Effects of Electric CurrentClass 10 Science Part 1 Chapter 4 Notes – Maharashtra Board

Summary: Effects of Electric Current

When you switch on a geyser, a fan or a bulb, the same electric current does very different jobs. In this chapter you find out where the energy comes from. The cell or mains supply pushes charge through a resistor, and the work done on that charge turns into heat, light or motion. The rate of this energy transfer is electrical power, P = VI, measured in watts.

The heating effect follows Joule's law: H = I²Rt. Heaters and irons use high-resistance nichrome coils, while bulbs use tungsten filaments that glow white-hot. Your electricity bill counts energy in 'units', where one unit is one kilowatt hour. Too much current from a short circuit or overloading can start a fire, so fuse wires and MCBs cut the supply in time.

In 1820 Oersted noticed that a compass needle turns near a current-carrying wire. Current creates a magnetic field! Around a straight wire the field lines are circles, found by the right hand thumb rule. A loop strengthens the field at its centre, and a solenoid behaves just like a bar magnet with a uniform field inside.

A current-carrying wire placed in a magnetic field feels a force, whose direction is given by Fleming's left hand rule. This idea runs the electric motor in your mixer and ceiling fan, and also the galvanometer, ammeter and voltmeter. The split ring reverses the current every half turn so the coil keeps spinning in one direction.

Faraday reversed the idea in 1831: a changing magnetic field through a coil produces a current. This is electromagnetic induction, and Fleming's right hand rule gives the direction of the induced current. Generators use it to turn mechanical energy into electricity. Slip rings give alternating current (AC, 50 Hz in India), while a split ring gives direct current (DC).

Key points to remember

• Energy given to a resistor = V × Q; electrical power P = VQ/t = VI. SI unit of power is the watt (1 W = 1 J/s = 1 V × 1 A).
• Heat produced in a resistor: H = VIt = I²Rt = V²t/R. H = I²Rt is Joule's law of heating.
• Ohm's law: V = IR. Combining gives P = I²R = V²/R.
• 1 kWh (one 'unit' of electricity) = 1000 W × 3600 s = 3.6 × 10⁶ J.
• Heating devices use nichrome (high resistivity alloy); bulbs use tungsten, which glows at about 3400 °C without melting.
• Home wiring has live, neutral and earth wires; live and neutral differ by 220 V. Fuse wires and MCBs protect against short circuit and overloading.
• Oersted (1820) discovered that an electric current produces a magnetic field; the unit of magnetic field intensity is named oersted.
• Field lines around a straight conductor are concentric circles; the field is stronger for larger current and weaker farther from the wire.
• Right hand thumb rule: thumb along current, curled fingers show the direction of magnetic field lines.
• A solenoid carrying current acts like a bar magnet; the field inside it is uniform (parallel lines).
• Force on a current-carrying conductor in a magnetic field is maximum when current is perpendicular to the field; direction by Fleming's left hand rule.
• Electric motor converts electrical energy into mechanical energy; the split ring reverses current every half rotation.
• Electromagnetic induction (Faraday, 1831): a current is induced in a coil whenever the magnetic lines of force through it change. Direction by Fleming's right hand rule.
• AC changes direction periodically (50 Hz in India); DC flows in one direction. AC generator uses slip rings; DC generator uses a split ring.

Effects of Electric Current – Notes

1. Energy transfer in an electric circuit and electric power

• In a circuit, the end of a resistor joined to the positive terminal (A) is at a higher potential than the end joined to the negative terminal (B).
• When a charge Q moves from A to B, work V_ABQ is done on it. The cell supplies this energy through the moving charge.
• In a resistor this energy turns into heat and its temperature rises. In a motor, the same energy becomes mechanical energy (motion).
• Electrical power is the rate at which electrical energy is supplied: P = V_ABQ/t = V_ABI, since I = Q/t.
• Unit: 1 volt × 1 ampere = (1 J/1 C) × (1 C/1 s) = 1 J/s = 1 watt (W). Bigger unit: 1 kW = 1000 W.
• Just as mechanical power is work done per second, electrical power is electrical work done per second.

2. Heating effect of electric current and Joule's law

• When current flows through a resistor, heat is produced in it. This is the heating effect of electric current.
• Heat produced in time t: H = P × t = VIt. Using Ohm's law V = IR, H = I²Rt = V²t/R.
• Joule's law of heating: H = I²Rt — heat is proportional to the square of current, to resistance and to time.
• Water heaters, electric cookers, irons, toasters and bulbs use this effect. They use conductors of high resistivity.
• Nichrome (an alloy) is used in heater coils: high resistivity, high melting point and it does not oxidise easily when red-hot.
• Tungsten is used as a coiled-coil filament in bulbs: it heats to nearly 3400 °C and glows. The bulb is filled with nitrogen or kept as vacuum so the filament does not burn away.
• At a fixed voltage, P = V²/R, so a coil with lower resistance (e.g. a shorter length of the same wire) gives more heat.

3. Commercial unit of energy, short circuit, overloading and fuse

• The watt is a small unit, so in practice we use kilowatt (kW). Energy used = power × time.
• 1 kWh = energy used by a 1 kW appliance in 1 hour = 1000 W × 3600 s = 3.6 × 10⁶ J. The electricity bill calls 1 kWh '1 unit'.
• House wiring has three wires: live, neutral (220 V between them) and earth (connected to the ground).
• Short circuit: if insulation wears off and live touches neutral, resistance becomes very low, a very large current flows, heat is produced and nearby wood, cloth or plastic may catch fire.
• Overloading: on hot summer evenings fans, ACs and lights all run together; the transformer supplies too much current and its fuse melts, cutting the supply.
• A fuse wire (low melting point) melts when current exceeds a safe limit, breaking the circuit and preventing accidents.
• MCB (miniature circuit breaker) switches off automatically when current suddenly rises; it can simply be switched on again after the fault is fixed.

4. Magnetic effect of current: straight conductor and right hand thumb rule

• A magnetic needle near a wire deflects when current flows and deflects the opposite way when the current is reversed. So current produces a magnetic field.
• Hans Christian Oersted discovered this in 1820. The unit of magnetic field intensity is named the oersted in his honour.
• Iron filings sprinkled on a cardboard around a vertical current-carrying thick wire arrange themselves in concentric circles centred on the wire.
• The field is stronger near the wire and weaker farther away, so the circles are drawn wider apart as distance increases. More current gives a stronger field.
• Right hand thumb rule: hold the conductor in your right hand with the thumb pointing in the direction of current; the curled fingers show the direction of the magnetic lines of force.
• This rule is also known as Maxwell's corkscrew rule.

5. Magnetic field due to a circular loop and a solenoid

• Bend a straight wire into a loop. Each small part of the loop produces circular field lines around itself.
• Near the centre, the circles become so large that the lines look straight. By the right hand thumb rule, all parts of the loop add their field in the same direction at the centre.
• A coil of n turns produces a field n times that of a single turn.
• A solenoid is an insulated copper wire wound closely in many loops like a spring.
• A current-carrying solenoid behaves like a bar magnet: one end acts as a north pole and the other as a south pole.
• Inside the solenoid the field lines are parallel, meaning the magnetic field is uniform there. Placing a soft iron core inside makes a strong electromagnet, as used in cranes and electric bells.

6. Force on a current-carrying conductor, Fleming's left hand rule and electric motor

• A flexible copper wire hanging between the poles of a horseshoe magnet bends when current flows. Reversing the current, or the magnet, reverses the direction of bending.
• The force is perpendicular to both the current and the magnetic field, and is maximum when the current is perpendicular to the field.
• Fleming's left hand rule: stretch the thumb, index finger and middle finger of the left hand mutually perpendicular. Index finger → magnetic field, middle finger → current, thumb → force (motion) on the conductor.
• An electric motor converts electrical energy into mechanical energy. Used in fans, mixers, washing machines, pumps, refrigerators and computers.
• Construction: rectangular insulated coil ABCD between N and S poles; ends joined to the two halves X and Y of a split ring fixed on the axle; stationary carbon brushes E and F touch the split ring.
• Working: current in AB and CD flows in opposite directions, so one arm is pushed down and the other up — the coil turns (anticlockwise in the textbook set-up).
• After every half rotation, the split ring halves swap brushes, reversing the current in the coil. So the forces keep turning the coil in the same direction continuously.

7. Galvanometer and electromagnetic induction

• A galvanometer detects small currents (around 1 mA). It works like a motor: a coil between magnetic poles rotates in proportion to the current, moving a pointer that can deflect on either side of zero.
• Ammeters and voltmeters work on the same principle.
• Moving a wire across a bar magnet's pole, or moving the magnet near a fixed wire, makes the galvanometer deflect. Current is produced by relative motion — electromagnetic induction, discovered by Michael Faraday in 1831.
• Switching a solenoid's current on or off makes a galvanometer joined to a nearby coil flick to one side and then back to zero; switching the other way makes it flick in the opposite direction.
• Moving the current-carrying solenoid towards or away from the coil also gives a deflection; faster motion gives a larger deflection.
• Faraday's law of induction: whenever the number of magnetic lines of force passing through a coil changes, a current is induced in the coil. This is the induced current.
• Fleming's right hand rule: thumb, index and middle fingers of the right hand mutually perpendicular. Thumb → motion of conductor, index finger → magnetic field, middle finger → induced current. Induced current is maximum when motion is perpendicular to the field.

8. Alternating current (AC) and direct current (DC)

• Direct current (DC) flows in one direction only, as from a cell. It may rise, stay steady or fall, but never reverses.
• Alternating current (AC) changes both size and direction after equal time intervals. It rises to a maximum, falls to zero, rises to a maximum in the reverse direction and returns to zero — one cycle.
• AC varies sinusoidally with time and is shown by the symbol ~.
• In India, AC frequency is 50 Hz: 50 cycles per second, so one cycle takes 1/50 s.
• AC is preferred for long-distance transmission because power loss is minimum. Our household supply is AC.

9. Electric generator (AC and DC)

• An electric generator converts mechanical energy into electrical energy using electromagnetic induction. Turbines in power stations, and the dynamo on a bicycle, rotate coils in a magnetic field.
• Construction (AC): coil ABCD between pole pieces of a magnet; its ends connect to two slip rings R₁ and R₂ fixed on the axle (insulated from it); stationary carbon brushes B₁ and B₂ connect to the external circuit/galvanometer.
• When the axle is rotated, arm AB moves up and CD moves down. By Fleming's right hand rule, current flows A→B→C→D in the coil and from B₂ to B₁ outside.
• After half a rotation, the arms swap positions and the induced current in the coil becomes D→C→B→A. Since each arm stays joined to the same brush, the external current reverses — this is AC.
• Using a coil of many turns instead of a single loop gives a current many times larger.
• DC generator: replace slip rings with a split ring (commutator). The arm moving up is always connected to one brush and the arm moving down to the other, so the external current flows in one direction only.
• Motor vs generator: a motor turns electrical energy into motion (left hand rule); a generator turns motion into electrical energy (right hand rule).

Formulas at a glance

Key terms

Effects of Electric Current – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Effects of Electric Current – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Effects of Electric Current

⏱ About 5 hours over 4 days

1. Day 1 — Power and heating effect
   Derive P = VI, then H = VIt, H = I²Rt and H = V²t/R in your notebook from memory. Learn the watt unit derivation and why nichrome and tungsten are used. Solve 6 numericals on power, current and resistance.
2. Day 2 — kWh, bills and safety
   Convert 1 kWh to joules yourself. Solve 4 bill problems (units and cost). Learn short circuit, overloading, fuse and MCB as 'cause → effect → protection'. Check your home's electricity bill for 'units consumed'.
3. Day 3 — Magnetic effect and motor
   Draw field patterns for a straight wire, a loop and a solenoid. Practise the right hand thumb rule and Fleming's left hand rule with your hand. Draw and label the electric motor; write its working in 6–8 points, stressing the split ring.
4. Day 4 — Induction, AC/DC, generator and self-test
   Write Faraday's law and Fleming's right hand rule. Draw the AC/DC graph and the AC generator; write how a DC generator differs. Make a motor vs generator comparison table. Finish with our 20 flashcards and the 15-question quiz.

✍️ Diagrams to practise

• Electric circuit with cell, resistor, ammeter, voltmeter and plug key
• Magnetic lines of force around a straight current-carrying conductor (concentric circles on cardboard)
• Right hand thumb rule and magnetic field due to a circular loop
• Magnetic field of a current-carrying solenoid
• Fleming's left hand rule and Fleming's right hand rule (hand diagrams)
• Electric motor with coil, magnet poles, split ring and carbon brushes
• AC generator with slip rings and DC generator with split ring
• Graph of AC and DC against time