When electric current flows through a conductor, it produces several effects. The two most important are the magnetic effect and the heating effect of electric current.
Electric Current and Circuits
Electric current (I): The flow of electric charges (electrons) through a conductor. Measured in amperes (A).
Voltage (V): The force that drives current through a circuit. Measured in volts (V).
Resistance (R): Opposition to the flow of current. Measured in ohms (Ω).
Ohm's Law: V = I × R (Voltage = Current × Resistance)
In a circuit, current flows from the positive terminal to the negative terminal of a battery (conventional current direction).
Magnetic Effect of Electric Current
When current flows through a wire, it creates a magnetic field around the wire. This was discovered by Hans Christian Oersted in 1820, who noticed a compass needle deflecting near a current-carrying wire.
- Electromagnet: A coil of wire (solenoid) wound around an iron core. When current passes through the coil, it acts as a magnet. When the current stops, the magnetism disappears. Properties:
- Strength increases with more turns of wire.
- Strength increases with greater current.
- An iron core strengthens the magnet significantly.
Applications of electromagnets: Electric bells, cranes in scrap yards, MRI machines, loudspeakers, electric motors, relays.
Electric bell mechanism: Current flows through an electromagnet, which attracts a metallic strip (armature). As the strip moves, it hits the bell (gong) and simultaneously breaks the circuit. The electromagnet loses magnetism, the spring pulls the armature back, the circuit closes again, and the process repeats rapidly, producing a ringing sound.
Heating Effect of Electric Current
When current flows through a conductor with resistance, electrical energy is converted to heat energy. This is called the Joule heating effect.
Joule's Law of Heating:
Heat produced (H) = I2 × R × t
Where I = current (A), R = resistance (Ω), t = time (s), H = heat in joules (J).
Important note: Higher resistance → more heat produced (for the same current and time).
- Applications of heating effect:
- Electric heater / room heater: High-resistance nichrome wire heats up.
- Electric iron: Nichrome coil heats the soleplate.
- Electric bulb (incandescent): Tungsten filament (very high melting point: 3422 °C) heats to ~2500 °C and glows.
- Electric fuse: A thin wire with low melting point. Melts and breaks the circuit when excess current flows, protecting appliances.
- MCB (Miniature Circuit Breaker): Modern safety device that trips (switches off) automatically under overload, replacing traditional fuses.
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Calculating resistance using Ohm's Law — A bulb has voltage 12 V across it and a current of 2 A flowing through it. Find resistance. R = V / I = 12 / 2 = 6 Ω.
Heat produced — A resistor of 10 Ω carries a current of 3 A for 5 seconds. H = I2 × R × t = 32 × 10 × 5 = 9 × 10 × 5 = 450 J.
Why does a bulb filament glow? Tungsten has very high resistance and a very high melting point. When current flows, enormous heat is produced (H = I2 Rt), raising temperature to ~2500 °C at which tungsten glows white-hot, emitting visible light.
Fuse rating — A household circuit is rated for 5 A. A fuse wire rated at 5 A is used. If a fault causes 8 A to flow, the fuse wire heats up rapidly (H = I2 Rt, with higher I), melts, and breaks the circuit, preventing fire or appliance damage.
Electromagnet strength — A student winds 20 turns of insulated wire around an iron nail and connects it to a battery. It picks up 10 paper clips. She increases to 40 turns and the same battery. It picks up 18 clips. More turns = stronger electromagnet.
Electric bell — In a doorbell circuit, pressing the switch completes the circuit. The electromagnet pulls the striker, which hits the bell and simultaneously breaks the contact, de-energising the magnet. The spring restores contact, and the cycle repeats, producing continuous ringing.
Nichrome vs copper — Nichrome has much higher resistance than copper. A nichrome heater wire of 5 Ω carrying 4 A for 60 s produces H = 42 × 5 × 60 = 4800 J of heat. A copper wire of 0.01 Ω with the same current produces only 42 × 0.01 × 60 = 9.6 J. This is why heating elements use nichrome, not copper.
Key Formulas
Key formulas
Common mistakes
Students often apply Joule's law incorrectly by forgetting to square the current (I2). Also, increasing resistance increases heat for the same current, which seems counterintuitive but follows from H = I2 Rt.
Summary
Electric current produces a magnetic effect (used in electromagnets, electric bells, motors) and a heating effect (used in heaters, bulbs, fuses). Ohm's Law relates voltage, current, and resistance. Joule's Law quantifies the heat produced by current flow.