Categories

# Magnetism : Magnetic field and Force

CONTENT

• Patterns of Magnetic Field around Current Carrying Conductors
• Magnetic Field around a Straight Conductor Carrying Current
• Magnetic Field around a Circular Conductor Carrying Current
• Magnetic Field around a Solenoid.
• Force between two Parallel Current Carrying Conductors.

## PATTERNS OF MAGNETIC FIELD AROUND CURRENT CARRYING CONDUCTORS

A straight conductor carrying current can be shown that it has magnet filed around it.  Allow a thick isolated copper wire to pass vertically through a hole in a card board sheet.  As shown below, sprinkle some iron fillings uniformly on the cardboard around the vertical wire connect the ends of the wire to a battery, switch on the current and place some compass needles around the wire.  Note the direction to which the compass needle point.  Switch on the current and note the swing of the needles and how they point. It will be observed that when current is switched on and the card board is gently tapped, the fillings arrange themselves in a series of concentric circles about the wire as centre.  Also as soon as the current is switch on, the needles will swing around and form a circle with the wire as centre.  The direction of the iron fillings depends on the direction of flow of the current.

The direction and pattern of the magnetic field around a straight conductor carrying current can be determined using the the following rules.

• Maxwell’s Cork Screw Rule:which states that when a right handed cork screw is turned, the direction of motion of the cork represents the direction of current while the direction of rotation of the cork represents the direction of the magnetic field.
• Right Hand Grip Rule (or Clenched Fist Rule):Grip the wire with your right hand so that your thumb points in the direction of flow of the current, your fingers will then point in the direction of the magnetic field around the wire.

## FORCE BETWEEN TWO PARALLEL CURRENT CARRYING CONDUCTORS.

When two current carrying wires are placed parallel to each other, their magnetic fields interact.

When current flows in the same direction, the conductors will repel each other, and when current flows in the same direction in the conductors, they attract.

New School Physics for Senior Secondary Schools (M.WAnyakohaPages 425 -428)

GENERAL EVALUATION

1. A uniform meter rule  AB is balanced on a knife edge which is 55cm from B. If a mass of 10g is hung at P, which is 10cm from A. Calculate the mass of the meter rule
2. Masses m1 and m2are hung at the 20cm and 60cm mark respectively of a uniform meter rule freely suspended at its centre of gravity. If the meter rule balances horizontally, determine the ratio of m1 to m2.

WEEKEND ASSIGNMENT

1.         Alloys for making powerful permanent magnets are these except — (a) alcomax   (b) alnico (c) mumetal(d) ticonal

2.         A magnetic substance can be demagnetized by (a) dropping on the floor (b) hammering while hot (c) divided touch (d) single touch

3.         Which of the following pairs of metals will be picked up by an electromagnet? (a)Aluminium and Copper            (b) Brass and Copper   (c) Iron and Steel         (d)Aluminium and Brass

4.         Which question can be asked to determine if a material is magnetic or non-magnetic? (a) Is it a metal or a non-metal? (b) Is it a conductor or an insulator?         (c) Is it a solid or a gas (d)Does it affect the direction of a compass needle?

5.         One of the following rules can be used to determine the direction of the magnetic field around a straight current carrying conductor. (a) Fundamental law of magnetism(b) Clenched fist rule  (c) Fleming’s right hand rule          (d) Fleming’s left hand rule

THEORY

1. State Maxwell’s cork screw rule.
2. Draw the diagram showing the magnetic field around a solenoid.