CLASS 8 SCIENCE · CHAPTER 4 · STUDY NOTES
Electricity: Magnetic and Heating Effects
Understand electricity: magnetic and heating effects through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
Current can deflect a compass
Explain why a compass can reveal current in a wire.A wire changes the compass direction
- Magnetic field
- The region where a magnetic effect can be detected.
- Magnetic effect of current
- A magnetic field produced by electric current flowing in a conductor.
A compass needle is a small magnet. Near a current-carrying wire it can turn from its original direction. When current stops, that wire’s magnetic effect disappears.
Oersted’s 1820 investigation helped establish the connection between electricity and magnetism. The compass shows an effect; it does not measure every detail of the current.
Take a moment. What do you think?
PAUSE & TRY · think first, then check with a free account
Question
A compass turns when a nearby circuit closes and returns when it opens. What does the change suggest?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 2, 3.
EXPLORE
A coil becomes an electromagnet
Identify the coil, iron core and current in an electromagnet.See what each part contributes
- Electromagnet
- A current-carrying coil that behaves as a magnet.
Winding wire into a coil combines its magnetic effect. An iron core usually makes the electromagnet stronger. Even without the iron, the coil has a magnetic effect while current flows.
For a suitable coil, more turns or a larger current can strengthen the effect. Reversing current reverses its north and south poles. Do not assume every extra cell is safe for any circuit.
PAUSE & TRY · think first, then check with a free account
Question
The iron core is removed from a working coil. Must all magnetic effect disappear?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 4, 5, 6.
EXPLORE
Switch the lifting force on and off
Explain why a scrap-yard magnet is controllable.Control the load
- Magnetic material
- A material, such as iron, that can be attracted strongly by a magnet.
A crane can lift iron or steel with a strong electromagnet. Current on gives a magnetic effect; current off lets the load be released.
This does not lift every metal equally: aluminium and copper are not attracted like iron. Electromagnets also play roles in bells, motors and loudspeakers.
PAUSE & TRY · think first, then check with a free account
Question
Why is an electromagnet useful when a crane must pick up and release iron?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 6, 7.
EXPLORE
Current can transfer energy as heat
Connect a warm wire with electrical energy and resistance.Energy changes form
- Resistance
- Opposition offered by a conductor to electric current.
- Heating effect of current
- Warming of a conductor when electrical energy is transferred as heat.
A wire carrying current can warm up. For the same current, a wire with higher resistance transfers more energy as heat. Nichrome is useful for heating elements.
Heating depends on current, resistance and how long current flows. Never touch a powered heating wire or use household mains for a student experiment.
PAUSE & TRY · think first, then check with a free account
Question
Two wires carry the same current for the same time. Why can the higher-resistance wire become hotter?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 7, 8.
EXPLORE
Useful heat can also cause damage
Distinguish intended heating from unwanted overheating.Where should the heat be?
- Heating element
- A conductor designed to produce heat in an electrical appliance.
Kettles, irons and room heaters use heating elements. Wires, sockets and plugs can also overheat when used beyond their rating or when faulty.
Suitable wires and protective devices help reduce risk. A damaged or unusually hot appliance needs an adult to disconnect it safely and arrange qualified inspection.
PAUSE & TRY · think first, then check with a free account
Question
Is a hot kettle element always a fault, and is a melting plug normal?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 8, 9, 16.
EXPLORE
A cell converts chemical energy
Identify the electrodes, electrolyte and outer circuit.Look inside a simple cell
- Electrode
- A conducting part that connects the cell’s chemical system to the circuit.
- Electrolyte
- A substance containing mobile ions that can conduct within a cell.
Two suitable different metals and an electrolyte can form a cell. Chemical reactions provide energy for current in a complete outer circuit. The electrolyte is not simply a store of pre-made electricity.
In a lemon cell, the lemon juice is the electrolyte. Several cells may be needed to light an LED, and the LED must be connected the right way. Such a model cannot power a household appliance.
PAUSE & TRY · think first, then check with a free account
Question
In a lemon cell using two metals, what job does the juice do?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 10, 11.
EXPLORE
Dry cells are not completely dry
Distinguish a primary dry cell from a rechargeable battery.A zinc-carbon cell cutaway
- Dry cell
- A cell with a moist paste electrolyte rather than a free-flowing liquid.
- Rechargeable battery
- A battery whose chemical system is designed to be charged and used again.
A common zinc-carbon dry cell has a zinc container, a central carbon rod and a paste electrolyte. Its metal cap is the positive terminal and its zinc container connects to the negative terminal.
Many phone batteries are rechargeable. Ordinary single-use cells must not be recharged. Batteries can contain hazardous and valuable materials even when they stop working; use suitable collection and recycling.
PAUSE & TRY · think first, then check with a free account
Question
A used single-use dry cell no longer lights a lamp. Is it safe to cut it open because it is “dead”?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 12, 13, 16.