What is magnetic force on a current carrying conductor?

What is magnetic force on a current carrying conductor?

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What is magnetic force on a current carrying conductor?

The Force on a Current Carrying Conductor in a Magnetic Field

  1. Figure shows the power window of a car. The glass sheet can slide up or down when the switch is pulled.
    magnetic force on a current carrying conductor 1
  2. The movement of the glass sheet is produced by an electric motor in the door of the car.
  3. In an electric motor, forces are exerted on coils that carry current in a magnetic field.
  4. Electrical meters such the ammeter and voltmeter in the laboratory also make use of the force on a current-carrying conductor.
  5. The force on a current-carrying conductor in a magnetic field is due to the resultant magnetic field produced by the combination of
    (a) the magnetic field due to the current in the conductor
    (b) and an external magnetic field due to the permanent magnets.
  6. The resultant magnetic field has magnetic field lines that are stretched round the conductor. It is known as a catapult field.
  7. The magnetic field is stronger on one side of the conductor than on the other side. This causes a resultant force to act on the conductor, as shown in Figure.
    magnetic force on a current carrying conductor 2

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The Direction of the Magnetic Force

  1. The direction of the force can be determined using Fleming’s left-hand rule.
    magnetic force on a current carrying conductor 3
  2. The following steps are used, as shown in Figure (b):
    (a) Point the first finger of the left hand in the direction of the magnetic field from the North pole towards the South pole.
    (b) The left hand is then rotated so that the second finger points in the direction of the current.
    (c) The thumb indicates the direction of the magnetic force. This is also the direction of motion if the conductor is free to move.
  3. Another rule that can be used is the right-hand slap rule, as shown in Figure (c):
    (a) Point the four fingers of the right hand in the direction of the field.
    (b) Rotate your hand until the thumb points in the direction of the current.
    (c) Do a slapping action. The direction of the slap is the direction of the magnetic force.

Magnetic Force on a Current Carrying Conductor Experiment

Aim: To study the magnetic force acting on a current-carrying conductor.
Materials: Bare copper wires (s.w.g. 20 or thicker) two thumbtacks, a pair of magnadur magnets, U-shaped steel yoke, wooden block, connecting wires
Apparatus: Low voltage d.c. power supply
Method:magnetic force on a current carrying conductor 4

  1. The apparatus is set up as shown in Figure.
  2. The d.c. power supply is switched on and the motion of the short copper wire is observed.
  3. The power supply is switched off and the connections to the power supply are interchanged.
  4. The power supply is switched on again and the motion of the short copper wire is observed.
  5. The power supply is switched off and the U-shaped steel yoke with the magnadur magnets is inverted.
  6. The power supply is switched on and the motion of the short copper wire is observed.

Observations:

  1. The observation in Step 2 and Step 4 are as shown in Figure.
    magnetic force on a current carrying conductor 5magnetic force on a current carrying conductor 6
  2. The observation in step 4 and step 6 are as shown in Figure.
    magnetic force on a current carrying conductor 7magnetic force on a current carrying conductor 8

Discussion:

  1. Before the power supply is switched on, there is a magnetic field between the magnadur magnets.
  2. The current in the short copper wire produces a magnetic field around the wire.
  3. The motion of the short copper wire shows that a force is exerted on the wire.
  4. The force on the wire is due to the interaction between the magnetic field due to the magnadur magnets and the magnetic field due to the current in the wire.
  5. When the connections to the power supply were interchanged, the current in the short copper wire changes direction. This causes the direction of the force on the wire to change.
  6. The direction of the magnetic field is from the North pole to the South pole. A change in the direction of the magnetic field causes a change in the direction of the force.

Magnetic Force on a Current Carrying Conductor Examples

  1. Figure shows the set-up of apparatus to study a current-carrying conductor in a magnetic field.
    magnetic force on a current carrying conductor example
    (a) State the direction of the magnetic field.
    (b) When the switch is closed, state the direction of
    (i) the current,
    (ii) the motion of the copper wire.
    Solution:
    (a) A
    (b) (i) C  (ii) E
  2. Figure shows a current-carrying wire between the poles of a pair of permanent magnets.
    magnetic force on a current carrying conductor example 1
    (a) State the direction of the magnetic field due to the permanent magnets.
    (b) Describe the magnetic field pattern produced by the current.
    (c) Compare the strength of the resultant magnetic field at point X and point Y.
    (d) State the direction of the magnetic force on the wire.
    Solution:
    (a) From North to South
    (b) Concentric circles in a clockwise direction
    (c) Magnetic field at X is stronger
    (d) Downwards

How does a current carrying conductor produces a magnetic field?

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Magnetic Field Due to a Current Carrying Straight Conductor

The Magnetic Field Due to a Current in a Straight Wire:

  1. The magnetic field lines are concentric circles as shown in Figure.
    current carrying conductor produces a magnetic field
  2. The spacing between the circles increases as you move away from the wire. This shows that the strength of the magnetic field decreases as the distance from the wire increases.
  3. At a certain point in the magnetic field, the strength of the field will increase if the current is increased.
  4. The current can be increased by:
    (a) Adding more cells in series at the power supply
    (b) Reducing the resistance of the rheostat
    (c) Using a shorter wire to reduce the resistance
    (d) Using a thicker wire to reduce the resistance
  5. The direction of the magnetic field can be determined using Ampere’s right-hand grip rule. Imagine the wire is gripped using your right hand with your thumb pointing in the direction of the current. The other four fingers show the direction of the magnetic field round the wire, as shown in Figure (a).
    current carrying conductor produces a magnetic field 1
  6. The direction of the magnetic field can also be determined by Maxwell’s Corkscrew Rule- Imagine a screw being turned into the wire along the direction of the current. The direction of rotation of the screw is the direction of the magnetic field as shown in Figure (b).
  7. Figure shows the magnetic field due to a straight wire seen from various angles.
    current carrying conductor produces a magnetic field 2
  8. If two straight wires are placed side by side, the magnetic fields produced by the currents in them will combine to form a resultant field.
    (a) Figure shows the magnetic field for two straight parallel wires carrying current in the same direction. The wires attract with each other.
    current carrying conductor produces a magnetic field 3(b) Figure shows the magnetic field for two straight parallel wires carrying current in opposite directions. The wires repel with each other.
    current carrying conductor produces a magnetic field 4

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Magnetic Field Due to a Current Carrying Straight Conductor Experiment

Aim: To study the pattern and direction of the magnetic field due to a current in a straight wire.
Material: Iron filings
Apparatus: Cardboard, thick insulated copper, 4 plotting compasses, low voltage high current d.c. power supply, connecting wires
Method:current carrying conductor produces a magnetic field 5

  1. The apparatus is set up as shown in Figure.
  2. The d.c. power supply is switched on and the direction shown by the compasses is noted.
  3. The d.c. power supply is switched off. The compasses are removed and some iron filings are sprinkled on the cardboard.
  4. The d.c. power supply is switched on again. The cardboard is tapped a few times and the pattern formed by the iron filings is noted.

Observation:current carrying conductor produces a magnetic field 6

Discussion:

  1. The iron filings show the pattern of the magnetic field while the compasses show the direction of the field.
  2. A thick copper wire is used so that a larger current will flow to produce a stronger magnetic field.
  3. The d.c. power supply should be switched off after making the observations so that the copper wire will not be overheated.

Magnetic Field Due to Current through a Circular Loop

The Magnetic Field Due to a Current in a Coil:

  1. Figure shows the magnetic field produced by a current in a circular coil.
    current carrying conductor produces a magnetic field 7
  2. The magnetic lines are closest to each other at the centre of the coil. This shows that at the centre of the coil, the magnetic field is the strongest.
  3. The strength of the magnetic field increases when:
    (a) The current in the coil is increased
    (b) The coil has more turns
    (c) A coil of smaller radius is used

Magnetic Field Due to Current through a Circular Loop Experiment

Aim: To study the pattern and direction of the magnetic field due to a current in a circular coil.
Material: Iron filings
Apparatus: Circular coil made of insulated copper wire mounted on a plastic frame, 3 plotting compasses, low voltage high current d.c. power supply, connecting wires
Method:
current carrying conductor produces a magnetic field 8

  1. The apparatus is set up as shown in Figure.
  2. The d.c. power supply is switched on and the direction shown by the compasses is noted.
  3. The d.c. power supply is switched off. The compasses are removed and some iron filings are sprinkled on the plastic frame.
  4. The d.c. power supply is switched on again. The plastic frame is tapped a few times and the pattern formed by the iron filings is noted.

Observation:current carrying conductor produces a magnetic field 9Discussion:

  1. It is better to have more turns of the copper wire. The magnetic field from each turn of wire will combine together to form
  2. a stronger field. This enables the iron filings to show clearly the pattern of the field.
  3. The d.c. power supply should be switched off after making the observations so that the copper wires will not be overheated.

Magnetic Field Due to a Solenoid Carrying Current

The Magnetic Field Due to a Current in a Solenoid:

  1. The magnetic field for a solenoid has a similar pattern to the magnetic field of a bar magnet, as shown in Figure. One end of the solenoid is a North pole while the other end is a South pole.
    current carrying conductor produces a magnetic field 10
  2. The polarity at the ends of the solenoid can be determined by:
    current carrying conductor produces a magnetic field 11(a) Using the right-hand grip rule. Hold the solenoid using your right hand with your four fingers curled around the solenoid along the direction of the current. The thumb will point to the end that is the North pole.
    (b) Looking at the end of the solenoid. A clockwise current indicates a South pole while an anticlockwise current indicates a North pole.

Magnetic Field Due to a Solenoid Carrying Current Experiment

Aim: To study the pattern and direction of the magnetic field due to a current in a solenoid.
Material: Iron filings
Apparatus: Solenoid made of insulated copper wire mounted on a plastic frame, 4 plotting compasses, low voltage high current d.c. power supply, connecting wires
Method:current carrying conductor produces a magnetic field 12

  1. The apparatus is set up as shown in Figure.
  2. The d.c. power supply is switched on and the direction shown by the compasses is noted.
  3. The d.c. power supply is switched off. The compasses are removed and some iron filings are sprinkled on the plastic frame.
  4. The d.c. power supply is switched on again. The plastic frame is tapped a few times and the pattern formed by the iron filings is noted.

Observation:
current carrying conductor produces a magnetic field 13

Discussion:

  1. It is better to have more turns of thick copper wire. The thicker wire has a smaller resistance so that the current will be larger. Each turn of wire forms its magnetic field. The magnetic field from all the turns of wire will combine together to form a stronger field. This enables the iron filings to show clearly the pattern of the field.
  2. The d.c. power supply should be switched off after making the observations so that the copper wires will not be overheated.