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

Magnets and Magnetic Field

A magnet attracts iron, nickel, cobalt and steel and, when freely suspended, settles in the north-south direction. Like poles repel and unlike poles attract. Poles of a magnet cannot be separated; breaking a magnet gives two smaller magnets.
The magnetic field is the region around a magnet where its force can be felt. Field lines show its direction: they leave the north pole, enter the south pole outside the magnet and run from south to north inside, forming closed curves. They never cross, they tend to shorten along their length, and they repel each other sideways. Where lines are crowded the field is strong. A region with no lines is a neutral region.

Field of a Current-Carrying Conductor

Oersted showed that a current produces a magnetic field. For a straight wire the field lines are circles around the wire in a plane perpendicular to it. The direction follows the right-hand thumb rule: point the thumb along the current and the curled fingers give the field direction.
Field of a long straight wire: \( B = \dfrac{\mu_0 I}{2\pi r} \) with \( \dfrac{\mu_0}{4\pi} = 10^{-7} \) T m/A. It is proportional to the current and inversely proportional to the distance.
Circular coil of n turns, radius r: field at the centre \( B = \dfrac{n\mu_0 I}{2r} \).
Solenoid (long coil) with n turns per unit length: field well inside is uniform, \( B = \mu_0 nI \), and about half of this at the ends. Its field looks like that of a bar magnet.

Electromagnets and Permanent Magnets

A soft iron core inside a solenoid becomes strongly magnetised while the current flows: this is an electromagnet. It is used in electric bells, cranes, relays and loudspeakers. A steel core, or an alloy such as alnico, stays magnetised and gives a permanent magnet, used in meters, microphones and compasses.

Force on a Current-Carrying Conductor and a Moving Charge

A wire of length l carrying current I in a field B at angle \( \theta \) feels the force \( F = BIl\sin\theta \). It is maximum (BIl) when the wire is perpendicular to the field and zero when parallel. A charge q moving with speed v feels \( F = qvB\sin\theta \), and in a perpendicular field it moves in a circle of radius \( r = \dfrac{mv}{qB} \).
Direction: Fleming's left-hand rule. Stretch the forefinger, middle finger and thumb of the left hand at right angles. If the forefinger points along the field and the middle finger along the current, the thumb shows the force.

Electric Motor

A DC motor turns electrical energy into mechanical energy. A rectangular armature coil is placed between the poles of a strong magnet. The two sides perpendicular to the field feel equal and opposite forces that form a couple and turn the coil. A split-ring commutator with carbon brushes reverses the current in the coil every half turn so that the torque always has the same sense and rotation continues. The speed grows with stronger field, larger current and more turns.

Electromagnetic Induction

A changing magnetic flux through a coil induces an EMF and a current in it. Faraday's laws: an EMF is induced whenever the flux changes, and its size is proportional to the rate of change of flux: \( e = N\dfrac{\Delta\Phi}{\Delta t} \). Flux \( \Phi = BA\cos\theta \) (unit weber, 1 Wb = 1 T m\( ^2 \)).
Lenz's law: the induced current opposes the change that causes it. This agrees with conservation of energy, since mechanical work is converted into electrical energy. Fleming's right-hand rule gives the direction of the induced current in a moving conductor: forefinger along the field, thumb along the motion, middle finger shows the current.
If a magnet is held stationary near a coil, there is no change of flux and no induced current.

AC and DC

AC can be generated cheaply, stepped up and down by transformers and sent over long distances with low loss, and converted to DC by rectifiers. Its drawbacks: it cannot be used for electrolysis or electroplating, and it is more dangerous than DC at the same voltage. In India the AC supply is at 220 V and 50 Hz, so the current reverses 100 times each second.

Domestic Electricity, Fuse and Earthing

Houses receive three wires: live (red), neutral (black) and earth (green). The potential difference between live and neutral is 220 V. Appliances are connected in parallel so that each gets the full voltage and works independently.
A fuse is a thin wire of low melting point alloy (tin-lead) fitted in series with the live wire. When overloading or a short circuit makes the current too large, it melts and cuts off the supply. Short circuiting happens when live and neutral touch directly, so the resistance is almost zero and the current is very large.
Earthing connects the metal body of an appliance through the third pin to the earth wire. If the live wire touches the body, the large current flows safely into the ground through this low-resistance path instead of through the person, and the fuse melts.