Magnetic Effects of Electric Current — Important Questions

Answer: (a) Concentric circles centred on the wire - lying in a plane perpendicular to the conductor; their direction is given by the right-hand thumb rule.

Answer: (a) Fleming's left-hand rule - forefinger points along the field, middle finger along the current and the thumb gives the direction of the force. (Fleming's right-hand rule is used for induced current.)

Answer: (a) Uniform and directed along the axis - the field lines inside a solenoid are parallel straight lines, showing that the field is the same at all points inside, like that of a bar magnet.

Answer: (a) Electromagnetic induction - a changing magnetic field linked with a coil induces an EMF (and current) in it, converting mechanical energy into electrical energy.

Answer: (a) Both A and R are true and R is the correct explanation of A - since a current-carrying solenoid acts like a bar magnet with a north and a south pole, when suspended freely it aligns itself along the Earth's magnetic north-south direction.

Right-hand thumb rule: if a straight current-carrying conductor is held in the right hand such that the thumb points in the direction of the current, then the direction in which the fingers curl round the conductor gives the direction of the magnetic field lines. It helps us determine the direction of the magnetic field produced around a current-carrying conductor.

(a) The force is maximum when the conductor is placed at right angles (perpendicular) to the magnetic field, and it is zero when the conductor is placed parallel to the field.
(b) The magnitude of the force depends on (i) the magnitude of the current, (ii) the strength of the magnetic field, and (iii) the length of the conductor lying in the field.

(a) See the diagram: outside the magnet the field lines emerge from the north pole and enter the south pole, forming closed loops (inside the magnet they run from south to north).
(b) Properties: (i) Two magnetic field lines never intersect each other - if they did, there would be two directions of the field at that point, which is impossible. (ii) The field lines are closer together where the field is stronger (near the poles) and farther apart where it is weaker; they form continuous closed loops.

(a) An electromagnet is a temporary magnet made by winding an insulated coil around a soft-iron core; it behaves as a magnet only as long as current flows through the coil.
(b) An electromagnet is a temporary magnet whose strength can be varied (by changing the current or number of turns) and whose polarity can be reversed; a permanent magnet (made of steel) retains its magnetism and its strength/polarity cannot be easily changed.
(c) Uses: (i) in electric bells, cranes for lifting heavy iron loads, and loudspeakers; (ii) in electric motors, generators, relays and telephones. (any two)

Applying Fleming's left-hand rule (forefinger = field, pointing downwards; middle finger = current, pointing north), the thumb - and hence the force - points horizontally towards the west.
The force can be increased by: (i) increasing the current in the conductor, (ii) using a stronger magnetic field, or (iii) increasing the length of the conductor lying in the field. (any two)

(a) An electric motor has a rectangular coil ABCD of insulated wire placed between the poles of a magnet. The ends of the coil are connected to the two halves of a split ring (commutator), which touch two conducting brushes connected to a battery. When current flows, the two sides of the coil carry current in opposite directions and, by Fleming's left-hand rule, experience forces in opposite directions. This produces a torque that rotates the coil.
(b) (i) The split-ring commutator reverses the direction of current in the coil after every half rotation, so the coil keeps rotating in the same direction. (ii) The brushes conduct current from the external battery to the split rings (coil).
(c) It is used in electric fans, washing machines, mixers/grinders, water pumps, etc. (any one)

(a) Electromagnetic induction is the production of an induced EMF (and current) in a coil due to a change in the magnetic field linked with it.
(b) Connect a coil to a galvanometer. When a bar magnet is moved towards the coil, the galvanometer shows a deflection (induced current); when it is moved away, the deflection reverses; when the magnet is held stationary, there is no deflection. This shows that a changing magnetic field induces a current.
(c) Fleming's right-hand rule: stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular; the forefinger points along the field, the thumb along the motion of the conductor, and the middle finger gives the direction of the induced current.
(d) An AC generator uses two slip rings (each end of the coil connected to its own ring), so the output current reverses direction every half rotation (alternating current). A DC generator uses a single split-ring commutator, which reverses the connections every half rotation so that the output current always flows in one direction (direct current).

(i) The potential difference between the live and neutral wires is about 220 V.
(ii) The earth wire provides a low-resistance path to the ground; if the live wire accidentally touches the metal body of an appliance, the current flows safely to the earth instead of through the user, preventing a fatal electric shock.
(iii) Short-circuiting occurs when the live and neutral wires come into direct contact (very low resistance), causing a sudden, very large current to flow.
(iv) Overloading occurs when too many appliances are connected to a single socket/circuit (or the supply voltage rises), so the current exceeds the safe value and the wires overheat.