4.5.4 Force on a Current-Carrying Conductor & 4.5.5 D.C. Motor
Motor Effect
When a current flows through a conductor placed in a magnetic field, the wire experiences a force. This phenomenon is known as the motor effect.
Factors Affecting the Force
- Force increases with higher current.
- Force increases with a stronger magnetic field.
- Force is greatest when the wire is perpendicular to the magnetic field lines.
- Force is zero when the wire is parallel to the magnetic field lines.
Reversing the Direction of Force
- Reverse the current direction in the wire.
- Reverse the magnetic field direction.
Direction of Force
The direction of the force is always perpendicular to both the magnetic field and the current.
Fleming’s Left-Hand Rule
- Hold your left hand with thumb, first finger, and second finger at right angles.
- First finger → direction of magnetic field (N → S).
- Second finger → direction of current (positive → negative).
- Thumb → direction of the force (motion).
Force on a Beam of Charged Particles
A beam of electrons (as in cathode rays) passing through a magnetic field is deflected due to the motor effect.
- The beam bends according to Fleming’s Left-Hand Rule.
- Increasing the magnetic field strength increases the amount of deflection.
- Using an electromagnet instead of a permanent magnet produces the same effect.
Interaction of Magnetic Fields
When a current-carrying wire is placed in a magnetic field, the field lines from the wire interact with those from the magnet. They cancel on one side and reinforce on the other, pushing the wire toward the weaker side of the field.
Principle of the D.C. Motor
A current-carrying coil in a magnetic field experiences a turning effect due to the motor effect.
Increasing the Turning Effect
- Increase the number of turns in the coil.
- Increase the current through the coil.
- Use a stronger magnet to increase field strength.
Construction of a D.C. Motor
- A rectangular armature coil of insulated wire is placed between two magnetic poles.
- The coil is connected to the power supply through carbon brushes and a split-ring commutator.
Working of a D.C. Motor
- When current flows, each side of the coil experiences an opposite force due to the motor effect.
- These forces form a couple, causing the coil to rotate.
- The split-ring commutator reverses the current every half turn, keeping the rotation in the same direction.
- Thus, electrical energy is converted into kinetic energy.
Controlling the Motor
- Speed of rotation is controlled by changing the current magnitude.
- Direction of rotation is reversed by reversing the current direction.
Comparison of Slip Rings and Split Rings
| Feature | Slip Rings (A.C. Generator) | Split Rings (D.C. Motor) |
|---|---|---|
| Contact Type | Smooth continuous surface | Two half rings with a gap |
| Maintenance | Low wear and long life | Wear quickly; need frequent replacement |
| Output | Alternating current | Pulsating direct current |
| Brush Contact | Continuous and stable | Brushes strike edges causing sparks and wear |
Exam Guidance
- Use Fleming’s Left-Hand Rule to predict force direction.
- Explain rotation in terms of forces on opposite sides forming a couple.
- State that the split-ring commutator keeps current direction consistent with motion.
- Describe energy conversion: electrical → kinetic.
- Include methods to increase motor speed and control rotation direction.
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