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Resistance
- All materials oppose current flow
- Some oppose less, some more
- This opposition is called resistance
- Copper has very low resistance
- Nichrome has very high resistance
- low electrical resistance
- current passes easily
- heats up very little
- an alloy of high resistance
- opposes current strongly
- used as a resistance wire
- Nichrome is an alloy of nickel, chromium, iron and manganese.
- A wire of high-resistance material is called a resistance wire.
- A short, thick wire has less resistance; a long, thin wire has more.
Heating Effect of Current
- Current passes through a high-resistance wire
- The wire becomes hot
- This is the heating effect of current
- Electric energy changes into heat energy
- Heat depends on resistance and current
- high resistance opposes current
- energy turns into heat
- the wire becomes red hot
- very low resistance
- negligible heat produced
- so it does not glow
- Heat produced depends on the resistance of the wire and the magnitude of current.
- Greater the resistance, greater the heat for a given current.
- Greater the current through a wire, greater the heat produced.
Connect the nails to a cell through a switch and switch on the current for a few seconds.
Touch the nichrome wire for a moment.
- Current passes through nichrome.
- The wire becomes hot.
- Electric energy becomes heat.
Name the following: The metal which is used to make the heating elements of electrical heating appliances like an electric iron.
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Explanation
- Nichrome has very high resistance, so it gets very hot on passing current.
Fill in the blank: An electric current flowing in a wire produces a ________ effect.
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Explanation
- Current through a high-resistance wire produces heat.
Explain why, the current that makes the heater element very hot (red hot), only slightly warms the connecting wires leading to the heater.
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Explanation
- The heater element is made of high-resistance nichrome, so a lot of heat is produced and it becomes red hot.
- The connecting wires are copper with very low resistance, so negligible heat is produced.
- So the same current makes the element red hot but only slightly warms the wires.
A student connects a nichrome wire between two terminals of a battery and observes that after a few seconds, the wire becomes hot. The student then replaces the wire with a thicker nichrome wire of the same length and later with a copper wire of the same thickness and length. (a) Why did the nichrome wire become hot? (b) Predict what will happen to the heating effect if the wire is made thicker but the same length and material is used. (c) If the nichrome wire is replaced with copper of the same dimensions, what difference will you observe in heating? Explain why.
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Explanation
- (a) Nichrome has high resistance, so current passing through it produces a lot of heat.
- (b) A thicker wire has lower resistance, so it produces less heat — the heating effect decreases.
- (c) Copper has very low resistance, so it produces almost no heat and barely warms up compared to nichrome.
The 'element' of an electric iron is made of:
- (a) iron
- (b) nickel
- (c) nichrome
- (d) tungsten
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Explanation
- Nichrome's high resistance makes the element red hot on passing current.
An electric current flows through a nichrome wire for a short time. A. The wire becomes warm. B. A magnetic compass placed below the wire is deflected. Choose the correct option: (NCERT Book Question)
- (a) only A is correct
- (b) only B is correct
- (c) both A and B are correct
- (d) both A and B are not correct
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Explanation
- Current produces a heating effect (wire warms) and a magnetic effect (compass deflects) at the same time.
We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome. When current is passed through the coils, compass needles placed near the coils will show deflection. (NCERT Book Question)
- (a) only in circuit A
- (b) only in circuits A and B
- (c) only in circuits A, B and C
- (d) in all four circuits
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Explanation
- Any current-carrying coil produces a magnetic field, so the compass deflects regardless of the wire material.
A straight conductor is placed horizontally, connected to a battery and switch, with the current flowing from left to right. A magnetic compass is placed directly below the conductor, with its needle initially pointing North-South. (a) Why did the compass needle deflect? (b) What will happen if the battery connections are reversed? (c) What will happen if the current in the wire is doubled?
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Explanation
- (a) The current-carrying wire produces a magnetic field, which deflects the compass needle.
- (b) Reversing the battery reverses the current direction, so the needle deflects in the opposite direction.
- (c) Doubling the current makes a stronger magnetic field, so the needle deflects more.
