomes magnetic.
2) Double-Touch Method
- Two magnets are used, rubbing their opposite poles from the center to the edges of an iron piece.
- Produces a stronger magnet.
3) Using Electricity (Electromagnets)
- Wrapping a coil of wire around a piece of soft iron and passing electric current creates an electromagnet.
6. Types of Magnets Based on Usage
1) Permanent Magnets
- Magnets that keep their magnetism permanently.
- Example: Bar magnets, fridge magnets.
2) Temporary Magnets
- Objects that become magnets only when near a magnet or electricity.
- Example: Electromagnets, iron nails near a magnet.
3) Electromagnets
- Made by passing electricity through a coil around soft iron.
- Used in electric bells, cranes, speakers, and MRI machines.
7. Uses of Magnets in Daily Life
✔ Navigation – Compass needles always point North-South.
✔ Electronics – Used in TVs, speakers, motors, and generators.
✔ Medical Field – MRI machines use strong magnets to scan the human body.
✔ Industries – Magnets help in lifting heavy objects like scrap metal.
✔ Household Items – Used in fridge doors, toys, and mobile phones.
8. How Did Ancient People Use Magnets?
- Ancient merchants and sailors used natural magnets (lodestones) to find directions during long travels.
- The discovery of magnetic compasses helped in sea navigation.
9. How to Protect Magnets from Losing Strength?
✔ Keep magnets away from heat and strong impacts.
✔ Store bar magnets with keepers (soft iron bars) to maintain strength.
✔ Avoid dropping or hammering magnets.
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