Class 10 Physics • Chapter 10 (Deep Detail)
Oersted's Experiment: A current carrying wire produces a magnetic field around it.
Right Hand Thumb Rule: If we hold the current carrying conductor in right hand such that thumb points in direction of current, then the curled fingers indicate direction of magnetic field lines.
A Solenoid behaves like a bar magnet.
CONCEPTUAL If current flows from East to West in a wire, what is direction of magnetic field at a point directly below it?
Ans: Towards North. (Using Right Hand Thumb Rule).
Q: How does the polarity of a solenoid turn depend on current direction?
Ans: Clockwise current $\to$ South Pole. Anti-clockwise current $\to$ North Pole.
Electromagnet: Temporary. Variable strength. Polarity can be reversed. (e.g., Electric Bell, Cranes).
Permanent Magnet: Permanent. Constant strength. Fixed polarity. (e.g., Speakers).
CONCEPTUAL A circular loop carries current in a clockwise direction. What is the polarity of the face of the coil facing you?
Ans: South Pole. (Clock Rule: Clockwise Current $\implies$ South Pole. Field lines enter into the face).
RECALL How can you increase the strength of the magnetic field of a solenoid?
Ans: 1. Increase current ($I$). 2. Increase number of turns ($N$). 3. Insert Soft Iron Core.
A current carrying conductor placed in a magnetic field experiences a force.
Force is maximum when conductor is perpendicular to field ($\theta = 90^\circ$).
Force is zero when conductor is parallel to field ($\theta = 0^\circ$).
Q: When is the force on a current carrying conductor maximum?
Ans: When the conductor is perpendicular ($90^\circ$) to the magnetic field.
Stretch Thumb, Forefinger and Central finger of Left Hand mutually perpendicular.
Application: DC Motor.
BOARD CHECK What is the function of the split ring commutator in a DC motor?
Ans: It reverses the direction of current in the coil after every half rotation to keep the torque unidirectional (continuous rotation).
REASONING An alpha particle (positively charged) enters a magnetic field moving towards East. The field acts acting Vertically Upwards. What is the direction of force?
Ans: Using Fleming's Left Hand Rule: Field (Forefinger) = Up. Current (Center Finger) = East (Motion of +ve charge). Thumb points South. Force acts towards South.
CONCEPTUAL An electron beam enters a magnetic field acting vertically downwards. The electron beam is moving horizontally from South to North. What is the direction of force?
Ans: Current is South (Opposite to electron). Field is Down. Using FLHR, Force is East.
The phenomenon of production of induced EMF (and induced current) in a coil when the magnetic flux linked with it changes.
Lenz's Law: The direction of induced current is such that it opposes the cause which produces it. (Based on Conservation of Energy).
Application: AC Generator.
CONCEPTUAL Which rule is used to determine the direction of induced current?
Ans: Fleming's Right Hand Rule (FRHR). FLHR is for Force (Motor), FRHR is for Induced Current (Generator).
REASONING Why is it difficult to push a magnet into a coil connected to a closed circuit?
Ans: Induced current creates a magnetic field that repels the motion (Lenz's Law). Work must be done against this repulsion, which converts to electrical energy.
A device which changes low voltage AC to high voltage AC and vice-versa. Works on Mutual Induction.
Step-up Transformer: $N_s > N_p$. Increases Voltage, Decreases Current.
Step-down Transformer: $N_s < N_p$. Decreases Voltage, Increases Current.
THINKING Why is electrical energy transmitted at very high voltages over long distances?
Solution: High Voltage $\implies$ Low Current.
Heat Loss ($I^2R$) is minimized.
NUMERICAL A transformer lowers voltage from 220 V to 11 V. Primary turns = 200. Find secondary turns.
Solution: $N_s/N_p = E_s/E_p \implies N_s/200 = 11/220 \implies N_s = 200 \times (1/20) = 10 \text{ turns}$.