Electricity: Magnetic and Heating Effects Class 8: Electromagnets, Heating Effect and Cells

What Electricity: Magnetic and Heating Effects is about

Key concepts

Magnetic effect of electric current

NCERT section 4.1 Does an Electric Current Have a Magnetic Effect?

Activity 4.1: a wire stretched between two nails on cardboard is connected through a switch to a cell, with a magnetic compass placed just below the wire. When the switch is ON, the compass needle deflects; when it is OFF, the needle returns to its original direction. Repeating ON and OFF gives the same result.

A compass needle is a tiny magnet, so its deflection shows that the current-carrying wire acts like a magnet. The space around a magnet, or around a wire carrying current, in which a compass needle is affected is called a magnetic field.

Magnetic effect of electric current: a current flowing through a conductor produces a magnetic field around it; the field disappears when the current stops.

History: Hans Christian Oersted, a professor in Denmark, noticed in 1820 that a nearby compass deflected whenever he closed or opened a circuit. He checked it carefully before publishing, and other scientists repeated his experiment, discovering that electricity and magnetism are linked.

Uses: electromagnets, electric bells, motors, fans, loudspeakers and more.

Worked example

In Activity 4.1 you have no bulb in the circuit. How can you tell whether a current is flowing? Explain your reasoning.

Try it first, then show the solutionSolution

1. Place a magnetic compass just below the wire.
2. If the needle deflects when the switch is ON and returns when it is OFF, a current is flowing.
3. A current produces a magnetic field around the wire, which acts on the compass needle (a tiny magnet).

Answer: Use a compass: its deflection shows the magnetic field produced by the current.

Electromagnets

NCERT section 4.1.1 Electromagnets

Activity 4.2: about 50 cm of insulated wire wound tightly around an iron nail and connected to a cell makes the nail pick up iron paper clips; when the cell is disconnected, the clips fall. (Connect only for a few seconds, or the cell weakens quickly.)

Activity 4.3: about 50 turns of insulated wire are wound on a paper cylinder (a coil). With current, the coil deflects compass needles at both ends; without current, the needles return. With an iron nail inside the coil, the deflection is much larger and the ends attract clips.

A current-carrying coil that behaves as a magnet is an electromagnet. Most practical electromagnets have an iron core, which makes them much stronger. They are temporary magnets: they work only while current flows, and can be switched on and off.

Worked example

Why does an iron nail with a coil around it attract more clips than the coil alone, and why do the clips fall when the cell is disconnected?

Try it first, then show the solutionSolution

1. The iron core makes the electromagnet much stronger than the coil alone, so it attracts more clips.
2. The magnetism exists only while current flows through the coil.
3. When the cell is disconnected, the current and its magnetic field stop, so the clips fall.

Answer: The iron core strengthens it; with no current there is no magnetic field.

Poles and strength of an electromagnet

NCERT section 4.1.1 Electromagnets

Activity 4.4: label the ends of the electromagnet A and B and bring a compass near each end with the current on. Unlike poles attract: if the north pole of the compass needle points towards end A, then A is a south pole. End B turns out to have the opposite polarity. So an electromagnet has two poles, north and south, like a bar magnet.

Reversing the current (turning the cell around) reverses the poles.

Making it stronger:

• increase the current, for example by using a battery of 2 or 4 cells instead of 1;
• increase the number of turns of the coil;
• use an iron core.

A stronger electromagnet deflects the compass more and attracts more clips.

Earth as a magnet: moving liquid iron deep in Earth's core creates electric currents that produce Earth's magnetic field. Migratory birds, fish and other animals use it to navigate, and it shields life from harmful particles from space.

Worked example

The north pole of a compass needle is attracted to end A of an electromagnet. What is the pole at A and at B? What happens if the cell connections are reversed?

Try it first, then show the solutionSolution

1. Unlike poles attract, so end A is a south pole.
2. The other end, B, is a north pole.
3. Reversing the current reverses the poles: A becomes north and B becomes south, so the compass needle turns round.

Answer: A south, B north; reversing the current swaps them.

Lifting electromagnets and other uses

NCERT section 4.1.2 Lifting electromagnets

Lifting electromagnets are strong electromagnets hung from cranes. The operator switches the current ON to lift iron and steel objects and OFF to drop them, so they are used in factories and scrap yards to move, lift and sort heavy iron and steel items (aluminium, copper, plastic and wood are left behind because they are not attracted).

Other devices using the magnetic effect: electric bells, motors, fans and loudspeakers.

Looking ahead: just as a current produces magnetism, a moving magnet can produce an electric current; this link is the basis of motors and power generators, which you will study in higher grades.

Worked example

A scrap yard has a mixed heap of iron rods, aluminium cans and copper wires. How can a lifting electromagnet help sort them, and why not use a permanent magnet?

Try it first, then show the solutionSolution

1. With the current ON, the electromagnet lifts only the iron rods (iron is magnetic; aluminium and copper are not).
2. The crane moves them to another pile, and switching the current OFF releases them.
3. A permanent magnet could not easily release the iron, because it cannot be switched off.

Answer: It picks out the iron and drops it at will; a permanent magnet cannot be switched off.

Heating effect of electric current

NCERT section 4.2 Does a Current Carrying Wire Get Hot?

Activity 4.5 (with a teacher): a 10 cm nichrome wire (about 0.3 mm thick) is tied between two nails 5 cm apart on cardboard and connected through a switch to a cell. After about 30 s with the switch ON, the wire feels warm when touched briefly (do not hold it). With 2 cells, it gets hotter in the same time.

Why: a conductor offers some resistance to the flow of current, and this converts part of the electrical energy into heat. This is the heating effect of electric current. Different conductors have different resistance: nichrome has a higher resistance than a copper wire of the same size and length, so it heats more, which makes it ideal for heating elements.

The heat produced depends on: the current (more cells, more heat), the material, the thickness and length of the wire, and the time for which current flows.

Appliances using it contain a heating element (a coil or rod): room heaters, electric stoves, kettles, irons, immersion rods, hair dryers; an incandescent bulb glows because its filament gets very hot. In industry, electric furnaces melt and recycle scrap steel.

Worked example

Two identical nichrome wires are connected for 30 seconds, one to 1 cell and one to 2 cells. Which gets hotter and why? Name two other factors that affect the heat produced.

Try it first, then show the solutionSolution

1. The wire with 2 cells gets hotter, because more cells give a larger current, and more current produces more heat.
2. Other factors: the time for which current flows, and the material, length and thickness of the wire.

Answer: The 2-cell wire; heat depends on current, time, material, length and thickness.

Problems of unwanted heating and safety

NCERT section 4.2 Does a Current Carrying Wire Get Hot?

The heating effect is useful, but it can also cause problems:

• energy loss as heat in wires carrying electricity over long distances;
• overheating in appliances, plugs and sockets, which can melt plastic parts or even start fires.

Safety:

• use wires, plugs and sockets rated for the current they will carry;
• household circuits have safety devices to reduce such accidents;
• do not overload sockets with many high-current appliances;
• in activities, touch a heated wire only briefly and under supervision.

Compared with burning wood or charcoal, electric heating gives no smoke in the home, is easier to control and is cleaner indoors, though the electricity itself must be generated somewhere.

Worked example

Why can a thin extension cord become dangerously hot if a room heater, an iron and a kettle are all plugged into it at once?

Try it first, then show the solutionSolution

1. Together the appliances draw a large current through the cord.
2. A large current produces a lot of heat in the cord's wires (heating effect).
3. If the cord is not rated for that current, it overheats, its plastic can melt and it may cause a fire.

Answer: Too much current heats the cord beyond its rating; use properly rated wiring and avoid overloading.

Voltaic cells and the lemon cell

NCERT section 4.3 How Does a Battery Generate Electricity?

A Voltaic (Galvanic) cell has two rods of different metals (the electrodes) partly dipped in a liquid (the electrolyte), usually a weak acid or salt solution, in a container. A chemical reaction between the electrodes and the electrolyte produces electricity. In the external circuit, current flows from the positive terminal through the circuit to the negative terminal. When the chemicals are used up, the cell is 'dead'.

History: in the late 1700s Luigi Galvani saw a dead frog's leg twitch when touched with two different metals and thought the electricity came from the frog. Alessandro Volta replaced the frog's leg with paper soaked in salt water and still got a current, showing that two metals and a liquid make electricity: the first battery.

Activity 4.6, lemon cell: push a copper wire and an iron nail, a little apart, into each of five or six lemons; join the copper of one lemon to the iron of the next; connect an LED between the first copper wire and the last nail. The LED glows if its longer leg (positive) is on the positive side; if not, reverse it. The copper and iron are the electrodes and lemon juice is the electrolyte (salt solution also works). Pure water does not work well as an electrolyte.

Common metal pairs: zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, lead/copper. Copper usually acts as the positive electrode and zinc as the negative one.

Worked example

In a lemon cell, name the electrodes and the electrolyte. The LED does not glow when connected; give two possible reasons and fixes.

Try it first, then show the solutionSolution

1. Electrodes: copper wire and iron nail; electrolyte: lemon juice.
2. Reason 1: the LED is connected the wrong way round; fix: reverse it so the longer leg goes to the positive (copper) side.
3. Reason 2: one lemon gives too little; fix: join more lemons in a chain (copper to iron), or check that the metals are not touching inside the lemon.

Answer: Copper and iron electrodes, lemon juice electrolyte; reverse the LED or add more lemons.

Dry cells and rechargeable batteries

NCERT section 4.3 How Does a Battery Generate Electricity?

Dry cell: the everyday 'battery' in torches and remotes. Its electrolyte is a moist paste, not a liquid, so it is portable and does not spill.

• A zinc container acts as the negative terminal.
• A carbon rod in the centre, with a metal cap, is the positive terminal.
• The paste electrolyte surrounds the carbon rod.

A dry cell is single-use: once used up, it must be disposed of.

Rechargeable batteries can be charged and reused many times, saving money and reducing waste: in watches, phones, laptops, cameras, inverters and electric vehicles. They too wear out after many charges (a phone battery needs charging more often after a year or two).

Lithium-ion batteries are the most common rechargeable type; they need metals like lithium and cobalt, mined in only a few places, so recycling matters. Solid-state batteries with solid electrolytes may be safer, charge faster and last longer.

Used batteries are not harmless: they may contain acids and metals like lead, cadmium, nickel or lithium, so they should be disposed of or recycled properly, not thrown in ordinary waste.

Worked example

Give two differences between a Voltaic cell and a dry cell, and one advantage of rechargeable batteries.

Try it first, then show the solutionSolution

1. A Voltaic cell has a liquid electrolyte in an open container; a dry cell has a moist paste, so it does not spill and is more portable.
2. In a dry cell the zinc container itself is the negative terminal and a carbon rod is the positive terminal; a simple Voltaic cell has two separate metal rods.
3. Rechargeable batteries can be reused many times, saving money and reducing waste.

Answer: Liquid versus paste electrolyte; container as electrode; rechargeables are reusable.

Science in real life: Electricity: Magnetic and Heating Effects

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries
• Magnetic Effect of Electric Current and Electromagnets

Electricity: Magnetic and Heating Effects: formulas and key facts

Magnetic effect

$\text{current in a conductor}\to \text{magnetic field around it}$

Electromagnet strength

$\text{stronger with more current, more turns, iron core}$

Poles

$\text{reverse the current}\Rightarrow \text{poles reverse}$

Heating effect

$\text{heat depends on current, material, length, thickness, time}$

Cell

$\text{two different metals (electrodes)}+\text{electrolyte}\to \text{electricity by chemical reaction}$

Dry cell terminals

$\text{zinc container}=(-),\text{carbon rod with metal cap}=(+)$

LED rule

$\text{longer leg}\to (+),\text{shorter leg}\to (-)$

Electricity: Magnetic and Heating Effects: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Electricity: Magnetic and Heating Effects

The question patterns to expect, and the method that works for each.

Tricks to remember Electricity: Magnetic and Heating Effects

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Current makes a compass dance

  Switch on, the needle swings; switch off, it settles back.

  Use it when: Magnetic effect.

• CTI for a strong electromagnet

  Current up, Turns up, Iron in.

  Use it when: Ways to increase strength.

• Nichrome for heat, copper for carrying

  High-resistance nichrome heats; low-resistance copper carries current with little heating.

  Use it when: Choosing wire materials.

• Zinc outside, carbon inside

  Dry cell: zinc can is negative outside, carbon rod is positive inside.

  Use it when: Labelling a dry cell.

• Long leg, plus side

  LED's longer leg goes to the positive terminal.

  Use it when: Lemon cell and LED circuits.

Electricity: Magnetic and Heating Effects: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Electricity: Magnetic and Heating Effects

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying a current-carrying wire becomes a permanent magnet

  The temporary nature is forgotten.

  Wrong working

  Once current has flowed, the wire stays magnetic.

  Right working

  The magnetic field exists only while current flows; it disappears when the current stops.

  ✓Do this: Always add 'only while current flows'.

  Usually costs: 1 mark.

• ✗Mistake: Getting the pole wrong from the compass

  Like and unlike poles are mixed up.

  Wrong working

  The compass's north pole points to end A, so A is north.

  Right working

  Unlike poles attract, so end A is a south pole.

  ✓Do this: The end that attracts the compass's north is south.

  Usually costs: 1 mark.

• ✗Mistake: Thinking more cells weaken an electromagnet

  Students guess without reasoning.

  Wrong working

  With 2 cells the electromagnet attracts fewer clips than with 1 cell.

  Right working

  2 cells give more current, so a stronger magnetic field and more clips.

  ✓Do this: More current means a stronger electromagnet.

  Usually costs: 1 mark in true/false items.

• ✗Mistake: Saying nichrome is used because it is a good conductor

  Heating is linked to conducting well.

  Wrong working

  Nichrome is used in heaters because it conducts electricity very easily.

  Right working

  Nichrome has high resistance, so it produces more heat for a given current.

  ✓Do this: Link heating to resistance.

  Usually costs: 1 mark.

• ✗Mistake: Reversing the LED is ignored

  LEDs are treated like bulbs.

  Wrong working

  The lemon cell is not working because the LED does not glow.

  Right working

  An LED lights only one way round; check that the longer leg is on the positive side before concluding.

  ✓Do this: Check LED polarity first.

  Usually costs: 1 mark.

NCERT exercise guide: Electricity: Magnetic and Heating Effects

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Electricity: Magnetic and Heating Effects

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   An electromagnet loses its magnetism when
   (a) more turns are added
   (b) the current is switched off
   (c) an iron core is inserted
   (d) more cells are added

   Hint

   Temporary magnet.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   In a dry cell, the positive terminal is the
   (a) zinc container
   (b) paste
   (c) carbon rod with metal cap
   (d) paper lining

   Hint

   Centre of the cell.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerExam level

   Name the liquid used in a Voltaic cell.

   Hint

   Weak acid or salt solution.

   Answer

   The electrolyte.

4. Q4 · 2 marks · Very short answerExam level

   State two problems caused by the heating effect of current.

   Hint

   Wires and appliances.

   Answer

   Energy is lost as heat in transmission wires; overheating can melt plugs and sockets or cause fires.

5. Q5 · 2 marks · Very short answerHarder exam level

   Why should the wire in Activity 4.2 be connected to the cell for only a few seconds?

   Hint

   Cell and wire.

   Answer

   A coil connected directly across a cell draws a large current, which drains (weakens) the cell quickly and also heats the wire.

6. Q6 · 2 marks · Very short answerHarder exam level

   In Activity 4.5, the wire heats more with 2 cells than with 1. Name the quantity that changed and two other factors that affect heating.

   Hint

   More cells.

   Answer

   The current increased; other factors: time for which current flows, and the wire's material, length or thickness.

7. Q7 · 3 marks · Short answerHarder exam level

   Explain how Volta showed that electricity in Galvani's experiment came from the metals, not the frog.

   Hint

   Salt water paper.

   Answer

   Volta replaced the frog's leg with paper soaked in salt water between two different metals and still got an electric current, so the metals and the liquid, not the frog, produced the electricity. This led to the first battery.

8. Q8 · 2 marks · Very short answerHarder exam level

   True or false, with reason: (i) The electrolyte in a dry cell is a liquid. (ii) Reversing the current in a coil reverses its magnetic poles.

   Hint

   Paste; poles.

   Answer

   (i) False: a dry cell uses a moist paste, which is why it does not spill. (ii) True: reversing the direction of current swaps the north and south poles.

9. Q9 · 3 marks · Short answerHarder exam level

   Why do rechargeable batteries help the environment, and why must even they be disposed of carefully?

   Hint

   Reuse and harmful contents.

   Answer

   They can be recharged and reused many times, so fewer batteries are thrown away. But they eventually wear out and contain metals such as lithium, nickel or cobalt (and sometimes acids), which can harm soil and water, so they should be recycled at proper collection points.

10. Q10 · 5 marks · Long answerHarder exam level

    Plan an experiment to test whether the number of cells affects the strength of an electromagnet. State what you change, keep the same and measure, and predict the result.

    Hint

    Count clips.

    Answer

    Change: number of cells (1, 2, 3) joined as a battery. Keep the same: the same coil, number of turns, iron nail, clips and time. Measure: number of clips lifted (or compass deflection at a fixed distance), repeated three times each. Prediction: more cells give more current, so more clips are lifted.

11. Q11 · 3 marks · Short answerHOTS / competency

    A compass placed below a wire does not deflect when the switch is closed. Give three possible reasons.

    Hint

    Check the circuit and placement.

    Answer

    The circuit may be open (loose connection or faulty switch), the cell may be dead so no current flows, or the compass is too far from the wire (or the wire already runs along the needle's direction so the effect is hard to see).

12. Q12 · 2 marks · Very short answerHOTS / competency

    Why does a phone battery need charging more often after a year or two? What future battery technology may help?

    Hint

    Wear and solid-state.

    Answer

    Rechargeable batteries slowly wear out after being charged and used many times, so they hold less charge. Solid-state batteries, with solid electrolytes, may be safer, charge faster and last longer.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What happens to a compass needle near a wire when current flows?Show answer

   It deflects, and returns when the current stops.

2. 2.What is a magnetic field?Show answer

   The region around a magnet or current-carrying wire where its magnetic effect can be felt.

3. 3.What is an electromagnet?Show answer

   A current-carrying coil that behaves like a magnet, usually with an iron core.

4. 4.How can the poles of an electromagnet be reversed?Show answer

   By reversing the direction of current.

5. 5.Why does Earth behave like a magnet?Show answer

   Moving liquid iron in Earth's core creates electric currents that produce a magnetic field.

6. 6.What is the heating effect of electric current?Show answer

   The heating of a conductor when current flows through it, because of its resistance.

7. 7.Name four appliances that use the heating effect.Show answer

   Electric iron, kettle, room heater, immersion rod (also stove, hair dryer).

8. 8.What are the parts of a Voltaic cell?Show answer

   Two different metal electrodes and an electrolyte in a container.

9. 9.Which part of a dry cell is the negative terminal?Show answer

   The zinc container.

10. 10.Name the most common type of rechargeable battery today.Show answer

    The lithium-ion battery.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you describe and explain Activity 4.1?
• ☐Can you state what Oersted discovered and when?
• ☐Can you make an electromagnet and explain why it is temporary?
• ☐Can you find the poles of an electromagnet with a compass?
• ☐Can you give three ways to make an electromagnet stronger?
• ☐Can you explain how a lifting electromagnet sorts scrap?
• ☐Can you explain why a nichrome wire gets hot?
• ☐Can you list factors that affect the heat produced?
• ☐Can you label a Voltaic cell and a dry cell?
• ☐Can you explain why an LED in a lemon circuit might not glow?

Electricity: Magnetic and Heating Effects: frequently asked questions