CBSE Class 10 · Science · board exam 2027
Chapter 12: Magnetic Effects of Electric Current — notes
The chapter’s 7 parts in the book’s order — the first idea of each and the points Medha Ma’am writes on the board — and the summary at the end. The full lesson and a check question for each part are in the app.
1. Oersted's experiment: a current has a magnetic effect
The figure.
- • Set-up: straight copper wire along north-south, compass needle just below it
- • Circuit: battery + plug key + copper wire, in series
- • No current: the needle points north-south
- • Current north to south: the needle's north pole turns east
- • Current reversed, south to north: the needle's north pole turns west
- • Reverse the current, the deflection reverses
- • Conclusion: a current-carrying conductor produces a magnetic field around it
- • Electricity and magnetism are related (Oersted)
- • A compass needle is a small bar magnet
2. Magnetic field and field lines
A freely suspended bar magnet always comes to rest pointing north-south.
- • A freely suspended magnet rests along north-south
- • North-seeking end = north pole (N); south-seeking end = south pole (S)
- • Like poles repel, unlike poles attract
- • Magnetic field: the region around a magnet in which its force can be detected
- • The lines along which iron filings align = magnetic field lines
- • Direction of the field = the way a compass needle's north pole points
- • Magnetic field: magnitude and direction
- • Property 1: outside from N to S, inside from S to N
- • So field lines are closed curves
- • Property 2: lines closer together ⇒ stronger field (strongest near the poles)
- • Property 3: no two field lines cross
- • Reason: at a crossing, a compass would point two ways, not possible
- • Uniform field: straight, parallel, equally spaced lines, arrows one way
3. Field due to a straight conductor and the right-hand thumb rule
The shape of the field round a straight wire.
- • Activity: straight wire XY through a horizontal cardboard, at right angles to it
- • Iron filings form concentric circles round the wire
- • Field lines of a straight conductor = concentric circles, centred on the wire
- • Current increased ⇒ deflection increases ⇒ stronger field
- • Farther from the wire ⇒ deflection decreases ⇒ weaker field
- • Current reversed ⇒ field reversed
- • Right-hand thumb rule
- • Hold the conductor in the right hand, thumb along the current
- • The curled fingers give the direction of the field lines
- • Current from X (top) to Y (bottom): thumb down
- • Fingers curl clockwise, seen from above
- • Example: horizontal wire, current east to west
- • Seen from the east end: the field goes clockwise
- • Directly below the wire: north to south
- • Directly above the wire: south to north
4. Field due to a current through a circular loop
Bend the wire into a circular loop and pass it through a cardboard at two points, P and Q. See the figure.
- • Circular loop: current out at P (⊙), in at Q (⊗)
- • Concentric circles round P and round Q
- • At the centre: the field lines are nearly straight
- • Every part of the loop gives the field in the same direction at the centre
- • n turns ⇒ field n times that of one turn
- • Field at the centre increases with: more current, more turns, smaller radius
5. The solenoid and the electromagnet
Wind many circular turns of insulated copper wire closely, in the shape of a cylinder.
- • Solenoid: a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder
- • Outside: like a bar magnet, one end N, the other end S
- • Inside: parallel straight lines ⇒ uniform field (same at all points)
- • Electromagnet: soft iron inside a current-carrying solenoid
- • Magnet only while the current flows
- • Current reversed ⇒ N and S exchange places
- • | Current-carrying solenoid | Bar magnet |
- • | Field inside uniform (parallel lines) | Field not uniform |
- • | Strength changes with current, turns | Strength fixed |
- • | Polarity reverses with the current | Polarity fixed |
- • | Temporary magnet | Permanent magnet |
6. Force on a current-carrying conductor in a magnetic field
A current pushes on a magnet.
- • Activity: aluminium rod AB between the poles of a horse-shoe magnet, field upwards
- • Rod in series with a battery, a key and a rheostat
- • Current B to A ⇒ rod displaced to the left
- • Current reversed ⇒ force reversed (rod to the right)
- • Field reversed ⇒ force reversed
- • Direction of force: depends on the current and the field
- • Both reversed ⇒ force unchanged
- • Current ⊥ field ⇒ force maximum; current ∥ field ⇒ force zero
- • Magnitude of the force increases with:
- • current, strength of the field, length of the conductor
- • Fleming's left-hand rule
- • Forefinger = magnetic field
- • Middle finger = current
- • Thumb = force (motion) on the conductor
- • Electron: current is opposite to its motion
- • Electron up, field to the left ⇒ current down
- • Left-hand rule ⇒ force into the paper
- • Positive charge: current along its motion
- • Case I (along the field): force zero, minimum
- • Case III (at right angles): force maximum
- • Case II (at a slant): in between
7. Domestic electric circuits
First, the kind of current.
- • DC (direct current): always in one direction, from a cell or a battery
- • AC (alternating current): reverses direction after equal intervals
- • In India, AC reverses every 1/100 s
- • Time of one cycle T = 2 × 1/100 s = 1/50 s
- • Frequency f = 1/T = 50 Hz
- • Advantage of AC: long-distance transmission without much loss of energy
- • Domestic supply: 220 V AC, 50 Hz
- • Red insulation = live wire (positive)
- • Black insulation = neutral wire (negative)
- • Potential difference between live and neutral = 220 V
- • Green insulation = earth wire, joined to a metal plate deep in the earth
- • Metal-bodied appliances (press, refrigerator) are earthed
- • Leakage current ⇒ low resistance path to the earth ⇒ body at earth's potential
- • So the user does not get a severe shock
- • 15 A circuit: high-power appliances (geyser, cooler)
- • 5 A circuit: bulbs, fans
- • Appliances in parallel ⇒ each gets the same potential difference
- • Electric fuse: in series, in the live wire
- • Unduly high current ⇒ Joule heating ⇒ fuse melts ⇒ circuit breaks
- • Protects the circuit and the appliances
- • Short circuit: live and neutral wires come in direct contact
- • Overloading: too many appliances on one socket, or a sudden rise in supply voltage
- • Both ⇒ the current increases abruptly
- • Example: P = 1 kW, V = 220 V. Rating of the circuit?
- • P = 1 kW = 1000 W
- • I = P/V = 1000/220 ≈ 4.55 A
- • 4.55 A < 5 A ⇒ a 5 A circuit is enough
Summary
Very good! You learnt a lot today, so let us revise it together once. Oersted showed that a current produces a magnetic field. Field lines go from N to S outside and S to N inside, they are closed curves, and they never cross. Round a straight wire they are concentric circles, and the right-hand thumb rule gives their direction. A loop gives a straight field at its centre, and a solenoid behaves like a bar magnet, with a uniform field inside. Then a conductor in a field feels a force, largest at right angles, and Fleming's left-hand rule gives its direction. For an electron, take the current opposite to its motion. Finally, AC and DC, fifty hertz, and the domestic circuit, with the live, neutral and earth wires, the fuse, short circuit and overloading. Draw the solenoid, the straight wire and the loop once in your notebook without looking, with arrows for the current and the field. Today's homework is on your screen. If anything was not clear, press the Ask button and ask me. Done, you worked very well today!
AskMedha · Medha Ma’am — CBSE Class 10 Science, Chapter 12. Written from the textbook, checked twice.
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