CBSE Class 10 Science · 2026–27
A compass turns near a current-carrying wire. A coil can lift a paper clip, and a motor can turn a fan. Connect these familiar effects by learning to read fields, follow current and predict force—then explore how motion can generate an electrical output.
- Read magnetic field-line direction and relative strength without treating lines as physical objects.
- Use the right-hand grip rule for a straight wire, loop and solenoid.
- Predict changes when current, distance, turns or core conditions change.
- Apply Fleming’s left-hand rule and distinguish it from induction’s right-hand rule.
- Explain AC/DC, 50 Hz cycles/reversals and high-voltage transmission reasoning.
- Explain domestic parallel branches and protective functions without attempting mains work.
- Explore motor, induction and generator energy conversions and simple mechanisms as formative learning.
01A compass reveals the magnetic field
A small compass near a bar magnet turns without touching it. The surrounding region has a magnetic field, and the needle responds to that field. The direction at a point is defined by the direction in which the north-seeking end of a small compass points.
Field lines are a drawing model for representing this direction and relative strength. Outside a bar magnet, arrows run from its north pole towards its south pole; inside the magnet they continue from south to north. Together they form closed loops. They are not physical strings leaving a pole.
Where lines are drawn closer together, the field is stronger. Lines do not cross: crossing would give two field directions at the same point. The pattern is three-dimensional even when we draw one flat section.
To map a field in a teacher-guided activity, place a bar magnet beneath a sheet and use a small compass at several positions. Mark the direction of the needle’s north end each time; do not merely join arbitrary decorative curves. Keep magnets away from devices or materials that they could damage, and use sealed iron-filings viewers if filings are shown.
Earth’s field also affects the compass. Far from a classroom magnet, the needle returns approximately to the local north–south direction. Near the magnet, it follows the resultant field.
Pause. Think. Try.
02Current makes a field around a wire
Close a switch in a suitable battery circuit near a compass and the needle can turn. Reverse the current and the deflection reverses. This connects electricity with magnetism: a current-carrying conductor produces a magnetic field.
A long straight wire has concentric circular field lines in planes perpendicular to it. Use the right-hand thumb rule: point your right thumb along conventional current; your curled fingers show the magnetic-field direction.
For a wire through a horizontal sheet, current upwards towards you is represented by a dot, like an arrowhead. Viewed from above, its field runs anticlockwise. Current downwards away from you is represented by a cross, like an arrow’s tail, and gives a clockwise pattern. Always state the viewing direction.
For the same long straight wire, the field is stronger when current is larger and weaker farther from the wire. In the simple long-wire model, doubling current doubles the field at a fixed distance; doubling distance halves it at fixed current. No advanced field-formula derivation is needed to explain those comparisons.
In a practical version, a teacher checks a low-voltage circuit with appropriate current limiting. Close the switch only briefly: a wire connected across a cell without suitable resistance can heat rapidly. A wire in a wall is not a safe substitute.
Pause. Think. Try.
03A loop combines the fields from its wire
Bend a current-carrying wire into a circular loop. Each small part produces a field, and at the centre the contributions point along the loop’s axis. Near the centre, the field lines are approximately straight and perpendicular to the plane of the loop.
The two faces behave like opposite magnetic poles. Look straight at one face: if the current appears anticlockwise, that face behaves as a north pole; if it appears clockwise, it behaves as a south pole. Looking from the other side reverses the apparent turning direction, so the same loop is not “north on both sides”.
A coil contains several turns carrying current in the same sense. If the turns have the same radius and are close together, their fields reinforce one another. More turns or more current makes the field stronger when other conditions are held fixed. Increasing the loop’s radius while holding current and turns fixed reduces the field at its centre.
Use these relationships for predictions rather than assuming that a bigger drawing means a stronger magnet. For a fair comparison, specify the current, turn count, radius and observation point.
The right-hand grip idea still works: curl fingers in the current’s direction around the loop; the extended thumb points along the field through its centre and towards the north face.
Pause. Think. Try.
04A solenoid gives a controllable magnet
A solenoid is a long coil of many turns. When current passes, its overall field resembles that of a bar magnet. Inside a sufficiently long solenoid, away from its ends, the field is approximately uniform: lines are nearly parallel and equally spaced.
Use the grip rule around its turns to find its north end. Reverse the current and the north and south ends exchange. Do not confuse this field-direction rule with Fleming’s left-hand force rule.
Inserting a suitable soft-iron core can make a strong electromagnet. Soft iron is useful because it magnetises readily and loses most of the induced magnetism when the current is removed. Avoid the absolute claim that every trace of magnetism always vanishes.
For a given coil and geometry, increasing current strengthens the field. Increasing the turns per unit length also strengthens it if current and core conditions are comparable. Simply making a coil longer while keeping the total turn count unchanged is not the same as adding more turns to the same length.
An electromagnet can switch a lifting force on and off, for example in a model crane. A teacher-led low-voltage version needs insulated wire, a checked current limit and short operating periods to prevent heating. A paper prediction or simulation can test the same reasoning without constructing a coil.
Pause. Think. Try.
05A current-carrying wire can feel a force
Place a current-carrying conductor in a magnetic field and it can experience a force. The direction depends on both the conventional current and the external field. For the simple school setup, the force is greatest when current is perpendicular to the field and zero when it is parallel.
Fleming’s left-hand rule keeps three perpendicular directions organised. Stretch the first finger, middle finger and thumb of the left hand mutually perpendicular. The first finger points along the magnetic field, the middle finger along conventional current, and the thumb gives the force or motion tendency.
As a direction example, take east, north and upwards as three perpendicular directions. If current is east and field is north, the force is upwards. Reverse current alone and the force is downwards. Reverse the field alone and it is also downwards. Reverse both and the force returns upwards.
The field belongs to the magnet or other source around the conductor; the force is not found by pointing the thumb along the wire’s circular self-field. Identify the given external field first.
This is the key physical idea behind a motor. For now practise predicting directions with diagrams or three labelled arrows. The grip rule finds field from current; the left-hand rule finds force from current and field. They answer different questions.
Pause. Think. Try.
06AC completes cycles; DC keeps one direction
A cell supplies direct current (DC) through a resistive circuit: its direction stays the same. DC need not be perfectly constant in magnitude, but it does not periodically reverse direction.
Alternating current (AC) reverses periodically. The frequency is the number of complete cycles each second, measured in hertz (Hz). The standard school model for India’s mains supply is 50 Hz: 50 complete cycles per second. Each sinusoidal cycle has two direction reversals, so that is 100 reversals per second.
The period T is the time for one complete cycle: T = 1/f. At 50 Hz, T=1/50=0.020 s. Consecutive direction reversals are separated by half a period, 0.010 s. Do not call 0.010 s the full period.
AC is convenient for conventional power systems because transformers can step its voltage up or down. For a given transmitted power, raising voltage lowers current. Smaller current means less resistive heating in transmission lines because loss is I²R. The key is lower current at higher voltage, not a claim that AC can never waste energy.
Use paper waveforms to count cycles and reversals. A graph of AC current crossing zero does not mean the supply is safe to touch at that instant. Learners do not measure or experiment with household mains.
Pause. Think. Try.
07Explain household protection without doing household wiring
A simplified domestic circuit has live and neutral supply conductors, with appliances connected in parallel. Each branch receives the supply p.d. and can have its own switch. Neutral is near earth potential in the ideal normal model, but it is never a wire for learners to touch or test. A conductor’s name or insulation colour does not establish safety.
A switch and overcurrent protection are placed in the live path so that opening them disconnects that path to the load. A short circuit creates an unintended very low-resistance path and can produce a large current. Overloading means the current demand exceeds what a circuit is designed to carry; it can happen without a direct live–neutral contact.
A correctly selected fuse melts to break excessive current; an MCB trips and can be reset after the fault is resolved. Neither should be bypassed. An earth connection on an appropriate metal-cased appliance provides a protective fault-current path and supports disconnection by suitable protection. It reduces risk; it is not a promise that every earthed object is always safe. A residual-current device detects an imbalance between outgoing and returning current, which is a different protective function from ordinary overcurrent protection.
For students: observe only supplied diagrams or label photographs. Keep water and wet hands away from electrical equipment, report damaged cords or hot sockets to an adult, and leave repairs to a competent electrician. Do not open plugs, change fuse wire, probe sockets or build a mains circuit. A dry wooden board or ordinary rubber gloves do not make such work a learner activity.
Pause. Think. Try.
08Explore a motor: turn electrical input into motion
Formative learning: this topic helps you understand and discuss an electric motor; it is part of the current CBSE formative material rather than the listed year-end core.
In a simple DC motor, a rectangular coil carries current between magnetic poles. Opposite sides of the coil have opposite current directions. The external field therefore exerts opposite forces on those sides, creating a turning effect about the axle. Electrical energy is transferred into mechanical energy, with some energy also dissipated as heat.
After half a turn, each side has moved to the other side of the rotation. A split-ring commutator reverses the current through the coil at that point. This reverses the forces on the moving sides, so the turning effect continues in the same rotational sense. The current inside the coil reverses every half-turn even though the battery’s external DC connection keeps its direction.
Stationary brushes maintain contact with the rotating split ring. More suitable turns, stronger field and a soft-iron core can improve the practical motor’s turning effect; real motors have more complex construction than the one-coil drawing.
For a self-check, explain the energy direction, the role of the forces, and why the ring is split. Do not simply memorise a circular arrow. A teacher demonstration or an unpowered labelled model is enough to explore the mechanism.
Pause. Think. Try.
09Explore induction: change the magnetic link
Formative learning: electromagnetic induction connects changing magnetic conditions with an induced potential difference.
Connect a coil to a sensitive galvanometer and move a bar magnet towards it. The pointer can deflect, showing an induced current in the closed circuit. Stop the magnet relative to the coil and the steady deflection returns to zero. Pull it away and the induced current reverses. Moving the coil relative to the magnet can also produce the effect.
The important idea is a change in the magnetic flux linked with the coil. At this level, think of a changing magnetic field through its area. A strong but unchanging field through a stationary coil does not sustain an induced current. An induced p.d. can exist even if the circuit is open; current needs a closed conducting path.
Faster relative motion, more turns or a stronger magnet can give a larger induced effect in an otherwise comparable setup. In a second demonstration, changing current in one coil changes its field and can induce a p.d. in a nearby coil. A constant current after switch-on does not keep causing a steady induction in the second coil.
Fleming’s right-hand rule can predict the induced conventional-current direction for a conductor moving across a magnetic field: first finger field, thumb motion, middle finger induced current. It answers a generation question, not the motor-force question answered by the left hand.
Pause. Think. Try.
10Explore a generator: supply the mechanical work
Formative learning: a generator uses electromagnetic induction to transfer mechanical energy into electrical energy. It does not create energy from nothing, and a magnet alone is not a fuel.
In the simple AC model, a coil is rotated in a magnetic field. As its orientation changes, the magnetic flux through it changes and an emf is induced. With a closed external circuit, current can flow. Every half-turn, the induced-current direction reverses.
Two separate complete slip rings, one attached to each end of the coil, maintain contact with stationary brushes. They preserve the alternating output. Contrast this with the split ring of a simple DC motor. A split-ring commutator on the corresponding simple generator model can give an external current that keeps one direction but varies in magnitude.
The mechanical input may come from a water turbine, a wind turbine, or a steam turbine heated using another energy source. In a real power-station alternator, the magnetic-field system may rotate while large output coils remain stationary; the simple rotating-coil picture shows the principle, not every machine’s layout.
Finish by comparing three ideas: current produces a field; a current in an external field can feel a force; a changing magnetic flux can induce a p.d. Use those sentences to choose the correct direction rule and energy conversion instead of memorising similar-looking diagrams.
Pause. Think. Try.
Your formula shelf
| Idea | Relation | Remember |
|---|---|---|
| AC period | T = 1/f | T in seconds, f in hertz. Complete cycles, not direction reversals. |
| Sinusoidal reversal count | Reversals per second = 2f | Two direction reversals per complete sinusoidal cycle; 50 Hz gives 100 per second. |
| Long-wire comparison | Field strength ∝ I/r | Relative comparison for the same long straight wire at distance r; no advanced derivation required. |
| Resistive line heating | Ploss = I²R | Compare same line resistance; for fixed transmitted power, higher voltage permits lower current. |
| Motor and generator | Motor: electrical → mechanical; generator: mechanical → electrical | Useful conversions with losses; formative models, not energy creation. |
Work through the numbers
Find the period and time between direction reversals of a 50 Hz sinusoidal current.
- T=1/f=1/50=0.020 s per complete cycle.
- Each cycle has two reversals, so the interval is T/2=0.010 s.
- Reversal count is 2f=100 per second.
A hypothetical resistive 2.0 kW load is shown connected to a 220 V branch rated for 5 A in a paper problem. Find the expected current and explain the issue. This is not a fuse-selection or wiring instruction.
- P=2.0 kW=2000 W.
- I=P/V=2000/220≈9.1 A.
- This exceeds the stated 5 A rating. The model illustrates overloading; the load should not be operated on that branch.
- Do not bypass or increase protection yourself; real installations require qualified assessment.
Bring it all together
Mix the ideas. Try each question before opening its explanation.
Write it. Explain it.
Use paper for a calculation, a labelled diagram or a short explanation. Then compare the reasoning. Suggested marks help you check how complete your answer is.
Viewed from above, a straight wire carries current downwards through a tabletop. Draw or describe its field direction. State what happens if current reverses and what happens if observation distance doubles while current stays fixed. · 3 marks
Downwards means away from the viewer (a cross). The circular field is clockwise viewed from above. Reversing current makes it anticlockwise. For the long-straight-wire model, doubling distance at fixed current halves field strength.
- 1 mark: clockwise for stated view.
- 1 mark: reversal gives anticlockwise.
- 1 mark: half field at twice distance, with model/constant current.
A solenoid-and-compass paper model is powered by a low-voltage battery. Predict the field direction when battery terminals reverse, and the strength when current increases in the same coil. Explain why a soft-iron core helps a switchable magnet. · 4 marks
Reversing terminals reverses current and therefore the field and solenoid poles. Increasing current in the same geometry strengthens the field. Soft iron magnetises readily and loses most induced magnetism after switch-off, making it suitable for a controllable magnet.
- 1 mark: current reversal.
- 1 mark: field/pole reversal.
- 1 mark: stronger field with current at fixed coil/geometry.
- 1 mark: appropriate soft-iron response.
Distinguish overloading and short circuit. Then explain why a protective earth must not be described as the normal neutral return. · 3 marks
Overloading is excessive total current demand for a circuit’s design. A short circuit is an unintended low-resistance path that can produce excessive current. Protective earth is a safety fault-current path on appropriate equipment; normal operating current returns through neutral in the simple domestic model.
- 1 mark: overload distinction.
- 1 mark: short-circuit distinction.
- 1 mark: earth is protective, not the normal operating return.
A sinusoidal supply has frequency 50 Hz. Find its full period and number of reversals in 1 s. Why can raising transmission voltage reduce resistive line losses for the same power? · 4 marks
T=1/50=0.020 s. There are 2×50=100 reversals in 1 s. For the same transmitted power, P=VI means higher V requires lower I. Line heating loss I²R then falls when line resistance is unchanged.
- 1 mark: 0.020 s full period.
- 1 mark: 100 reversals.
- 1 mark: lower current for given power.
- 1 mark: I²R loss reasoning.
A straight conductor carries current east in an external field directed north. Using east, north and up as perpendicular directions, state the force direction. Then predict the result of reversing only current and of reversing both current and field. · 3 marks
The force is upwards. Reversing current alone makes it downwards. Reversing both restores the upwards direction. This follows the mutually perpendicular assignments in Fleming’s left-hand rule.
- 1 mark: upwards.
- 1 mark: current-only reversal gives downwards.
- 1 mark: both reversed gives upwards with rule.
Classroom extension
Explore the mechanism beyond the year-end revision route.
Compare the contact arrangements and energy directions in a simple DC motor and rotating-coil AC generator. Why is a stationary magnet near a stationary coil insufficient for continuous generation?
The simple motor takes electrical input and uses a split ring to reverse coil current each half-turn. A simple AC generator takes mechanical input and uses two full slip rings to deliver alternating output. A stationary magnet/coil with unchanged flux does not sustain induction.
- Energy directions correct.
- Split ring and slip rings distinguished.
- Change in linked magnetic flux identified.