Halliday Physics: Magnetism

Cheatsheet Content

### Magnetism Basics - **Definition:** A substance that attracts magnetic materials like iron, nickel, and cobalt, and has directional properties is called a magnet. - **Poles:** Magnets have two poles, designated as the North Pole (N) and the South Pole (S). - **Reason for Naming:** When a bar magnet is suspended, it aligns itself in the north-south direction. The part that points north is called the North Pole, and the part that points south is called the South Pole. This occurs because the Earth has a magnetic field. #### Interactions between Poles - **Attraction:** When two unlike poles (North and South) of two magnets are brought close together, they attract each other. - **Repulsion:** When two like poles (both North or both South) of two magnets are brought close together, they repel each other. ### Magnetic Field - **Definition:** The region around a magnet where its magnetic force can be experienced. - **Direction of Magnetic Field:** It is defined as the direction in which a small free-moving North Pole would move when placed in the magnetic field. - **Representation:** Magnetic field lines are used to represent the magnetic field. They originate from the North Pole and end at the South Pole. #### Characteristics of Magnetic Field Lines - A larger number of magnetic field lines will be concentrated where the magnetic field is stronger. - Magnetic field lines never intersect each other. - They form closed loops. - Outside the magnet, they run from the North Pole to the South Pole, and inside the magnet, from the South Pole to the North Pole. ### Electromagnetism - **Principle:** A wire carrying an electric current creates a magnetic field around it. - **Magnetic field around a straight wire:** Concentric circles around the wire. The direction can be found using the Right-Hand Grip Rule: If the thumb points in the direction of current, the fingers curl in the direction of the magnetic field. #### Solenoid - **Definition:** A coil of insulated wire wound in the shape of a cylinder. - **Magnetic field of a solenoid:** Similar to a bar magnet. The strength depends on the number of turns, current, and core material. - **Electromagnet:** A temporary magnet created by inserting a piece of iron into a current-carrying solenoid. Used in motors, generators, and lifting heavy objects. #### Effect of a Magnetic Field on a Current-Carrying Wire - **Principle:** A current-carrying wire placed in a magnetic field experiences a force. The direction of this force is given by Fleming's Left-Hand Rule. - **Characteristics:** - A larger number of magnetic field lines will be created below the wire and a smaller number above, which will push the wire upwards. - If the direction of the current in the wire is reversed, the direction of the magnetic field and the force will change. - The density of the magnetic field lines on the wire will increase the current. ### D.C. Motor - **Principle:** A D.C. motor converts electrical energy into mechanical energy. It uses the principle that a current-carrying conductor placed in a magnetic field experiences a force. - **Components:** - **Armature:** A coil of insulated wire wound on a soft iron core. - **Commutator:** A split ring that reverses the direction of current in the coil every half rotation. - **Brushes:** Carbon contacts that supply current to the commutator. - **Magnets:** Provide the magnetic field. - **Working:** When current flows through the coil, the magnetic field exerts a force, causing the coil to rotate. The commutator ensures continuous rotation in one direction. #### Electromagnetic Induction - **Definition:** The phenomenon where an electromotive force (EMF) is generated inside that loop, which can then induce current. - **Uses:** Electromagnetic induction is used to create generators, where electricity is generated by changing the magnetic field inside a conductive wire. ### Generator - **Principle:** A generator converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction. - **Working:** Rotating the armature of a D.C. motor will provide D.C. current flow, and rotating an AC motor will provide AC current flow in the same way. #### Transformer - **Definition:** A device that changes the voltage of an alternating current. It consists of two coils (primary and secondary) wound around a soft iron core. - **Energy Conversion:** A generator converts mechanical energy into electrical energy. - **Equation:** $\frac{V_p}{V_s} = \frac{N_p}{N_s}$ where $V$ is voltage and $N$ is the number of turns. ($p$ for primary, $s$ for secondary) #### Types of Transformers - **Step-up Transformer:** Increases voltage ($N_s > N_p$, $V_s > V_p$). - **Step-down Transformer:** Decreases voltage ($N_s #### Motors vs. Generators - **Motors:** Convert electrical energy to mechanical energy. - **Generators:** Convert mechanical energy to electrical energy. - **Use of Motors in Fans:** Motors are used in electric fans because when electricity flows through the motor, its shaft rotates. #### Power of a Transformer - In an ideal transformer, the power in the primary coil equals the power in the secondary coil. - The current in the primary and secondary coils are inversely proportional to their respective voltages.