- A d.c. A motor works on the principle that a current-carrying conductor placed normally in a magnetic field experiences a force, producing rotational motion.
- The split ring commutator reverses the direction of current in the coil after every half rotation, so that the coil continues to rotate in the same direction.
- The armature coil experiences an anticlockwise couple due to equal and opposite forces on its arms, causing continuous rotation of the coil.
- In a d.c. Motor, electrical energy supplied by the battery is converted into mechanical energy.
Definitions [10]
Definition: Magnetic Force
The force experienced by a moving charge in the presence of a magnetic field, which depends on charge q, velocity v and magnetic field B, and which is opposite in direction on a negative charge compared to a positive charge, is called the magnetic force.
Definition: Lorentz Force
When both electric and magnetic fields act on a charge, the total force is called the Lorentz force.
Definition: Right Hand Thumb Rule
If a current-carrying straight conductor is held in the right hand such that the thumb points in the direction of the electric current, then the fingers curled around the conductor show the direction of the magnetic field.
This is called the Right-Hand Thumb Rule.
OR
If you hold a current-carrying conductor in your right hand with the thumb pointing in the direction of the current, then the curled fingers show the direction of the magnetic field (lines of force) around the conductor.
Definition: Solenoid
If a conducting wire is wound in form of a cylindrical coil whose diameter is less in comparison to its length, the coil is called a solenoid.
OR
A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
OR
When a copper wire with a resistive coating is wound in a chain of loops (like a spring), it is called solenoid.
Define electric motor.
lt is a device to convert electrical energy into mechanical energy. It is based on the principle that when a current-carrying coil is placed in a magnetic field, it experiences a force.
Definition: Electric Motor
A device changing electrical energy into mechanical energy is known as electric motor.
Definition: Electromagnetic Induction
Electromagnetic induction is the production of an electromotive force across an electrical conductor in a changing magnetic flux or magnetic field.
Definition: Magnetic Flux
The total number of magnetic field lines passing perpendicularly through a given surface area.
Definition: A.C. Generator
An a.c. generator is a device which converts the mechanical energy into the electrical energy using the principle of electromagnetic induction.
Definition: Simple D.C. Motor
An electric motor is a device which converts the electrical energy into the mechanical energy.
Formulae [9]
Formula: Electric Field Due to a Point Charge
\[\vec{E}=\frac{1}{4\pi\varepsilon_0}\frac{Q}{r^2}\hat{r}\]
Formula: Maximum Magnetic Force
Maximum magnetic force (when v ⊥ B): Fmax = qv B
Formula: Magnetic Force
Vector Form: \[\vec F\] = q(\[\vec v\] × \[\vec B\])
Magnitude Form: F = qv B sin θ
Where:
- q = charge on the particle
- v = speed of the particle
- B = magnetic field strength
- θ = angle between \[\vec v\] and \[\vec B\]
Formula: Lorentz Force
\[\vec F\] = q(\[\vec E\] + \[\vec v\] × \[\vec B\])
Formula: Magnetic Field on the Axis of a Circular Loop
\[\vec{B}=\frac{\mu_0IR^2}{2(x^2+R^2)^{3/2}}\hat{i}\]
Where:
- I = current
- R = radius of loop
- x = distance from centre along axis
- μ0 = permeability of free space
Formula: Force on a Current-carrying Conductor in a Magnetic Field
F = IL × B
Formula: Motion of Charged Particle
Centripetal force provided by magnetic force: \[\frac{mv^2}{r}=qvB\]
Angular speed = \[\omega=\frac{qB}{m}\]
Period of circular motion = \[T=\frac{2\pi m}{qB}\]
Frequency = \[f=\frac{qB}{2\pi m}\]
Formula: Magnetic Force on a Straight Current-Carrying Conductor
\[F=BIL\sin\theta\]
Vector Form:
\[\vec{F}=I(\vec{L}\times\vec{B})\]
Special Cases:
- θ = 90∘ → F = BILF
- θ = 0∘ → F = 0
Formula: Magnetic Flux
ΦB = \[\vec B\] ⋅ \[\vec A\] = B A cos θ
| Symbol | Meaning | SI Unit |
|---|---|---|
| \[Φ_B\] | Magnetic Flux | Weber (Wb) |
| B | Magnetic Field Strength | Tesla (T) |
| A | Area of the surface | m² |
| θ | Angle between B and the normal to the surface | degrees/radians |
Theorems and Laws [3]
Law: Fleming's Left-Hand Rule
If we stretch the index finger, middle finger and thumb of the left hand mutually perpendicular to each other such that the index finger points along the direction of the magnetic field and the middle finger along the direction of current (moving charge), then the thumb represents the direction of the force F experienced by the moving charge.
Fleming’s Left-Hand Rule
If the thumb, forefinger and middle finger of the left hand are stretched mutually perpendicular to each other, and the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, then the thumb gives the direction of the force acting on the conductor.
Faraday's Laws of Electromagnetic Induction
Faraday's First Law
Whenever the magnetic flux linked with a circuit changes, an EMF is induced in the circuit. The induced EMF lasts only as long as the change in flux is taking place.
Faraday's Second Law
The magnitude of the induced EMF in a circuit is directly proportional to the rate of change of magnetic flux through the surface enclosed by that circuit.
Key Points
Key Points: Bar Magnet and Solenoid Analogy
- A bar magnet behaves like a solenoid
- Both produce similar magnetic field patterns
- Solenoid Relation: M = NIA
Key Points: Force on a Current Carrying Conductor in a Magnetic Field
- A current-carrying conductor placed in a magnetic field experiences a force when the direction of current is not parallel to the magnetic field.
- The direction of force reverses when the direction of current or the direction of magnetic field is reversed, and no force acts when current flows parallel to the magnetic field.
Key Points: Electromagnetic Induction
- Electromagnetic induction requires a changing magnetic flux — a static field produces no induction
- The faster the change in flux, the greater the induced EMF (Faraday's Second Law)
- The induced EMF exists only during the change; it ceases when the flux becomes constant
- Both the motion of a conductor in a magnetic field and the change of current in a nearby circuit can cause induction
- The direction of the induced current can be found using Fleming's Right-Hand Rule or Lenz's Law
Key Points: Simple D.C. Motor
Important Questions [35]
- Magnetic Lines of Force Are Closed Continuous Curves.
- Write any three properties of magnetic lines of force.
- State the Right Hand Thumb Rule.
- Draw a neat diagram of a solenoid and name its various components.
- What is a solenoid?
- Observe the given figure of Fleming's Left Hand Rule and write the labels of 'A' and 'B':
- Differentiate between conductors and insulators.
- Name the following diagram and explain the concept behind it.
- Observe the Following Figure: If the Current in the Coil a is Changed, Will Some Current Be Induced in the Coil B? Explain.
- State Whether the Following Statements Are True Or False. an Electric Motor Converts Mechanical Energy into Electrical Energy.
- Name Any Four Appliances Where Electric Motor is Used.
- What is the Principle of Electric motor?
- Draw Figure ‘Electric Motor’ and Working
- Explain the Construction and Working of an Electric Motor.
- Label the Four Parts of an Electric Motor
- Tell the odd one out. Give a proper explanation. Loud speaker, Microphone, Electric motor, Magnet
- State Fleming’S Right Hand Rule.
- Answer the Following: State the Principles of the Electric Motor and Electric Generator.
- Observe the given figure of Fleming’s Right Hand Rule and write the labels of A and B correctly.
- State Three Differences Between Direct Current and Alternating Current.
- The Device Used for Producing Electric Current is Called
- State Fleming’S Right-hand Rule.
- Name the following diagram and explain the concept behind it.
- Explain the construction and working of an electric generator (AC) with the help of a neat diagram.
- State true or false: The frequency of AC is 50 Hz.
- The device used for producing electric current is called a ______.
- Write Two Uses of Dc Motor
- When Does Short Circuiting Take Place?
- What Happens to the Flow of Electric Current During a Short Circuit?
- What is Overloading?
- How Can the Effects of Overloading Be Avoided?
- Why Should the Wires Carrying Electricity Not Be Touched Barefooted?
- Name safety measures commonly used in electric circuits and appliances.
- Answer the Following Question: What is Overloading? When Does It Occur? How Can Overloading Be Avoided?
- What Happens to the Resistance of the Circuit During a Short Circuit?
Concepts [11]
- Magnetic force
- Bar Magnet and Solenoid Analogy
- Right-hand Thumb Rule
- Applications of Biot-Savart's Law > Magnetic Field at the Axis of a Circular Current-carrying Loop
- Applications of Ampere’s Circuital Law > Magnetic Field of a Long Straight Solenoid
- Force on a Current-Carrying Conductor Placed in a Uniform Magnetic Field
- Electric Motor
- Introduction to Electromagnetic Induction
- A.C. Generator
- Simple D.C. Motor
- Household Electrical Circuits
