Magnetic Effects of Electric Current Chapter Notes - Science Class 10

Introduction

• Magnet is any substance that attracts iron or iron-like substances.
• An electric current-carrying wire behaves like a magnet.
• Electromagnets and electric motors involve the magnetic effect of electric current, and electric generators involve the electric effect of moving magnets.
• Compass needle get deflected on passing an electric current through a metallic conductor.

Properties of Magnet

(i) Every magnet has two poles i.e. North and South.
(ii) Like poles repel each other.
(iii) Unlike poles attract each other.
(iv) A freely suspended bar magnet aligns itself in nearly north-south direction, with its north pole towards north direction.

Characteristics of Field Lines

• Field lines arise from North pole and end into South pole of the magnet.
• Field lines are closed curves.
• Field lines are closer in stronger magnetic field.
• Field lines never intersect each other as for two lines to intersect, there must be two north directions at a point, which is not possible.
• Direction of field lines inside a magnet is from South to North.
• The relative strength of magnetic field is shown by degree of closeness of field
• lines.

Magnetic Field of a Bar Magnet

• H. C. Oersted was the first person to state that electric current has magnetic field.
  

Right Hand Thumb Rule

• Imagine you are holding a current carrying straight conductor in your right hand such that the thumb is pointing towards the direction of current.
• Then the fingers wrapped around the conductor give the direction of magnetic field.

Magnetic Field due to Current through a Straight Conductor

• It can be represented by concentric circles at every point on conductor.
• Direction can be given by right hand thumb rule or compass.
• Circles are closer near the conductor.
• Magnetic field ∝ Strength of current.
• Magnetic field ∝ 1/Distance from conductor

Magnetic Field due to Current through a Circular Loop

• It can be represented by concentric circle at every point.
• Circles become larger and larger as we move away.
• Every point on wire carrying current would give rise to magnetic field appearing as straight line at centre of the loop.
• The direction of magnetic field inside the loop is same.

Factors affecting magnetic field of a circular current carrying conductor

• Magnetic field ∝ Current passing through the conductor
• Magnetic ∝ 1/Distance from conductor
• Magnetic field ∝ No. of turns in the coil

• Magnetic field is additive in nature i.e., magnetic field of one loop adds up to magnetic field of another loop. This is because the current in each circular turn has some direction.

Solenoid

• A coil of many circular turns of insulated copper wire wrapped closely in a cylindrical form.
• Magnetic field of a solenoid is similar to that of a bar magnet.
• Magnetic field is uniform inside the solenoid and represented by parallel field lines.

Direction of magnetic field

(i) Outside the solenoid: North to South
(ii) Inside the solenoid: South to North
Solenoid can be used to magnetise a magnetic material like soft iron.

Electromagnet 

• It is a temporary magnet, so, can be easily demagnetised.
• Strength can be varied.
• Polarity can be reversed.
• Generally strong magnet.

Permanent Magnet

• Cannot be easily demagnetised.
• Strength is fixed.
• Polarity cannot be reversed.
• Generally weak magnet.

Force on a Current carrying Conductor in a Magnetic Field

Andre Marie Ampere suggested that the magnet also exerts an equal and opposite force on a current carrying conductor.

• The displacement in the conductor is the maximum when the direction of current is at right angle to the direction of magnetic field.
• Direction of force is reversed on reversing the direction of current.

Fleming’s Left Hand Rule

• Stretch the thumb, fore finger and middle finger of your left hand such that they are mutually perpendicular. 
  • If fore finger points in the direction of magnetic field, middle finger in the direction of current then thumb will point in the direction of motion or force.
  • Heart and brain in the human body have significant magnetic field.
• MRI (Magnetic Resonance Imaging): Image of internal organs of body can be obtained using magnetic field of the organ.
• Galvanometer: Instrument that can detect the presence of current in a circuit. It also detects the direction of current.

Domestic Electric Circuits

• There are three kinds of wires used:
  (i) Live wire (positive) with red insulation cover.
  (ii) Neutral wire (negative) with black insulation cover.
  (iii) Earth wire with green insulation cover.
• The potential difference between live and neutral wire in India is 220 V.
• Pole ⇒ Main supply ⇒ Fuse ⇒ Electricity meter ⇒ Distribution box ⇒ To separate circuits

• Earth Wire: Protects us from electric shock in case of leakage of current especially in metallic body appliances. It provides a low resistance path for current in case of leakage of current.
• Short Circuit: When live wire comes in direct contact with neutral wire accidentally. The resistance of circuit becomes low which can result in overloading.
• Overloading: When current drawn is more than current carrying capacity of a conductor, it results in overloading.

Causes of overloading

• Accidental hike in voltage supply.
• Use of more than one appliance in a single socket.

Safety devices

• Electric fuse
• Earth wire
• MCB (Miniature Circuit Breaker)