Class 8 › Science › Chapter 4
Electricity: Magnetic and Heating Effects
Oersted's discovery, electromagnets and their strength, the heating effect of current and household appliances, and how cells and batteries (Voltaic, dry, rechargeable) work.
1. Magnetic Effect of Electric Current
Activity 4.1: current and a compass
Set-up: a wire stretched over a magnetic compass, connected through a switch to a cell.
Switch ON: the compass needle is deflected. Switch OFF: the needle returns to its original direction.
Meaning: a current-carrying wire has a magnetic effect on the needle (a tiny magnet), and the effect acts through non-magnetic materials in between.
Magnetic field: the region around a magnet or a current-carrying wire where its magnetic effect can be felt (e.g. by deflection of a compass needle). Magnetic effect of electric current: when current flows through a conductor, it produces a magnetic field around it; the field disappears when the current stops.
- Hans Christian Oersted (1777–1851, Denmark) discovered this in 1820: during a demonstration he noticed a compass needle near a circuit deflect whenever the circuit was closed or opened. He investigated, was certain, then published; other scientists repeated the experiment to check it.
- Uses: electromagnets, electric bells, motors, fans, loudspeakers.
2. Electromagnets
Electromagnet: a current-carrying coil that behaves as a magnet. Most electromagnets have an iron core to make them stronger.
Electromagnet: coil on an iron nail connected to a cell; clips fall when current stops.
| Observation (Activities 4.2–4.4) | Conclusion |
|---|---|
| Compasses near a coil deflect when current flows | The coil behaves like a magnet |
| Coil with iron nail inserted gives larger deflection, attracts clips | Iron core makes it a stronger magnet |
| Current stopped: needles return, clips fall | The coil loses its magnetism when current stops |
| One end attracts the compass's north pole, other end does the opposite | An electromagnet has two poles (N and S) like a bar magnet. If the north pole of the compass is attracted to end A, then A is a south pole |
Strength increases with
More current
A battery of more cells gives a larger current, a stronger field: larger compass deflection and more clips attracted.
Strength increases with
More turns of the coil
More turns also make a stronger electromagnet. Strength can be changed by current, turns, or both.
Reversing
Direction of current
Changing the direction of the current reverses the poles.
- Do not leave a coil connected to the cell for more than a few seconds or the cell weakens quickly.
- A step further: Earth behaves like a giant magnet because moving liquid iron in its core creates electric currents that generate a magnetic field. Migratory birds, fish and animals use it to navigate, and it shields life from harmful particles from space.
- Conversely, a moving magnet can produce electric current (higher grades): the basis of motors and power generators.
3. Lifting Electromagnets
- Strong electromagnets hung from cranes. The operator switches current ON to lift iron/steel objects and OFF to release them (the magnetic field disappears).
- Widely used in factories and scrap yards to move, lift and sort heavy metal items.
- An advantage over a permanent magnet: it can be switched on and off and made stronger or weaker.
4. Heating Effect of Electric Current
Heating effect of electric current: generation of heat in a conductor when current flows through it. Current faces opposition called resistance, and some electrical energy is converted into heat energy.
Activity 4.5 (under teacher supervision)
Set-up: a 10 cm nichrome wire (about 0.3 mm thick) tied between two nails, connected to a cell, holder and switch.
Result: after ~30 s with the switch ON, the wire feels warm. Touch only momentarily and never hold it (safety first).
With 2 cells: more heat in the same time, because heat depends on the magnitude of current.
- Different conductors offer different resistance: nichrome offers higher resistance than copper of the same size and length, so it heats more for a given current and is used in heating devices.
- Heat generated depends on: the amount of current, the material, thickness and length of the wire, and the time for which current flows.
- The wire ends of the electromagnet activity also warmed up for the same reason; an incandescent lamp glows because its filament is heated by current.
5. Heating Appliances, Uses and Safety
Uses
Household appliances
Electric room heaters, stoves, kettles, irons, immersion rods and hair dryers. All contain a rod or coil of wire called a heating element, sometimes seen glowing red hot.
Industry
Steel furnaces
High-temperature electric furnaces melt and recycle scrap steel into usable steel.
Problems
Unwanted heating
Energy loss in wires during transmission; overheating can melt plastic parts of plugs and sockets and cause fires.
- Prevention: use wires, plugs and sockets rated for the specified current; household circuits include safety devices to minimise such incidents.
- Electric heating is convenient compared with burning firewood or charcoal: no smoke, easy to control, cleaner indoors and less deforestation (society impact, exercise 6).
6. How a Battery Generates Electricity: Voltaic Cell
Cell / battery: a device that generates electric current because of chemical reactions inside it. (A battery is a combination of cells.)
Simple Voltaic cell (left) and dry cell (right).
- Voltaic (Galvanic) cell: two rods of different metals (electrodes) partly dipped in a liquid electrolyte (usually weak acid or salt solution) in a glass or plastic container. A chemical reaction between the rods and the electrolyte produces electricity.
- Current flows from the positive terminal through the circuit to the negative terminal. When the chemicals are used up the cell is 'dead'.
- History: named after Volta and Galvani (late 1700s). Galvani saw a dead frog's leg kick when touched with copper and iron and thought electricity came from the frog; Volta thought it came from the metals and showed it by using saltwater-soaked paper instead of the frog, still getting current. This led to the first battery.
- Lemon cell (Activity 4.6): copper wire and iron nail in each lemon (kept apart), lemons connected in a chain, LED connected across the ends. Electrodes: copper and iron; electrolyte: lemon juice (salt solutions also work). If the LED does not glow, reverse its connections: current passes through an LED only when its longer (positive) leg is connected to the positive terminal.
- Common metal pairs: zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, lead/copper. Copper acts as the positive electrode and zinc as the negative.
7. Dry Cells and Rechargeable Batteries
| Dry cell | Rechargeable battery | |
|---|---|---|
| Electrolyte | Thick moist paste (not liquid); more portable and convenient than a Voltaic cell | Many types (watches, phones, laptops, cameras, inverters, vehicles) |
| Terminals | Zinc container = negative; carbon rod with metal cap = positive | – |
| Use | Single-use: must be disposed of when used up | Can be recharged and reused many times: saves money, prevents waste |
| Limitation | Cannot be recharged | Slowly wear out after many charges (why an old phone battery needs frequent charging) |
- Lithium-ion (Li-ion) batteries are the most common rechargeable type, using metals like lithium and cobalt mined in limited parts of the world, so countries compete for supplies, recycle old batteries and develop new technology.
- Solid-state batteries (future): solid electrolyte instead of liquid or paste; safer, faster charging, longer life.
- Disposal: a 'dead' battery still contains acids and metals such as lead, cadmium, nickel or lithium, which can cause fires or harm the environment in ordinary garbage. Use e-waste recycling facilities; materials can be recovered and reused.
8. Quick Revision
| Concept | Key point |
|---|---|
| Magnetic effect | Current produces a magnetic field; Oersted, 1820; compass deflects |
| Electromagnet | Current-carrying coil (iron core makes it stronger); has N and S poles |
| Strength of electromagnet | Increases with current and number of turns; reverse current reverses poles |
| Heating effect | Resistance converts electrical energy to heat |
| Heat depends on | Current, material, thickness, length, time |
| Nichrome | High resistance, used in heating elements |
| Cell | Converts chemical energy to electricity; electrodes + electrolyte |
| Dry cell | Zinc (−), carbon rod (+), moist paste; single use |
Exercise hints
From 'Keep the curiosity alive'
Q1: (i) electrolyte, (ii) magnet. Q2: (i) False, (ii) True, (iii) False. Q3: (c): any current-carrying wire heats and also deflects a compass. Q4: Voltaic cell → chemical reactions; electric iron → heating effect; nichrome wire → best for electric heater; electromagnet → magnetic effect. Q5: (ii) generates more heat for a given current. Q10: still deflects but less (no iron core). Q11: (iv) all four coils carry current, so all deflect the compass. Q7(iii): reversing the battery reverses the poles and the direction of deflection.
Reasoning for Q8
Why did Sumana's magnet stop lifting?
Left ON, the cell's chemicals were used up and its current fell, weakening the magnetic field until it could not lift clips. The wire stays warm from the heating effect (and the cell may also be weak). This is my reasoning, not a textbook answer.
Common mistakes:
• Saying a coil is a magnet even with no current; it loses magnetism when current stops.
• Forgetting that an iron core makes the electromagnet stronger.
• Thinking the pole of an electromagnet cannot change; reversing the current reverses it.
• Saying nichrome heats more because it is a better conductor; it heats more because of higher resistance.
• Calling a dry cell rechargeable.
• Throwing used batteries in normal garbage.
• Touching a heated wire for a long time (burns).
• Saying a coil is a magnet even with no current; it loses magnetism when current stops.
• Forgetting that an iron core makes the electromagnet stronger.
• Thinking the pole of an electromagnet cannot change; reversing the current reverses it.
• Saying nichrome heats more because it is a better conductor; it heats more because of higher resistance.
• Calling a dry cell rechargeable.
• Throwing used batteries in normal garbage.
• Touching a heated wire for a long time (burns).