Effects of Electric Current Class 10 Notes, Formulas | SSC

This chapter studies what an electric current does as it flows through a circuit. It begins with the transfer of energy from a cell to a resistor, Joule's law of heating, electric power and the commercial unit kWh, along with short circuit, fuse and overloading. It then explores the magnetic effect of current: fields around a straight wire, a loop and a solenoid, the force on a current carrying conductor and Fleming's left hand rule, leading to the electric motor. Finally it covers Faraday's electromagnetic induction, Fleming's right hand rule, AC and DC, and the AC and DC generator.

Summary

When a charge Q flows from point A to point B across a resistor with potential difference V, work VQ is done on it, and the energy comes from the cell. Electrical power is energy per unit time, P = VQ/t = VI. The SI unit of power is the watt: 1 W = 1 V x 1 A = 1 J/s. In a resistor this energy is converted into heat, so the heat produced in time t is H = VIt. Using Ohm's law V = IR, this becomes H = I squared R t, which is Joule's law of heating, and also H = V squared t/R.

Heating effect of current is used in water boilers, cookers, irons, heaters and bulbs. Heating elements are made of alloys such as nichrome, which have high resistivity, while bulb filaments are made of tungsten, which glows at about 3400 degrees C. As the watt is small, power is measured in kW, and electrical energy in kilowatt hour: 1 kWh = 1000 W x 3600 s = 3.6 x 10^6 J, which is the 1 unit shown on electricity bills.

Household wiring has live, neutral and earth wires; the live and neutral differ by 220 V. If their insulation fails and they touch, a very large current flows (short circuit), producing heat that can start a fire. A fuse wire melts and breaks the circuit when excessive current flows; miniature circuit breakers (MCB) are switches that open when current rises suddenly. Overloading happens when too many appliances draw more current than a transformer can supply, and its fuse melts.

In 1820 Hans Christian Oersted found that a magnetic needle near a current carrying wire is deflected, showing that electricity and magnetism are related; the unit of magnetic field intensity is named oersted after him. Around a straight current carrying wire the magnetic lines of force are concentric circles in a plane perpendicular to the wire; the field is stronger for larger current and weaker farther from the wire. Its direction is given by the right hand thumb rule (Maxwell's cork-screw rule): hold the wire in the right hand with the thumb along the current, and the curled fingers give the direction of the field.

For a circular loop, every part of the loop produces field lines at the centre in the same direction, so near the centre the lines are nearly straight; with n turns the field is n times that of one turn. A solenoid is an insulated copper wire wound as a chain of loops like a spring. Its field resembles that of a bar magnet: one end acts as a north pole and the other as a south pole, and inside the solenoid the lines are parallel, meaning the field is uniform.

A current carrying conductor placed in a magnetic field experiences a force perpendicular to both the current and the field; reversing either the current or the field reverses the force, and the force 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; if the index finger points along the field and the middle finger along the current, the thumb gives the force. The electric motor converts electrical energy into mechanical energy using this force. A rectangular coil ABCD lies between magnetic poles; its ends connect to split ring halves X and Y which touch carbon brushes E and F. Forces on AB and CD act in opposite directions and rotate the coil; after every half rotation the split ring reverses the current in the coil so that it keeps rotating in the same direction.

In 1831 Michael Faraday showed that a current can be produced with the help of a moving magnet. A current is induced when a wire moves in a magnetic field, when a magnet moves near a coil, when current in a nearby solenoid is switched on or off or changed, or when the solenoid is moved relative to the coil; faster motion gives larger deflection of the galvanometer. Faraday's law of induction: whenever the number of magnetic lines of force passing through a coil changes, a current is induced in it. Fleming's right hand rule: with the right hand thumb, index and middle fingers perpendicular, the thumb shows the motion of the conductor, the index finger the field and the middle finger the induced current. The induced current is maximum when the motion is perpendicular to the field. A galvanometer, which works on the motor principle, detects small currents on both sides of zero.

Direct current (DC) flows in one direction and is not oscillatory, though its magnitude can vary. Alternating current (AC) changes its magnitude and direction after equal intervals of time in a sinusoidal way. In India AC has a frequency of 50 Hz, and AC is used for long distance transmission because power loss is minimum. An electric generator converts mechanical energy into electrical energy by rotating a coil in a magnetic field. In an AC generator the coil ends connect to two separate conducting rings (slip rings) R1 and R2 with brushes B1 and B2, so the current in the external circuit reverses every half rotation. In a DC generator a split ring replaces the slip rings, so the current in the external circuit always flows in one direction.

Mind map

Working of an electric motor

Important definitions

Formulas

Key points to remember

• Heat in a resistor comes from energy supplied by the cell through the moving charge.
• Nichrome (high resistivity alloy) is used in heating coils; tungsten in bulb filaments (about 3400 degrees C).
• Live and neutral wires have a potential difference of 220 V; the earth wire is connected to the ground.
• Field lines around a straight wire are concentric circles; field weakens with distance and strengthens with current.
• Field inside a solenoid is uniform; a solenoid behaves like a bar magnet.
• Force on a current carrying conductor is maximum when current is perpendicular to the field.
• Left hand rule is for motors (force); right hand rule is for generators (induced current).
• A motor uses a split ring; an AC generator uses two slip rings; a DC generator uses a split ring.
• Induced current is larger when the relative motion is faster.
• Frequency of AC in India is 50 Hz, so one cycle takes 1/50 s.
• Galvanometer, ammeter and voltmeter work on the same principle as the motor.

Solved examples

Example 1. A 6 m nichrome coil has resistance 22 ohm and is connected to 220 V. Will we get more heat if the wire is cut to half its length and made into a coil?

Whole wire: \( P = \dfrac{V^2}{R} = \dfrac{220^2}{22} = 2200 \) W. Half wire has half the resistance, 11 ohm: \( P = \dfrac{220^2}{11} = 4400 \) W. So the half length coil gives more heat (double).

Example 2. A resistance of 9 ohm produces 400 J of heat per second. Find the potential difference across it.

\( P = 400 \) W. \( P = \dfrac{V^2}{R} \Rightarrow V^2 = 400\times9 = 3600 \Rightarrow V = 60 \) V.

Example 3. An electric iron uses 1100 W at the high setting and 330 W at the low setting on 220 V. Find the current and resistance in each case.

High: \( I_1 = \dfrac{P}{V} = \dfrac{1100}{220} = 5 \) A, \( R_1 = \dfrac{V}{I_1} = \dfrac{220}{5} = 44\ \Omega \). Low: \( I_2 = \dfrac{330}{220} = 1.5 \) A, \( R_2 = \dfrac{220}{1.5} \approx 146.7\ \Omega \).

Example 4. A bulb on a 220 V supply draws 0.45 A. Find its power and the units used in 10 hours.

\( P = VI = 220\times0.45 = 99 \) W. Energy \( = 99\times10 = 990 \) Wh \( = 0.99 \) kWh, that is 0.99 unit.

Example 5. Heat is produced in a resistance at 100 W when 3 A flows. Find the resistance.

\( P = I^2R \Rightarrow R = \dfrac{P}{I^2} = \dfrac{100}{9} \approx 11\ \Omega \).

Example 6. Bulbs of 100 W and 60 W work on 220 V and are connected in parallel. Find the current in the main conductor.

\( I_1 = \dfrac{100}{220} \approx 0.4545 \) A, \( I_2 = \dfrac{60}{220} \approx 0.2727 \) A. Total \( I = I_1 + I_2 = \dfrac{160}{220} \approx 0.72 \) A.

Example 7. Which uses more energy: a 500 W TV for 30 minutes or a 600 W heater for 20 minutes?

TV: \( 500\times0.5 = 250 \) Wh. Heater: \( 600\times\dfrac{20}{60} = 200 \) Wh. The TV uses more energy.

Example 8. A 1100 W iron is used 2 hours daily. Find the cost for April at Rs 5 per unit.

Daily energy \( = 1.1\ \text{kW}\times2\ \text{h} = 2.2 \) kWh. April has 30 days: \( 2.2\times30 = 66 \) units. Cost \( = 66\times5 = \) Rs 330.

Important questions and answers

Q1. Give reason: Tungsten is used for the coil in an electric bulb.

The filament must get extremely hot (about 3400 degrees C) to emit light without melting. Tungsten has a very high melting point and high resistivity, so it can glow at such temperatures; it is coiled as a solenoid type coil.

Q2. Give reason: Nichrome, not a pure metal, is used in heating appliances like irons and heaters.

Heating devices need conductors of high resistivity so that more heat is produced by the current. Alloys such as nichrome have higher resistivity than pure metals, so more heat is produced and they are preferred.

Q3. Give reason: Copper or aluminium wires are used for electric power transmission.

Copper and aluminium have very low resistivity, so they are good conductors and very little energy is lost as heat during transmission.

Q4. Give reason: kWh is used to measure electrical energy in practice rather than joule.

The joule is a very small unit; household consumption would be huge numbers in joule. 1 kWh = 3.6 x 10^6 J, so kWh is a convenient practical unit.

Q5. How does a short circuit occur? What is its effect?

When the insulation on live and neutral wires is damaged, or due to a fault in an appliance, the two wires touch directly. The resistance becomes very small and a very large current flows, producing a lot of heat. This can set nearby wood, cloth or plastic on fire. A fuse or MCB is used to cut off the supply.

Q6. What is a solenoid? Compare its field with that of a bar magnet.

A solenoid is an insulated copper wire wound in a chain of loops like a spring. When current passes, its field is like that of a bar magnet: one end acts as a north pole, the other as a south pole, and the lines of force outside go from N to S. Inside, the lines are parallel, so the field is uniform.

Q7. Describe the magnetic field near a long straight current carrying conductor.

The magnetic lines of force are concentric circles with the wire at the centre, in a plane perpendicular to the conductor. The field is stronger near the wire and for larger current; its direction is given by the right hand thumb rule.

Q8. State Faraday's law of electromagnetic induction.

Whenever the number of magnetic lines of force passing through a coil changes, a current is induced in the coil. This current is called the induced current.

Q9. Distinguish between AC generator and DC generator.

AC generator: coil ends are connected to two separate slip rings, so after every half rotation the current in the external circuit reverses; it gives alternating current. DC generator: coil ends are connected to a split ring, so the arm going up is always connected to one brush and the arm going down to the other; current in the external circuit is in one direction only.

Q10. Distinguish between Fleming's left hand rule and right hand rule.

The left hand rule gives the direction of the force on a current carrying conductor in a magnetic field (thumb = force) and applies to the electric motor. The right hand rule gives the direction of the induced current in a conductor moving in a magnetic field (middle finger = induced current) and applies to the generator. In both, the index finger shows the magnetic field.

Q11. Explain the working of an AC generator.

A coil ABCD is rotated between magnetic poles by external mechanical means. Its ends are joined to slip rings R1 and R2 touching brushes B1 and B2. As AB moves up and CD down, by Fleming's right hand rule current is induced along ABCD and flows from B2 to B1 in the external circuit. After half a rotation the arms exchange positions, so current flows DCBA and from B1 to B2 outside. This reversal every half rotation gives alternating current.

Q12. Odd one out: voltmeter, ammeter, galvanometer, thermometer.

Thermometer. Voltmeter, ammeter and galvanometer are electrical measuring instruments working on the principle of the electric motor; a thermometer measures temperature.

Exam tips

• For diagrams of motor and generators label magnet poles, coil ABCD, axle, split ring or slip rings, brushes and direction of rotation.
• Choose the power formula by what is given: VI with current, V squared/R with voltage, I squared R with current and resistance.
• Convert minutes to hours before calculating kWh.
• Remember: Left hand for motor (force), Right hand for generator (induced current).
• In 'give reason' answers on heating appliances mention resistivity and melting point.

FAQs

What is 1 unit of electricity?

1 unit is 1 kWh, the energy used when a 1 kW appliance runs for 1 hour, equal to 3.6 x 10^6 J.

Why is AC used for long distance transmission?

Transmitting electric power in AC form results in minimum power loss over long distances.

Why is a split ring used in a motor?

It reverses the direction of current in the coil after every half rotation, so the forces keep turning the coil in the same direction.

Who discovered electromagnetic induction?

Michael Faraday, in 1831, showed that a moving magnet can produce a current in a conductor.

These notes were prepared with AI help from the Balbharati Science and Technology Part 1 textbook (2026-27) and checked against it.