Class 10 Science Notes Chapter 12 — Magnetic Effects of Electric Current#
- Hans Christian Oersted (1820) found that a current in a wire deflects a compass needle. This showed that electricity and magnetism are linked.
- A compass needle is a small bar magnet. Its north-seeking end is its north pole.
Magnetic field and field lines#
- Magnetic field — the region around a magnet where its force can be felt. It has both size and direction.
- Field lines show the field. The direction is the way the north pole of a compass would move.
Properties of magnetic field lines:
- Outside a magnet they go from north to south; inside, from south to north. So they are closed curves.
- Where lines are closer together, the field is stronger.
- Two field lines never cross. If they did, a compass at that point would point in two directions at once, which is impossible.
Field due to a current#
| Conductor | Field pattern | How strength changes |
|---|---|---|
| Straight wire | Concentric circles around the wire | Stronger with more current; weaker farther from the wire |
| Circular loop | Circles near the wire, nearly straight lines at the centre | Stronger with more current and with more turns ($n$ turns give $n$ times the field) |
| Solenoid (coil of many turns, shaped like a cylinder) | Like a bar magnet outside; uniform (parallel straight lines) inside | Stronger with more current and more turns |
Right-hand thumb rule: hold the current-carrying straight conductor in your right hand with the thumb pointing in the direction of the current. Your curled fingers show the direction of the magnetic field lines.
- Electromagnet — a soft iron core inside a solenoid. It is a strong magnet only while current flows.
Force on a current-carrying conductor#
- A current-carrying conductor in a magnetic field feels a force (shown by André-Marie Ampère).
- The force is greatest when the current is perpendicular to the field. It reverses if the current or the field is reversed.
Fleming’s left-hand rule: stretch the thumb, forefinger and middle finger of the left hand at right angles to each other.
- Forefinger → direction of the magnetic field
- Middle finger → direction of the current
- Thumb → direction of the force (motion) on the conductor
- Devices that use this force: electric motors, generators, loudspeakers, microphones and measuring instruments.
- MRI (magnetic resonance imaging) uses the very weak magnetic fields made by nerve impulses in the heart and brain to make images of the inside of the body.
Domestic electric circuits#
- Homes get AC power at 220 V and 50 Hz.
| Wire | Insulation colour | Role |
|---|---|---|
| Live (positive) | Red | Brings current in |
| Neutral (negative) | Black | Return path; potential difference with live = 220 V |
| Earth | Green | Safety; connected to a metal plate deep in the earth |
- Earthing: the metal body of appliances (iron, toaster, fridge) is connected to the earth wire. If current leaks to the body, it flows to the earth, so the user does not get a severe shock.
- Two separate circuits: a 15 A circuit for high-power appliances (geysers, air coolers) and a 5 A circuit for bulbs and fans.
- Appliances are connected in parallel, each with its own switch.
- Short-circuit — the live and neutral wires touch directly (damaged insulation or a fault). The resistance becomes very small and the current very large.
- Overloading — too much current, caused by short-circuiting, a sudden rise in supply voltage, or too many appliances on one socket.
- An electric fuse in series protects against both: it melts and breaks the circuit.
Common mistakes#
- Using the right hand for the force on a conductor. Fleming’s rule for force uses the left hand.
- Saying field lines go from south to north outside a magnet. Outside, they go north to south.
- Mixing up the wire colours. Red = live, black = neutral, green = earth.
- Saying the field inside a solenoid is non-uniform. It is uniform.