What this chapter is about
This chapter explores two important effects that electric current produces when it flows through a conductor: magnetic effects and heating effects. When current flows through a wire, it creates a magnetic field around it. This discovery, made long ago, connects electricity and magnetism and forms the basis of many devices we use daily, from electric bells to motors.
The chapter also explains how electric current heats up wires and filaments. This heating effect is what makes electric bulbs glow, geysers heat water, and irons press clothes. You will learn why some materials heat up more than others and how this property is used in household appliances.
After studying this chapter, you should be able to explain how an electromagnet works, understand why fuses protect our homes from electrical fires, and connect the science behind common appliances like heaters, toasters and electric bells.
Key ideas
- A wire carrying electric current produces a magnetic field around it. This is called the magnetic effect of current.
- An electromagnet is made by winding an insulated wire around an iron nail and passing current through it. The nail becomes a magnet only while current flows.
- The strength of an electromagnet depends on the number of turns in the coil and the amount of current flowing through it.
- When electric current passes through a conductor, some electrical energy changes into heat energy. This is the heating effect of current.
- Thin wires with high resistance heat up more than thick wires with low resistance when the same current flows through them.
- A fuse is a safety device containing a thin wire that melts and breaks the circuit when too much current flows, preventing fires.
- Electric bulbs use a thin tungsten filament that glows white-hot when current passes through it, producing light.
- MCB (Miniature Circuit Breaker) is a modern safety device that automatically switches off when current exceeds safe limits.
Formulas and facts to remember
- Magnetic field lines around a current-carrying straight wire form circles with the wire at the centre.
- An electromagnet loses its magnetism when the current is switched off, unlike a permanent magnet.
- Heat produced in a wire increases when current increases or when resistance increases.
- Heat produced = Current × Current × Resistance × Time (H = I² × R × t).
- Tungsten is used in bulb filaments because it has a very high melting point (about 3400°C).
- Nichrome (an alloy of nickel and chromium) is used in heating elements because it has high resistance and does not oxidise easily when heated.
- Fuse wire is made of materials with low melting points, such as alloys of tin and lead.
- The unit of electric current is ampere (A), named after the scientist André-Marie Ampère.
Worked examples
Example 1: Making an electromagnet stronger
Ravi wound 20 turns of insulated copper wire around an iron nail and connected it to a cell. The electromagnet picked up 5 iron pins. He wants it to pick up more pins. What should he do?
Solution: To make the electromagnet stronger, Ravi can: 1. Increase the number of turns of wire around the nail (say, from 20 to 40 turns). 2. Use more cells connected in series to increase the current. 3. Use a thicker iron nail as the core.
If Ravi doubles the turns to 40, the magnetic field becomes stronger, and the electromagnet can pick up more pins.
Example 2: Choosing the right fuse
An electric iron in Meena's house is rated 1000 watts and works on 250 volts. Which fuse should she use: 2 A, 5 A, or 15 A?
Solution: First, find the current the iron uses. Current = Power ÷ Voltage = 1000 ÷ 250 = 4 A
The fuse rating should be slightly higher than the normal current but not too high. A 2 A fuse will blow immediately (too low). A 5 A fuse is suitable (just above 4 A). A 15 A fuse is too high and won't protect properly.
Meena should use the 5 A fuse.
Example 3: Why does a heater coil glow but the connecting wires don't?
In a room heater, the coil glows red but the thick copper wires connecting it to the plug remain cool. Why?
Solution: Both the coil and the wires carry the same current. The heater coil is made of nichrome, which has high resistance. The copper wires have very low resistance. Heat produced depends on resistance (H = I² × R × t). Since the coil has much higher resistance, it produces much more heat and glows. The copper wires produce very little heat and stay cool.
Common mistakes
- Thinking an electromagnet stays magnetic after current stops → No, it loses magnetism immediately when current is switched off.
- Believing thick wires heat up more than thin wires → Actually, thin wires have higher resistance and heat up more for the same current.
- Confusing fuse rating with power → Fuse rating is in amperes (current), not watts (power).
- Thinking all metals can be used in bulb filaments → Only metals with very high melting points like tungsten work; others would melt.
- Assuming MCB and fuse work the same way → Fuse wire melts and must be replaced; MCB just trips and can be reset.
Quick revision
- Current in a wire creates a magnetic field around it.
- Electromagnet = coil around iron core + current; works only when current flows.
- Heating effect: thin wires with high resistance produce more heat.
- Fuse protects circuits by melting when current is too high.
- Tungsten filament in bulbs; nichrome wire in heaters.
- MCB is a reusable safety switch; fuse must be replaced after it blows.