Lesson 4: Effects of Electric Current

Rule 1: Power and Joule's law — A cell gives energy V·Q to a resistor and this becomes heat. Power P = V I. The heat produced is H = P t = V I t = I² R t; this is Joule's law of heating (जूलचा उष्णता नियम).

• Easy : A toaster of 880 W works on 220 V: I = P ÷ V = 880 ÷ 220 = 4 A, and R = V ÷ I = 220 ÷ 4 = 55 Ω.
• Medium : A current of 2 A flows through 10 Ω for 30 s: H = I² R t = 4 × 10 × 30 = 1200 J.
• Exam-type : If the current is doubled in the same resistor for the same time, H becomes 4 times (because of I²), so 1200 J becomes 4800 J.

In the figure : In the figure one unit of electricity in joules.

Rule 2: Unit of electricity — 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J, and this is 1 unit of electricity on the bill.

• Easy : A 60 W bulb used for 5 hours: energy = 0.06 kW × 5 h = 0.3 kWh.
• Medium : A 2000 W heater for 3 hours uses 2 × 3 = 6 kWh, which is 6 units.
• Exam-type : Bill: three 40 W fans run 10 hours a day for 30 days. Total power 120 W = 0.12 kW; energy = 0.12 × 10 × 30 = 36 units. At ₹7 per unit the bill is 36 × 7 = ₹252.

Spot the difference

Rule 1: Heating appliances — Heaters, cookers and irons use coils of nichrome, which has high resistivity. A bulb has a tungsten filament that reaches about 3400 °C and is kept in vacuum or nitrogen so that it does not burn.

• Easy : A nichrome coil heats up because its resistance is high, so I² R is large.
• Medium : A copper connecting wire has very low resistance, so for the same current it makes very little heat.
• Exam-type : Why is a tungsten filament placed in vacuum? Tungsten has a very high melting point, but at 3400 °C in air it would burn with oxygen.

Rule 2: Home wiring and fuses — Home wiring has live, neutral and earth wires; the potential difference between live and neutral is 220 V. If live and neutral touch, a short circuit (लघुपथन) gives a very large current and can start a fire. A fuse wire melts and breaks the circuit; an MCB switches off automatically. Overloading means drawing too much current.

In the figure : In the figure live, neutral and earth wires with a fuse.

• Easy : A 2000 W geyser on 220 V draws about 9 A, so a 10 A fuse suits it.
• Medium : In a short circuit the resistance is tiny. If R = 0.1 Ω on 220 V, then I = 220 ÷ 0.1 = 2200 A, which is enormous.
• Exam-type : Plugging a heater, an iron and a geyser into one socket draws too much current: that is overloading, and the fuse or MCB should cut the supply.

Spot the difference

Rule 1: Oersted and the thumb rule — Oersted (1820) saw that a current deflects a magnetic needle. A straight current-carrying wire makes circular magnetic field lines; the field weakens with distance and grows with the current. Right-hand thumb rule (Maxwell's cork-screw rule): point the thumb along the current and the curled fingers show the direction of the field.

In the figure : In the figure the right-hand thumb rule.

• Easy : Current upward in a vertical wire: the thumb points up, so the field circles around the wire in the direction of the curled fingers.
• Medium : If the current is reversed, the field direction is reversed; if the current is doubled, the field gets stronger.
• Exam-type : A compass needle placed far from the wire barely moves, which shows that the field weakens with distance.

Rule 2: Loop and solenoid — In a circular loop all field lines at the centre point the same way; n turns give n times the field. A solenoid (परिनालिका) is a long coil; it has a field like a bar magnet, uniform inside, with one end north and the other south. Putting a soft iron core inside makes an electromagnet (विद्युतचुंबक).

In the figure : In the figure a solenoid behaves like a bar magnet.

• Easy : A coil of 10 turns gives 10 times the field of a single loop for the same current.
• Medium : An iron nail with insulated wire wound on it and connected to a cell picks up pins; remove the cell and the pins fall.
• Exam-type : To make a stronger electromagnet: increase the number of turns, increase the current, and use a soft iron core.

Spot the difference

Rule 1: Force on a conductor — A current-carrying conductor in a magnetic field feels a force, greatest when the current is at right angles to the field. Fleming's left-hand rule: first finger = field, middle finger = current, thumb = force.

In the figure : In the figure Fleming's left-hand rule.

• Easy : Hold the thumb, first and middle fingers of the left hand at right angles to each other; they point to force, field and current.
• Medium : If only the current is reversed, the force is reversed; if both field and current are reversed, the force stays in the same direction.
• Exam-type : If the current is parallel to the field, the force is zero; at right angles it is maximum.

Rule 2: The electric motor — An electric motor changes electrical energy into mechanical energy. A rectangular coil between magnet poles, a split ring commutator (दिक्परिवर्तक) and carbon brushes work together: the split ring reverses the current every half turn, so the coil keeps turning in one direction.

In the figure : In the figure a motor changes electrical energy to motion.

• Easy : A table fan, a mixer and a washing machine all contain electric motors.
• Medium : If the split ring were replaced by two full rings, the coil would swing back and forth and stop instead of rotating.
• Exam-type : To make a motor spin faster: use a stronger magnet, more turns in the coil, or more current.

Spot the difference

Rule 1: Faraday's discovery — Faraday (1831): a current is induced in a coil when the number of magnetic lines through it changes. Fleming's right-hand rule: thumb = motion, first finger = field, middle finger = induced current. A galvanometer detects very small currents.

In the figure : In the figure electromagnetic induction.

• Easy : Push a bar magnet into a coil connected to a galvanometer: the needle deflects.
• Medium : Hold the magnet still inside the coil: no deflection, because the lines are not changing. Pull it out: the needle deflects the other way.
• Exam-type : Moving the magnet faster or using more turns gives a larger deflection, meaning a larger induced current.

Rule 2: AC and DC — DC (direct current, दिष्ट विद्युतधारा) flows in one direction only. AC (alternating current, प्रत्यावर्ती विद्युतधारा) changes direction periodically; in India its frequency is 50 Hz. AC is best for long-distance transmission and is used in our homes.

In the figure : In the figure direct and alternating current.

• Easy : A cell or battery gives DC; the wall socket at home gives AC.
• Medium : In 50 Hz AC one cycle takes 1 ÷ 50 = 0.02 s.
• Exam-type : Each cycle reverses direction twice, so 50 Hz AC changes direction 100 times in every second.

Rule 3: The generator — A generator (जनित्र) turns mechanical energy into electrical energy: a coil rotates in a magnetic field. With slip rings it gives AC; with a split ring it gives DC.

In the figure : In the figure a generator makes electricity from motion.

• Easy : In a hydro power station falling water turns the turbine, which turns the generator coil.
• Medium : A motor and a generator are opposite in action: the motor uses electricity to make motion; the generator uses motion to make electricity.
• Exam-type : Why is a bicycle lamp brighter when you pedal faster? The coil cuts the field lines faster, so the induced voltage is higher.

Spot the difference