Electricity and Magnetism

A. Electric Circuit & Schematic Symbols:

An Electric Circuit is a closed conducting path through which electric current flows from the positive terminal of a power source to the negative terminal.

• Electric Cell: Long thin line (positive terminal $+$) paired with a shorter thick line (negative terminal $-$).
• Battery: Combination of two or more cells connected in series.
• Switch / Key: Open switch (breaks circuit, current $= 0$); Closed switch (completes loop, current flows).
• Bulb: A looped filament inside a circle.

B. Conductors vs Insulators:

• Conductors: Materials containing abundant free electrons that permit electric current to flow with negligible resistance (e.g., copper, aluminum, silver, graphite, tap water).
• Insulators: Materials whose electrons are tightly bound to atomic nuclei, offering extremely high resistance to current flow (e.g., rubber, PVC plastic, glass, dry wood, porcelain). Electric wires are coated in PVC to prevent fatal electric shocks.

A. Joule's Law of Heating:

When an electric current flows through a high-resistance conductor, the moving electrons collide with metal ions, converting electrical energy into thermal energy:

• Heating Appliances (Irons, Heaters, Toasters): Employ heating elements made of Nichrome (an alloy of Nickel, Chromium, and Iron) because of its very high electrical resistance and exceptionally high melting point ($~1,400^\circ\text{C}$); it glows red-hot without oxidizing or melting.

B. The Electric Fuse (Safety Device):

• A safety device inserted in the live wire to protect circuits from overloading and short-circuiting.
• Material: A short piece of wire made from an alloy of Lead and Tin ($63\%\text{ Sn} + 37\%\text{ Pb}$) having a low melting point and relatively high resistance.
• Mechanism: When excessive current surges through the circuit, Joule heating causes the fuse wire to heat rapidly and melt, breaking the circuit and preventing electrical fires and appliance damage.
• Modern domestic circuits use Miniature Circuit Breakers (MCBs), which automatically trip an electromagnetic switch during current surges and can be reset without replacement.

A. Fundamental Laws of Magnetism:

1. Every magnet has two opposite poles: a North-seeking Pole ($N$) and a South-seeking Pole ($S$). Isolated magnetic monopoles do not exist!
2. Law of Attraction & Repulsion: Like poles repel each other ($N-N$ or $S-S$), whereas unlike poles attract each other ($N-S$).

B. Why REPULSION is the ONLY Sure Test of a Magnet:

A magnet attracts:

• The opposite pole of another magnet (unlike poles attract).
• Any ordinary unmagnetized magnetic material (such as an iron nail) due to induced magnetism!

Therefore, attraction can occur between a magnet and a non-magnetized piece of iron. However, REPULSION occurs exclusively between two like magnetic poles! Thus, repulsion is the only definitive test to confirm that a given metallic bar is an actual permanent magnet.

A. Electromagnet Construction & Properties:

An Electromagnet is a temporary magnet created by winding insulated copper wire around a soft iron core and passing an electric current through the coil.

• Its magnetic field vanishes completely the instant the current is switched off. Soft iron is used because it has high magnetic permeability and zero retentivity (loses magnetism immediately).
• Strengthening an Electromagnet: 1. Increase the magnitude of current, 2. Increase the number of turns in the coil.

B. Working Mechanism of the Electric Bell:

1. When the push-button switch is pressed, the circuit is completed, and current flows through the electromagnet coils.
2. The soft iron core becomes strongly magnetized and attracts the soft iron armature.
3. The hammer attached to the armature strikes the brass gong, producing a loud ringing sound.
4. As the armature moves forward, it pulls away from the adjustable contact screw, breaking the circuit at the contact point!
5. With current stopped, the electromagnet instantly loses its magnetism. A spring pulls the armature back against the contact screw.
6. This restores the electrical connection, current flows again, and the cycle repeats rapidly, producing continuous ringing!