Science · Class 8 · Chapter 4

Electricity: Magnetic and Heating Effects

Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
  • Magnetic effect of current — A current-carrying wire creates a magnetic field.
  • Electromagnet — A current-carrying coil that behaves like a magnet.
  • Heating effect of current — Heat generated in a conductor due to resistance.
  • Resistance — Opposition a conductor offers to current flow.
  • Electrolyte / Electrode — The conducting liquid / the metal rod in a cell.
  • Voltaic (Galvanic) cell — Generates current via a chemical reaction.
  • Dry cell — A single-use cell with a paste electrolyte.
  • Rechargeable battery — Can be recharged and reused many times.
40 min lesson

Chapter 4 · Concept 1 of 10

The Magnetic Effect of Electric Current

  • Place a compass under a wire, then switch the circuit ON — the needle deflects. Switch OFF, and it returns to normal.
  • Hans Christian Oersted, 1820 — Discovered this link between electricity and magnetism quite by accident, during a lecture demonstration.

Chapter 4 · Concept 2 of 10

Electromagnets

  • Wind wire into a coil, connect it to a cell, and the coil behaves like a magnet — an electromagnet.
  • An iron core makes it stronger — Slide an iron nail inside the coil, and the electromagnet becomes far stronger — the basis of nearly every practical electromagnet.

Chapter 4 · Concept 3 of 10

Controlling an Electromagnet

  • Two poles, like any magnet — An electromagnet has North and South poles, found by testing which end attracts a compass pole.
  • What changes its strength — More current (more cells) or more coil turns makes it stronger. Reversing the current's direction reverses its poles.

Chapter 4 · Concept 4 of 10

Lifting Electromagnets and Earth's Magnetism

  • Lifting electromagnets — Powerful electromagnets hung from cranes — switch current ON to lift iron or steel, OFF to drop it. Common in factories and scrap yards.
  • Earth, a giant magnet — Currents from moving liquid iron in Earth's core create its magnetic field — helping migratory animals navigate, and shielding us from harmful space particles.

Chapter 4 · Concept 5 of 10

The Heating Effect of Electric Current

  • A conductor resists the flow of current — and that resistance converts some electrical energy into heat.
  • Not all wires heat the same — A nichrome wire heats up more than a copper wire of the same size, because it offers higher resistance.

Chapter 4 · Concept 6 of 10

What Decides How Much Heat Is Generated

  • Current — More current, more heat.
  • Material — Higher resistance, more heat.
  • Thickness & length — Thinner, longer wires heat more.
  • Time — Longer current flow, more total heat.

Chapter 4 · Concept 7 of 10

Appliances That Use This Effect

  • Everyday appliances — Room heaters, stoves, kettles, irons, immersion rods, and hair dryers all contain a heating element — a rod or coil that heats up when current flows.
  • Safety and industry — Overheating can melt plugs or start fires — household circuits include safety devices. Industrially, electric furnaces reach very high temperatures to melt and recycle scrap steel.

Chapter 4 · Concept 8 of 10

The Voltaic Cell

  • Two different metal electrodes, dipped in an electrolyte, react chemically to produce electric current.
  • Even a lemon can power an LED — A copper wire, an iron nail, and lemon juice as the electrolyte can light an LED — the same principle behind every battery, named after Volta and Galvani.

Chapter 4 · Concept 9 of 10

Dry Cells

  • Paste, not liquid — A zinc container (negative terminal) and a central carbon rod (positive terminal), with a paste-like electrolyte — single-use, and must be disposed of once used up.

Chapter 4 · Concept 10 of 10

Rechargeable Batteries

  • Lithium-ion, today's standard — Used in phones, laptops, cameras, inverters, and electric vehicles — recharged and reused many times, though they eventually wear out too.
  • Handle with care — Used batteries contain valuable and harmful materials — they belong in e-waste recycling, never regular garbage.

Quick Recap

Check Your Understanding

  1. 1Fill in the blanks: (i) The solution used in a Voltaic cell is called ___. (ii) A current-carrying coil behaves like a ___.
  2. 2True or False: (i) Dry cells are less portable than Voltaic cells. (ii) A coil becomes an electromagnet only when current flows through it. (iii) A single-cell electromagnet attracts more paper clips than the same electromagnet with 2 cells.
  3. 3Current flows through a nichrome wire briefly. (i) The wire becomes warm. (ii) A compass below it deflects. Which is correct?