Knowt
Magnetism and Electromagnetics
Magnets and Magnetism
Magnets attract magnetic materials. The magnetic materials are:
Iron
Steel (because it contains iron)
Nickel
Cobalt
Poles of a Magnet: A magnet has two poles, a North pole and a South pole.
Magnetic forces are strongest at the poles.
Like poles repel, unlike poles attract. This is known as the Law of Magnetism.
The forces between magnets are non-contact forces.
Magnetic Fields: A magnetic field is the region around a magnet where a magnetic force is exerted on another magnet or magnetic material.
Magnetic fields are represented by magnetic field lines.
The direction of the field line shows the direction of the force on another north pole placed at that point. Field lines always go from the North pole to the South pole of a magnet.
The strength of the magnetic field is indicated by the spacing of the field lines - the closer the lines, the stronger the field. The magnetic field is strongest at the poles where field lines are closest together.
The magnetic field becomes weaker as distance from the magnet increases, as field lines become further apart.
Plotting Magnetic Fields: Magnetic fields can be plotted using a plotting compass. Place the compass at different points around a magnet, and mark the direction of the needle. Connecting these marks will show the field lines.
Earth's Magnetic Field: The Earth has its own magnetic field, which is why a compass needle points North. The Earth's core is made of iron, which contributes to this magnetic field.
Permanent and Induced Magnets:
A permanent magnet creates its own magnetic field. The magnetism of a permanent magnet cannot be turned on or off (e.g., bar magnet).
An induced magnet becomes magnetic when it is placed in a magnetic field. It loses most or all of its magnetism when removed from the field (e.g., iron filings). Induced magnets only attract other materials, they are never repelled.
Induced Magnetism: When a magnetic material enters a magnetic field, it becomes an induced magnet with the opposite pole induced closest to the permanent magnet. This is why magnets attract magnetic materials.
Magnetisation: Magnetic materials can be magnetized by:
Stroking with a magnet.
Leaving it near a magnet for a long time.
Electromagnetism
Magnetic Field around a Current-Carrying Wire: An electric current flowing through a wire produces a circular magnetic field around the wire.
The field is strongest close to the wire and weakens with distance.
Corkscrew Rule: This rule helps determine the direction of the magnetic field. If you point your thumb in the direction of the conventional current (positive to negative), your fingers curl in the direction of the magnetic field.
Reversing the current direction reverses the magnetic field direction.
Solenoids: A solenoid is a coil of insulated wire. It produces a stronger and more uniform magnetic field compared to a single wire.
The magnetic field inside and around a solenoid is similar in shape to that of a bar magnet.
The magnetic field is strong and uniform inside the solenoid.
Poles of a Solenoid: The poles of a solenoid depend on the direction of the current:
South pole: where the current flows clockwise.
North pole: where the current flows anticlockwise.
To increase the strength of the magnetic field of a solenoid:
Increase the current.
Increase the number of turns (coils).
Add an iron core.
Electromagnets: An electromagnet is a solenoid with an iron core.
The iron core becomes magnetized when current flows through the solenoid, significantly increasing the magnetic field strength.
Advantages of Electromagnets: Electromagnets can be turned on and off by switching the current on or off. Their strength can also be easily controlled.
Applications of Electromagnets:
Electric Bell: When the button is pressed, current flows through the electromagnet coil. The electromagnet attracts an iron bar, causing a clapper to hit the bell. This action also breaks the circuit. The electromagnet turns off, the bar returns to its original position, reconnecting the circuit, and the cycle repeats, causing the bell to ring continuously.
Relay: A relay uses a low voltage circuit to control a high voltage circuit. When a switch is closed in the low voltage circuit, the electromagnet attracts an iron arm, which then closes the contacts in the high voltage circuit, turning it on.
Loudspeaker: An alternating current in a coil of wire creates an electromagnetic field. This field interacts with the magnetic field of a permanent magnet. As the current changes direction, the coil is attracted and repelled by the permanent magnet, causing the speaker cone to vibrate and produce sound waves.
Magnetic Force and Motion (The Motor Effect)
The Motor Effect: When a current-carrying wire is placed in a magnetic field, it experiences a force. This is called the motor effect.
The force is strongest when the wire is perpendicular to the magnetic field. If the wire is parallel to the field, there is no force.
The direction of the force is perpendicular to both the direction of the current and the magnetic field.
Fleming's Left-Hand Rule: This rule helps determine the direction of the force on a current-carrying wire in a magnetic field.
Thumb: Represents the direction of the Motion (Force).
First Finger: Represents the direction of the Field (North to South).
Second Finger: Represents the direction of the Current (Conventional current, positive to negative).
Factors Affecting Force Strength: The strength of the force on the wire can be increased by:
Increasing the magnetic flux density (strength of the magnetic field) (B) in Tesla (T).
Increasing the current (I) in Amperes (A).
Increasing the length of the conductor (wire) (l) in meters (m) within the magnetic field.
Equation:
DC Motors: Electric motors use the motor effect to rotate a coil in a magnetic field.
A coil of wire is placed in a magnetic field.
When current flows through the coil, forces act on the sides of the coil that are in the magnetic field.
These forces are opposite on different sides of the coil, causing it to rotate.
A commutator is used to reverse the current direction every half turn to maintain continuous rotation in one direction.
Electromagnetic Induction
Induced Potential Difference: When a conductor moves relative to a magnetic field or if the magnetic field around a conductor changes, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced. 1 This process is called electromagnetic induction or the generator effect.
Factors Affecting Induced Potential Difference: The size of the induced potential difference can be increased by:
Increasing the speed of movement of the conductor or the magnet.
Increasing the strength of the magnetic field.
Increasing the number of turns in a coil (if using a coil).
Generators: Generators use electromagnetic induction to generate electricity.
A generator (dynamo) rotates a coil in a magnetic field.
As the coil rotates, it cuts through magnetic field lines, inducing a potential difference and current.
In an AC generator, the coil rotates continuously, inducing an alternating current.
In a DC generator, a commutator is used to produce a direct current.
Microphones: Microphones use the generator effect in reverse to convert sound waves into electrical signals. Sound waves cause a diaphragm to vibrate, which is connected to a coil in a magnetic field. The movement of the coil in the magnetic field induces a potential difference, creating an electrical signal that corresponds to the sound waves.
Transformers and the National Grid
Transformers: Transformers are used to change the potential difference (voltage) of an alternating current. They rely on electromagnetic induction.
Step-up transformers increase the voltage and decrease the current. They have more turns on the secondary coil than the primary coil.
Step-down transformers decrease the voltage and increase the current. They have fewer turns on the secondary coil than the primary coil.
Transformer Equation:
Where:
Vp = Primary voltage
Vs = Secondary voltage
Np = Number of turns on primary coil
Ns = Number of turns on secondary coil
Ip = Primary current
Is = Secondary current
National Grid: The National Grid is a network of cables and transformers that distributes electricity across long distances.
High Voltage Transmission: Electricity is transmitted at high voltages and low currents in the National Grid to reduce energy loss due to heating in the cables.
Step-up transformers are used at power stations to increase the voltage for efficient transmission.
Step-down transformers are used locally to reduce the voltage to safer levels for domestic use.
