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Force on a Current-Carrying Conductor (The Motor Effect)

🔬 What Is the Motor Effect?​

When a wire carrying an electric current is placed in a magnetic field, it experiences a force.
If the wire is free to move, it moves (or jumps) due to this force.

🧪 Experiment: Demonstrating the Motor Effect​

What You Need:​

  • A C-shaped magnet (permanent or electromagnet)

  • A flexible wire placed between the poles of the magnet

  • A power supply and switch

What Happens:​

  1. The wire is loosely placed inside the magnetic field.

  2. When the switch is closed, current flows through the wire.

  3. The wire jumps upwards (or downwards, depending on directions).

  4. If you:

    • Reverse the current, the wire moves in the opposite direction.

    • Reverse the magnetic field, the wire also moves in the opposite direction.

  5. The stronger the current or magnetic field, the bigger the force.

🧲 Why Does the Wire Move?​

  • The magnetic field from the magnet interacts with the magnetic field created by the current in the wire.

  • The result is a combined magnetic field with more field lines on one side of the wire.

  • These lines act like stretched rubber bands, pulling the wire to equalize the field, which causes a force.

  • The force is strongest when the wire is at right angles to the magnetic field.

✋ Fleming’s Left-Hand Rule​

Use this rule to figure out the direction of the force on the wire.

Hold your left hand like this:

  • First finger = direction of the Magnetic Field (Field)

  • Second finger = direction of the Current

  • Thumb = direction of the Force (Thrust)

🧠 TIP: Remember FBI:

  • F = Magnetic Field (First finger)

  • B = Current (seCond finger)

  • I = Force/Thrust (Thumb)

📌 The force is zero if the wire is parallel to the field.

⚡ Force on Charged Particles in a Magnetic Field​

What if it’s a beam of electrons or ions?​

  • Charged particles moving in a magnetic field also experience a force.

  • This force causes the particles to curve or move in circles.

  • The direction of the force is again found using Fleming’s Left-Hand Rule.

🧲 In diagrams, a magnetic field into the paper is shown using crosses (×)
(e.g., like the tail of an arrow going away from you).

IMPORTANT:​

  • For electrons (negative charge): treat the current as going in the opposite direction.

  • For positive particles: use the current’s actual direction.

✅ Summary​

ConceptKey Points
Motor effectCurrent + magnetic field = force on a conductor
DemonstrationWire jumps when current flows in a magnetic field
Direction of forceUse Fleming’s Left-Hand Rule
Strongest force when...Wire is at right angles to magnetic field
Charged particles in fieldDeflected due to magnetic force; curve path
Electron beam deflectionOpposite to conventional current (because electrons are negative)