Force on a Current-Carrying Conductor in a Magnetic Field

A current-carrying conductor produces its own magnetic field. When that conductor is placed in an external magnetic field, a magnetic force may act on it.

Because of this force, the conductor may move or be deflected from its original position. This principle forms the basis of an electric motor.

👉 Fleming's Left-Hand Rule (Fleming's Left-Hand Rule)

When the directions of the magnetic field and electric current are known, the direction of force or motion on a current-carrying conductor can be determined using Fleming's Left-Hand Rule.

Stretch the thumb, forefinger and middle finger of your left hand so that all three are mutually perpendicular. Then:

• Forefinger → direction of magnetic field
• Middle finger → direction of electric current
• Thumb → direction of force or motion

A simple way to remember it: Field – Current – Force correspond to forefinger – middle finger – thumb.

👉 What Does the Magnetic Force Depend On?

For a straight current-carrying conductor of length L, carrying current I, placed in a magnetic field of strength B, and making an angle θ with the magnetic field, the magnitude of the force in the ideal case is:

Thus, the magnitude of force depends on:

• Magnetic field strength - B
• Electric current - I
• Effective length of the conductor inside the magnetic field - L
• Angle between the current and magnetic field - θ

When is the force maximum?

When the current and magnetic field are perpendicular,

then,

When is the force zero?

When the current is parallel or antiparallel to the magnetic field,

then,

👉 Direction of Force When Current or Field is Reversed

• If the direction of current is reversed while the magnetic field remains unchanged, the direction of force reverses.
• If the direction of the magnetic field is reversed while the current remains unchanged, the direction of force reverses.
• If both the current and magnetic field are reversed together, the direction of force remains unchanged.

This direction-dependent behavior is very important for understanding motor rotation.

👉 Barlow's Wheel

Barlow's wheel is a historical device used to demonstrate the effect of force on a current-carrying conductor placed in a magnetic field.

It consists of a toothed metal wheel arranged so that one of its teeth can touch a conducting liquid and complete the circuit. The current-carrying part of the wheel is positioned between the poles of a strong magnet.

When electric current flows, the magnetic field exerts a force on the current-carrying tooth. According to Fleming's Left-Hand Rule, the tooth moves, and the next tooth completes the circuit. Repetition of this process causes the wheel to rotate continuously.

What factors affect the rotation?

• 🔹 Reversing the direction of current reverses the direction of rotation.
• 🔹 Reversing the direction of the magnetic field reverses the direction of rotation.
• 🔹 Increasing the current increases the magnetic force and turning effect.
• 🔹 Increasing the magnetic field strength increases the turning effect.
• 🔹 Removing the magnetic field removes the cause of rotation.

👉 Electric Motor

An electric motor is a device that converts electrical energy into mechanical energy.

It works on the following principle:

⭐ A current-carrying conductor placed in a magnetic field experiences a force.

In an electric motor:

⭐ Electrical energy → Mechanical energy

is converted.

👉 Main Parts of a Simple DC Motor

1. Field Magnet

A strong magnet produces the magnetic field required for the motor to operate.

2. Armature or Rotating Coil (Armature / Coil)

A rectangular coil of insulated wire is placed between the two poles of the magnet. In a common diagram, it is represented as coil ABCD. It can rotate about an axis.

3. Split-Ring Commutator

The two ends of the coil are connected to two separate half-rings. These two half-rings rotate with the coil.

After every half-turn, the contact with the brushes changes, causing the direction of current through the coil to reverse. As a result, the turning effect on the coil continues in the same rotational direction.

4. Brush

Two stationary conducting brushes maintain contact with the rotating split-ring commutator and deliver electric current from the external DC source to the coil.

👉 How Does the Motor Rotate?

Suppose the opposite sides AB and CD of the armature are placed in the magnetic field.

When electric current flows through the coil, the directions of current in AB and CD are opposite. In the same magnetic field, according to Fleming's Left-Hand Rule, opposite forces act on these two sides.

These opposite forces produce a turning effect (turning effect / torque), causing the coil to start rotating.

After the coil completes half a turn, the split-ring commutator changes its contact with the brushes. As a result, the direction of current in the coil is reversed.

Because the current is reversed, the directions of force on AB and CD also change at the correct moment, so the coil continues to rotate in the same overall direction.

This process repeats after every half-turn, allowing the motor to rotate continuously.

👉 How to Increase the Turning Effect of a Motor

The turning effect of a motor can be increased by:

• 🔹 Increasing the strength of the field magnet
• 🔹 Increasing the electric current through the coil
• 🔹 Increasing the number of turns in the coil
• 🔹 Increasing the area of the coil appropriately

👉 Uses of Electric Motors

Electric motors are used in many devices, such as:

• 🔹 Electric fans
• 🔹 Water pumps
• 🔹 Mixers and grinders
• 🔹 Toy cars
• 🔹 Electric vehicles
• 🔹 Various industrial machines

The basic function of a motor is to use electrical energy to produce rotational or other forms of mechanical motion.

👉 Why is a Split-Ring Commutator Needed in a DC Motor?

If the direction of electric current through the coil were not reversed after every half-turn, the turning effect (turning effect) on the coil would not continue to maintain the same direction of rotation.

The split-ring commutator (Split-ring Commutator) changes the connection of the coil after every half-turn and reverses the direction of electric current. As a result, the direction of force on the coil also changes at the correct moment, allowing a simple DC motor to continue rotating in the same direction.

🧠 Quick Revision

⭐ Magnetic field + current-carrying conductor → magnetic force

⭐ Fleming's Left-Hand Rule → determines the direction of force or motion

⭐ F = BIL sin θ

⭐ The force is maximum when electric current and magnetic field are perpendicular.

⭐ The force is zero when electric current and magnetic field are parallel.

⭐ Electric motor → converts electrical energy into mechanical energy

⭐ Armature (Armature) → rotating coil

⭐ Split-ring commutator (Split-ring Commutator) → reverses the direction of current in the coil after every half-turn

⭐ Brush (Brush) → maintains electrical contact between the external circuit and the rotating commutator

✍ Try It Yourself

🔹 1. Why does the direction of force on a conductor reverse when the direction of electric current is reversed?

🔹 2. What do the forefinger, middle finger and thumb represent in Fleming's Left-Hand Rule?

🔹 3. What is the magnitude of force on a current-carrying conductor when it is parallel to the magnetic field?

🔹 4. Under what condition is the magnetic force on a current-carrying conductor maximum?

🔹 5. Why is a split-ring commutator required in a DC motor?

🔹 6. How do opposite forces on the AB and CD sides of a motor produce a turning effect?

🔹 7. Write two ways to increase the turning effect of a motor.

🔹 8. What type of energy is converted into what type of energy in an electric motor?