Electricity: Magnetic and Heating Effects | CBSE Class 8 Science Notes

This note covers the magnetic effect of electric current, compass experiments, electromagnets, magnetic poles, lifting electromagnets, the heating effect of current, heating appliances, Voltaic cells, lemon cells, dry cells, rechargeable batteries, and battery disposal.

How can a compass show the magnetic effect of electric current?

What changes when current flows?

Electric current is the flow of electric charge. Electric charge is an electrical property of matter. A conductor is a material through which electric current can flow easily. When current flows through a conducting wire, it produces a magnetic effect around the wire.

A magnetic compass contains a tiny magnet that can turn. Its needle changes direction when a magnet is brought near it. This change from its original direction is called deflection. A current carrying wire can also deflect the needle.

Definition: A magnetic field is the region around a magnet or a current carrying wire where its magnetic effect can be felt, for example through the deflection of a compass needle.

An electric circuit is a path through which current can flow. A switch opens or closes that path. Closing the switch allows current to flow in the experimental circuit; opening it stops the current.

How is the observation checked?

1. Place an electric cell, a portable source of electricity, in a holder and connect it to a switch and connecting wires.
2. Stretch part of the longer wire between two nails fixed to cardboard, keeping it slightly above the cardboard.
3. Place a magnetic compass below this stretched wire and observe the initial direction of its needle.
4. Close the switch and watch the needle deflect. Open the switch and watch it return to its original direction.
5. Repeat switching ON and OFF a few times to confirm the connection between current and deflection.

Two views show a cell, connecting wire, compass and pin switch. The wire crosses above the compass. The switch positions and compass needle directions differ between the views.

See Fig. 4.1 in your NCERT textbook

The magnetic effect of electric current means that current in a conductor produces a magnetic field around it. This field disappears when the current stops. The compass therefore provides a way to detect current without relying on a glowing lamp.

How is an electromagnet made and tested?

What does winding a wire achieve?

A coil is wire wound into repeated loops, called turns. A current carrying coil behaves as a magnet. Such a coil is called an electromagnet. It loses its magnetic effect when the current stops flowing.

To make a simple electromagnet, tightly wind around 50 centimetres of flexible insulated wire around an iron nail and secure it with adhesive tape. A centimetre, written cm, is a unit of length. Insulated wire has a covering that prevents electrical contact through its covered surface.

Connect the ends to an electric cell and bring the nail near iron paper clips. The clips cling while current flows and fall when the wire is disconnected. Keep the connection for no more than a few seconds, because the cell may weaken quickly.

Is an iron nail essential for the magnetic effect?

A second arrangement uses around 100 cm of insulated wire wound in around 50 turns on a paper cylinder. The cylinder has a diameter roughly equal to the width of a pencil. The diameter is the distance across its circular opening through the centre.

1. Place a magnetic compass near each end of the coil and observe the needle positions before connecting the cell.
2. Connect the coil to the cell and observe the needles deflect as current flows through the coil.
3. Disconnect the cell and check that the needles return to their original positions.
4. Insert an iron nail into the centre of the coil and repeat the comparison.
5. Place iron paper clips near the nail ends and check their attraction while current flows.

The core is the material placed inside the coil. Inserting an iron nail makes the electromagnet stronger, producing much more compass deflection. For practical applications, most electromagnets have an iron core to make them stronger.

The pictures show compasses beside a coil, the coil connected to a cell, an iron nail inserted through the coil, and paper clips hanging from its ends.

See Fig. 4.3b to e in your NCERT textbook

How can an electromagnet's poles and strength be changed?

How are the poles identified?

Magnetic poles are the north and south ends of a magnet. An electromagnet also has a north pole and a south pole. Polarity means which end is north and which is south. Unlike poles, north and south, attract each other.

Label the coil ends A and B; these letters identify the two ends, rather than standing for physical quantities. Bring a compass near end A while the coil carries current. Observe which compass pole is attracted towards that end.

If the compass's north pole is attracted towards A, then A is the electromagnet's south pole. Repeat the test at B. Its polarity is opposite to A. Identifying a pole therefore requires attention to which compass pole moves towards the coil.

Which changes make the magnet stronger?

The strength depends on the amount of current through the coil and the number of turns. Compare the same coil using a single cell and a battery, a source of electricity that can contain connected cells. In this comparison, more cells give a larger current.

To compare the effect of cell number, use the same coil with 2 and 4 cells, connecting each for only a few seconds, as in the first activity, because the wire can become warm and the cells may weaken quickly. To investigate turns, use 2 cells with coils having different numbers of turns. These comparisons distinguish changes in current from changes in the winding.

Note: Removing the iron nail does not remove the coil's ability to produce a magnetic field. A current carrying coil still deflects a compass, but its effect is weaker than with the iron nail inserted.

Why are electromagnets useful for lifting objects?

How does switching control the load?

A lifting electromagnet is a strong electromagnet used to lift iron or steel objects. It may be suspended from a crane. Its useful feature is that the operator can control its magnetic action by switching the electric current ON and OFF.

When current flows, the electromagnet attracts and lifts iron or steel objects. When current is switched OFF, its magnetic field disappears and the objects are released. The same sequence can be demonstrated using a wire coil around an iron nail and iron paper clips.

1. Bring the electromagnet close to the iron or steel objects that are to be lifted.
2. Switch the current ON so that the coil produces a magnetic field and attracts the objects.
3. Move the lifted objects using the crane while the electromagnet holds them.
4. Switch the current OFF at the destination so that the objects are released.

Lifting electromagnets are widely used in factories and scrap yards to move, lift and sort heavy metal items. The load in this application consists of iron or steel objects. Switching controls both the attraction and the release of these objects.

Where else is the magnetic effect useful?

The magnetic effect of current has applications in electric bells, motors, fans and loudspeakers. An electric motor produces motion using electricity. These devices connect an electrical supply with a magnetic effect, though their detailed mechanisms differ.

The link between electricity and magnetism was discovered by Hans Christian Oersted in 1820. His finding was checked by other scientists who repeated the experiment and investigated the connection further. A compass beside a wire makes this link observable in a simple circuit.

The lifting model also shows why switching matters. Leaving a cell connected for too long may weaken it quickly. A weaker magnetic effect can reduce its ability to hold clips. This is a possible explanation to investigate, rather than proof of one particular fault.

Why does a wire become warm when current passes through it?

What is the heating effect?

Resistance is the opposition a conductor offers to the flow of current. Different conductors offer different levels of resistance. Because of this resistance, some electrical energy changes into heat energy, the energy responsible for warming the wire.

Definition: The heating effect of electric current is the warming of a conductor when current passes through it. Some electrical energy is converted into heat energy during this process.

Nichrome is a wire material used for heating. A nichrome wire offers higher resistance than a copper wire of the same size and length. Keeping those dimensions the same is important when making this comparison between the materials.

How is warming observed?

The heating experiment uses a cardboard piece about 10 cm long and 10 cm wide. Fix two nails about 5 cm apart. Stretch a 10 cm length of nichrome wire, about 0.3 millimetres thick, between the nails and connect it to a cell and switch.

A millimetre, written mm, is a unit of length used here for the wire's thickness. A second, written s, is a unit of time. Allow current to flow for about 30 s, then switch OFF.

Under teacher supervision, compare the wire's warmth before and after current flows by touching it only momentarily after switching OFF. Do not hold it or touch it for an extended period. Repeat the observation to confirm the change.

A wire is stretched between two nails on cardboard. Connecting wires join this arrangement to a cell and a pin switch on another piece of cardboard.

See Fig. 4.5 in your NCERT textbook

The wire feels warm after current has passed. This gives evidence of the heating effect. The same current can also produce a magnetic effect: a compass placed below a current carrying nichrome wire can deflect while the wire becomes warm.

What affects heating, and where is it useful or harmful?

Which factors influence the heat produced?

The amount of heat generated depends on the amount of current, the material, thickness and length of the wire, and the duration for which current flows. These factors must be considered when comparing the warming of different wires.

In the supervised experiment, using a battery of 2 cells produces more heat than using one cell for the same duration. This illustrates the dependence on current: the change in the source changes the heating while the wire and duration remain the same.

Wire comparisons can use two nichrome wires of equal length but different thicknesses, approximately 0.3 mm and 0.6 mm. Pass current through each for 30 s and compare their warming. Another comparison uses wires of the same diameter but different lengths.

How do heating appliances use this effect?

A heating element is the rod or coil of wire in an electrical heating appliance. It becomes hot when current passes. In some appliances where the element is visible, it can be seen glowing red hot.

A furnace is an enclosed space built to generate heat. Electrical heating in steel manufacturing converts scrap into usable steel. This is an industrial application of the same broad effect demonstrated by warming the nichrome wire.

Why must unwanted heating be limited?

Heating can cause energy loss in wires during transmission, the carrying of electrical energy from one place to another. Overheating may damage plugs and sockets, melt plastic parts, or even lead to fires. Household circuits include safety devices to minimise such incidents.

Use appropriate wires, plugs and sockets rated for the specified current, meaning designed for that current. Useful heating in an element and unwanted heating at an electrical connection are different outcomes of the heating effect.

How does a Voltaic cell generate electricity?

What are its main parts?

A Voltaic cell, also called a Galvanic cell, produces electricity through a chemical reaction. It contains two metal rods made of different materials and a liquid in a glass or plastic container. The rods are partly dipped in the liquid.

The rods are called electrodes. The liquid is the electrolyte, which is usually a weak acid or salt solution. An electrolyte helps conduct electricity. A chemical reaction between the electrodes and electrolyte produces the electricity supplied by the cell.

The terminals are the cell's connection points. They are identified as positive and negative. When a circuit is connected, electric current flows from the positive terminal through the circuit to the negative terminal.

Two labelled electrodes dip into liquid electrolyte in a glass container. Connecting wires join their upper ends through an electric lamp. Arrows labelled electric current show the direction around the external circuit.

See Fig. 4.7 in your NCERT textbook

Why does the cell eventually stop working?

Over time, the chemicals used in the cell's reaction get used up. The cell then stops supplying electricity and is called dead. Here, dead describes its inability to provide electricity; it does not mean that every material inside has disappeared.

This explains the connection between a cell's internal chemistry and its external effects. A cell supplies current because reactions occur inside it. The supplied current can then make a wire magnetic, warm a wire or light a suitable lamp.

The names Voltaic and Galvanic honour Alessandro Volta and Luigi Galvani. Galvani observed a frog's leg move when touched with two different metals. Volta investigated the role of the metals and liquid, using saltwater-soaked paper instead of the frog's leg.

Volta still obtained electric current. The investigation showed that the combination of metals and liquid could generate current and led to the first battery. The important connection is between a chemical arrangement inside the source and electricity supplied to a circuit.

How can lemons be used to make electric cells?

Which materials provide the electrodes and electrolyte?

A lemon cell uses a copper wire or strip and an iron nail as its two metal electrodes. Lemon juice acts as the electrolyte and helps conduct electricity. The metals are inserted a small distance apart in the lemon.

The activity uses five or six juicy lemons, copper wires or strips 1 to 2 mm thick, iron nails, connecting wires, and an LED. LED means light-emitting diode, a device that gives light when a suitable current passes through it in the correct direction.

1. Insert a copper wire or strip and an iron nail into one lemon, keeping the two electrodes a small distance apart.
2. Prepare the remaining lemons with the same two kinds of electrode.
3. Connect an iron nail in one lemon to the copper electrode in the next, making a chain of lemon cells.
4. Connect the LED between the copper electrode of the first lemon and the iron nail of the last lemon.
5. If the LED does not glow, reverse its connections and check again.

Why does the LED connection matter?

The LED's longer wire is its positive terminal; the shorter wire is its negative terminal. Current can pass through it only when its positive terminal connects to the battery's positive terminal and its negative terminal connects to the battery's negative terminal.

A glowing LED indicates that the cell arrangement is working. A non-glowing LED should prompt a check of the connections, including their direction. It should not immediately be taken as proof that the lemon arrangement cannot produce electricity.

Photograph: Electric cell made using lemons (a). Five lemons with metal electrodes are joined by wires to an LED. The accompanying drawing, captioned Connections in lemon cell (b), labels lemon, copper wire, iron nail and LED, with the LED connected across the two ends of the chain.

See Fig. 4.8 in your NCERT textbook

Salt solutions can also be used instead of lemon juice. Common metal pairs for Voltaic cells include zinc and copper, zinc and silver, aluminium and copper, iron and copper, magnesium and copper, and lead and copper. Some metals, like copper, act as positive electrodes, while others, like zinc, act as negative electrodes, because of their chemical properties.

How does a dry cell differ from a liquid-electrolyte cell?

What is inside a dry cell?

A dry cell contains a thick, moist electrolyte paste rather than a liquid electrolyte. The name does not mean that the inside is completely free of moisture. Dry cells are among the most widely used electric cells.

In the dry cell described here, the zinc container acts as the negative terminal. A carbon rod stands at the centre, surrounded by electrolyte paste. A metal cap over the carbon rod acts as the positive terminal.

An external view is placed beside a cutaway view. The cutaway labels the metal cap, central carbon rod, surrounding electrolyte and outer zinc container.

See Fig. 4.9 in your NCERT textbook

Why is the form of the electrolyte important?

Voltaic cells with liquid electrolyte were an important discovery, but they are not convenient for everyday use. The paste electrolyte distinguishes the dry cell from that arrangement. Dry cells are more portable than liquid-electrolyte Voltaic cells.

The described dry cell is a single-use cell. Once used up, it must be disposed of. This is different from a rechargeable battery, which can be charged and reused multiple times.

When identifying parts, distinguish the carbon rod from the metal cap covering it. The positive terminal is the metal cap; the negative terminal is the zinc container. Also distinguish the cell's useful lifetime from disposal: a used-up cell still needs appropriate handling.

Why are rechargeable batteries useful, and how should used batteries be handled?

What does recharging allow?

Rechargeable batteries can be charged and reused multiple times. This prevents wastage and saves money over time. They are increasingly used for several applications instead of relying on cells that must be discarded after use.

Different rechargeable batteries serve different purposes. Applications include watches, phones, laptops, tablets, cameras, inverters and electric vehicles. An inverter is equipment that uses stored battery electricity to provide an electrical supply.

Rechargeable batteries do not last forever. After repeated charging and use, they slowly wear out. The ability to recharge therefore extends their usefulness but does not give them an unlimited working life.

What materials and developments matter?

A lithium-ion battery, abbreviated Li-ion battery, is a type of rechargeable battery. These batteries rely on special metals such as lithium and cobalt. Recovering materials from used batteries helps make valuable materials available for reuse.

Solid-state batteries are batteries being developed with solid materials replacing liquid or paste-like electrolytes. This changes the material inside the battery. It is distinct from a dry cell, whose electrolyte is a moist paste.

Why does a dead battery still require care?

A battery that stops working is not completely dead in the sense of being harmless or empty. It could still contain acids and metals such as lead, cadmium, nickel or lithium. These materials may cause fires or harm the environment if discarded in regular garbage.

Recycling means recovering materials for further use. Many battery materials are valuable and could be recycled and reused. Special e-waste recycling facilities, which handle discarded electrical and electronic items, provide places for disposing of used batteries.

Note: A cell's failure to supply current does not establish that its contents are harmless. Use suitable e-waste recycling facilities for used batteries, and seek guidance when unsure where to dispose of them.

The useful life of a battery and the fate of its materials are separate questions. A rechargeable battery can be reused while it functions, and materials may still be recovered after it wears out.

Glossary

• Magnetic field — Region around a magnet or current carrying wire where its magnetic effect can be detected.
• Deflection — A change in the direction of a compass needle from its original position.
• Electromagnet — A current carrying coil that behaves as a magnet while electric current flows through it.
• Iron core — Iron placed inside a coil to make its magnetic effect stronger when current flows.
• Polarity — The identification of a magnet's ends as its north and south magnetic poles.
• Resistance — The opposition that a conductor offers to the flow of electric current through it.
• Heating effect — Warming of a conductor when current flows and some electrical energy changes into heat energy.
• Heating element — A rod or wire coil that becomes hot in an electrical heating appliance.
• Voltaic cell — An electric cell in which different metal electrodes and an electrolyte produce electricity through chemical reaction.
• Electrodes — The rods or other conducting parts that make contact with the electrolyte in a cell.
• Electrolyte — The material in a cell that helps conduct electricity; it may be a liquid, moist paste or solid.
• Dry cell — An electric cell whose electrolyte is a thick moist paste rather than a liquid.
• Rechargeable battery — A battery that can be charged and reused multiple times, although it eventually wears out.
• Dead cell — A cell that has stopped supplying electricity after the chemicals involved have been used up.

Common errors and misconceptions

• Misconception: A wire must be made of iron to produce a magnetic field. Correct: Current through a conductor produces a magnetic field. Iron, copper, aluminium and nichrome coils can deflect nearby compass needles when they carry current.
• Misconception: Every electromagnet must contain an iron core. Correct: A current carrying coil itself acts as an electromagnet. Most practical electromagnets contain iron cores because the cores make them stronger.
• Misconception: Reversing the current leaves the poles unchanged. Correct: Reversing current direction reverses the electromagnet's north and south poles.
• Misconception: Heating is always an unwanted effect of current. Correct: It is useful in heating elements, though unwanted heating can waste energy or damage connections.
• Misconception: A dry cell has no moisture inside it. Correct: Its electrolyte is a thick, moist paste rather than a liquid.
• Misconception: An LED connected either way will show whether lemon cells work. Correct: Its positive and negative terminals must be connected to the corresponding battery terminals.
• Misconception: Rechargeable batteries can be used forever. Correct: Repeated charging and use gradually wear them out, even though they can be reused multiple times.
• Misconception: A dead battery is harmless household rubbish. Correct: It could still contain materials that may cause fires or environmental harm and should go to suitable e-waste recycling facilities.

Exam-style questions with model answers

Q1. A compass is placed below a wire connected to a working cell and a switch. Its needle deflects when the switch closes and returns when the switch opens. Explain these two observations. [2 marks]

1. Closing the switch allows current through the wire. The resulting magnetic field deflects the compass needle, which is a tiny magnet.
2. Opening the switch stops the current. The wire's magnetic field disappears, so the needle returns to its original direction.

Q2. A current carrying coil deflects a nearby compass. Inserting an iron nail produces much greater deflection; stopping the current removes the coil's magnetic effect. Explain these three results. [3 marks]

1. The current carrying coil behaves as a magnet, called an electromagnet. Its magnetic field acts on the compass needle and causes the observed deflection.
2. The inserted iron nail forms an iron core. It makes the electromagnet stronger, which explains the much greater compass deflection.
3. When current stops flowing, the coil loses its magnetic effect. The electromagnet's action therefore depends on the flow of current.

Q3. The two ends of an electromagnet are labelled A and B. A compass's north pole is attracted towards A while current flows. Identify both poles and state the change when the current direction is reversed. [3 marks]

1. End A is the south pole because unlike magnetic poles attract. The attraction of the compass's north pole identifies the opposite pole at A.
2. End B is the north pole. The two ends of the electromagnet have opposite polarities, just as a magnet has north and south poles.
3. Reversing current direction reverses the electromagnet's poles. End A becomes north and end B becomes south.

Q4. A lifting electromagnet on a crane picks up iron or steel objects when current is ON and releases them when current is OFF. Explain the pickup, holding, release, control advantage and one place where such equipment is used. [5 marks]

1. When the current is switched ON, the coil behaves as an electromagnet. It produces a magnetic field that attracts the iron or steel objects.
2. While current continues to flow, the electromagnet holds the attracted objects. The crane can then move the objects that have been lifted.
3. Switching the current OFF makes the magnetic field disappear. The magnetic holding action stops, allowing the objects to be released.
4. The operator can control pickup and release by switching the current. This makes it possible to use the magnetic effect when required.
5. Such electromagnets are widely used in scrap yards. They help move, lift and sort heavy iron or steel items efficiently.

Q5. In a supervised experiment, the same nichrome wire carries current first from one cell and then from 2 cells for the same duration. The second trial generates more heat. Explain resistance and heating, interpret the comparison, and name the other wire and timing factors affecting heat generation. [4 marks]

1. Resistance is the opposition offered by the wire to current flow. Different conducting materials offer different levels of this opposition.
2. Because of resistance, some electrical energy is converted into heat energy. The wire therefore warms when electric current passes through it.
3. The greater heating with 2 cells shows that the heat generated depends on the amount of current. The wire and duration are kept the same in this comparison.
4. Heat generation also depends on the material, thickness and length of the wire, and on the duration for which current flows.

Q6. Five or six lemon cells contain copper electrodes and iron nails, with the nail of one lemon joined to the copper electrode of the next. An LED connects the first copper electrode and last iron nail. Identify the electrodes and electrolyte, explain the LED connections and a check if it does not glow, and state what glowing shows. [5 marks]

1. The copper wires or strips and iron nails are the electrodes. Each lemon contains two different metals placed a small distance apart.
2. Lemon juice is the electrolyte. It helps conduct electricity in the arrangement, which uses a chemical reaction to produce an electric current.
3. The LED's longer wire is its positive terminal and must connect to the battery's positive terminal. Its shorter negative wire connects to the negative terminal.
4. If the LED does not glow, reverse its connections and check again. Current can pass through the LED only with the correct terminal connections.
5. A glowing LED indicates that the cell arrangement is working. The connected lemon cells are supplying electricity to the external circuit.

Q7. Describe the electrolyte, negative terminal, positive terminal and reuse limitation of the single-use dry cell. [4 marks]

1. The electrolyte is a thick, moist paste around the central carbon rod. The word dry distinguishes this paste from a liquid electrolyte.
2. The zinc container acts as the negative terminal. It is both the outer container and a part of the cell's electrical arrangement.
3. A metal cap covers the carbon rod at the centre and acts as the positive terminal of the cell.
4. This dry cell is single-use. Once used up, it must be disposed of through an appropriate route rather than treated as a rechargeable battery.

Q8. Give one advantage of rechargeable batteries and one reason why used batteries should go to suitable e-waste recycling facilities. [2 marks]

1. Rechargeable batteries can be charged and reused multiple times, preventing wastage and saving money over time.
2. Used batteries could contain harmful materials, while valuable materials could be recovered through recycling instead of being discarded in regular garbage.

Key takeaways

• Current in a conductor produces a magnetic field, which can be detected by the deflection of a nearby compass needle.
• A current carrying coil is an electromagnet; most practical electromagnets use an iron core to increase their strength.
• More current or more turns can strengthen an electromagnet, while reversing the current reverses its magnetic poles.
• The heating effect converts some electrical energy into heat, with the amount depending on current, wire properties and duration.
• Heating elements use electrical heating, while unwanted overheating may damage plugs and sockets or even lead to fires.
• Voltaic cells use electrodes and an electrolyte to generate electricity through chemical reactions that eventually use up the chemicals.
• The described dry cell contains moist paste, a zinc container as its negative terminal and a metal cap as its positive terminal.
• Rechargeable batteries can be reused but gradually wear out; suitable recycling facilities help recover materials and limit disposal problems.

Test yourself

What happens to a wire's magnetic field when current stops?

The magnetic field produced by current in the wire disappears when the current stops flowing.

Does removing the iron nail stop a current carrying coil from deflecting a compass?

No. The current carrying coil still has a magnetic effect, but its effect is weaker without the iron nail.

The north pole of a compass is attracted to a coil end. Which pole is that end?

That end is the south pole of the electromagnet, because unlike magnetic poles attract each other.

Why compare nichrome and copper wires of the same size and length?

Keeping dimensions the same allows the comparison to focus on their materials. Nichrome offers higher resistance than copper under those conditions.

What is a heating element?

It is a rod or coil of wire that becomes hot when current passes through an electrical heating appliance.

Which substances are the electrodes and electrolyte in a lemon cell?

The copper wire or strip and iron nail are electrodes. Lemon juice is the electrolyte that helps conduct electricity.

Why is the name dry cell potentially misleading?

A dry cell contains a thick, moist electrolyte paste. It is not completely dry or without an electrolyte.

Why does a dead battery still need suitable disposal?

It could retain materials that may cause fires or harm the environment. Valuable materials could also be recovered through recycling.