CBSE Class 10 Physics Magnetic Effects of Electric Current Notes Set 03

Magnetic Effect of Current

Oersted found in 1820 that a wire carrying a current deflects a compass needle placed near it, so a current produces a magnetic field. The lines of force are circles centred on the wire. Their direction is given by Ampere's swimming rule, Maxwell's corkscrew rule (the screw turns in the sense of the lines when it advances along the current) and the right-hand grip rule (thumb along the current, fingers curl along the lines).

A circular loop sends lines of force in through one face and out of the other face. A coil of many turns, a solenoid, behaves like a bar magnet. Looking at one end, the end is a north pole if the current there is anticlockwise and a south pole if it is clockwise.

If a rod is placed inside a solenoid, a steel rod stays magnetised (permanent magnet) but a soft iron rod is magnetised only while the current flows. That is an electromagnet, used in cranes for lifting scrap iron.

Electric Motor

A current-carrying conductor in a magnetic field experiences a force. The two sides of a rectangular coil carry opposite currents and are pushed in opposite directions, so the coil rotates. The direction of the force is given by Fleming's left-hand rule: first finger along the field, middle finger along the current, thumb gives the motion. The motor converts electrical energy into mechanical energy; its power increases with a stronger field, more turns, a larger current and a soft iron armature.

Electromagnetic Induction and the Dynamo

Faraday showed that when a conductor cuts magnetic lines of force an emf is induced. The emf is larger with more turns, a stronger field and faster cutting. A dynamo rotates a coil of many turns in a magnetic field, and the emf is led out through brushes. It converts mechanical energy into electrical energy, the reverse of a motor. The direction of the induced emf is given by Fleming's right-hand rule.

The Transformer

A transformer has a laminated soft iron core carrying two insulated coils, the primary connected to the a.c. supply and the secondary giving the output. The changing flux set up by the primary induces an emf in the secondary: Vs / Vp = Ns / Np. A step-up transformer has Ns > Np, so the output voltage is larger and the output current smaller, because power in = power out for an ideal transformer. Example: secondary 500 V with 2000 turns and primary of 40 turns gives Vp = 500 × 40 / 2000 = 10 V.

Power Transmission

Electrical power is generated at a convenient a.c. voltage, stepped up for transmission through cables to reduce the power loss, and stepped down at substations to 220 to 230 V for homes. Domestic supply uses the live wire, the neutral wire (earthed at the substation) and an earth wire connected to appliance bodies as a safeguard against shock. The meter records kilowatt hours, and the supply passes through the main fuse and switch to the distribution board with a fuse on each line.