Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Magnetic fields
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Magnetic fields
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Subject: Physics
Class: Senior Secondary 3
Term: 1st Term
Week: 1
Theme: Fields At Rest And In Motion
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Plot the magnetic field around;- a bar magnet- a straight conductor carrying current- a solenoid. Make a magnet form a soft iron bar Make an electro-magnet Describe the working principles of an electric bell and a telephone ear piece Locate the earth's magnetic north-south direction. Explain the magnetic for ce on a moving charge. State the relation between the magnetic for ce and the motion of a charge in a magnetic field.
2. 1. Magnetic Field A magnetic field is a region around a magnet or a current-carrying conductor where magnetic effects (forces) can be detected. It is a vector quantity, possessing both magnitude and direction. Magnetic fields are typically represented by magnetic field lines (also known as lines of force).
Properties of Magnetic Field Lines: They emerge from the North pole and enter the South pole outside the magnet, forming continuous closed loops within the magnet (from South to North). They never intersect each other. The density of the lines indicates the strength of the magnetic field; closer lines mean a stronger field. The tangent to a field line at any point gives the direction of the magnetic field at that point. 2.
2. Plotting Magnetic Fields a.
Around a Bar Magnet: To plot the magnetic field around a bar magnet, place the magnet on a sheet of paper. Sprinkle iron filings uniformly around the magnet. Gently tap the paper; the filings will align themselves along the magnetic field lines. Alternatively, use a small plotting compass to trace the field lines: place the compass at various points, mark the direction indicated by the North pole of the compass, and connect the dots to form continuous lines.
Pattern: Concentrated at the poles, diverging from the North and converging into the South.
Neutral Points: If two bar magnets are placed close to each other, points where the magnetic fields cancel out can be observed (zero net magnetic field). This occurs where the field lines from both magnets oppose each other. b. Around a Straight Conductor Carrying Current: In 1820, Oersted discovered that an electric current produces a magnetic field.
Experiment: Pass a straight wire vertically through a piece of cardboard. Connect the wire to a power supply. Place plotting compasses or sprinkle iron filings on the cardboard. When current flows, the compasses align in concentric circles around the wire, or the iron filings form circular patterns. Right-Hand Grip Rule (Maxwell's Corkscrew Rule): If the wire is gripped with the right hand such that the thumb points in the direction of current flow, then the fingers curled around the wire indicate the direction of the magnetic field lines (concentric circles).
Pattern: Concentric circles around the wire, with the plane of the circles perpendicular to the wire. Direction is clockwise or anti-clockwise depending on the current direction. c.
Around a Solenoid: A solenoid is a long coil of insulated wire wound in the form of a helix.
Experiment: Pass current through a solenoid. Use a plotting compass or iron filings to observe the field pattern.
Right-Hand Solenoid Rule: If the fingers of the right hand are curled in the direction of the current flow around the solenoid, the thumb points in the direction of the North pole of the solenoid.
Pattern: The magnetic field inside a long solenoid is uniform and parallel to the axis, similar to that of a bar magnet. Outside, the field lines resemble those of a bar magnet. 2.
3. Making a Magnet from a Soft Iron Bar a.
Stroking Method: Single Touch Method: Take a strong magnet and stroke a soft iron bar (e.g., a nail) repeatedly in the same direction, using one pole of the magnet, lifting the magnet high after each stroke. The end of the iron bar where the stroking begins acquires the opposite polarity to the stroking pole, and the end where it finishes acquires the same polarity.
Double Touch Method: Place two strong magnets with opposite poles (e.g., N and S) at the centre of the soft iron bar. Stroke outwards simultaneously in opposite directions, lifting the magnets high after each stroke. The ends of the soft iron bar will acquire polarity opposite to the poles that touched them at the start. The centre will become the opposite pole to the ends. b.
Electrical Method (using a Solenoid): Place the soft iron bar inside a solenoid. Pass a direct current (DC) through the solenoid for a few minutes. The soft iron bar becomes magnetized. To make a permanent magnet, use steel and demagnetize with alternating current Earth's magnetic North. Note that this is not precisely the geographic North. 2.
7. Magnetic Force on a Moving Charge When a charged particle moves through a magnetic field, it experiences a force, provided its velocity is not parallel or anti-parallel to the magnetic field. This is known as the Lorentz force.
Formula: The magnitude of the magnetic force (F) on a charge (q) moving with velocity (v) in a magnetic field (B) is given by: $F = qvB \sin\theta$ Where: $F$ is the magnetic force (in Newtons, N) $q$ is the magnitude of the charge (in Coulombs, C) $v$ is the speed of the charge (in meters per second, m/s) $B$ is the magnetic field strength or magnetic flux density (in Tesla, T) $\theta$ is the angle between the velocity vector ($\vec{v}$) and the magnetic field vector ($\vec{B}$). Direction of Force (Fleming's Left-Hand Rule): This rule is used to determine the direction of the force on a current-carrying conductor (or a moving positive charge) in a magnetic field.
Thumb: Points in the direction of the Force (motion).
Forefinger: Points in the direction of the external Field (magnetic field lines North to South).
Middle Finger: Points in the direction of the Current (or velocity of a positive charge). For a negative charge (e.g., electron), the direction of the middle finger should be opposite to the actual velocity of the electron, or the resulting force will be opposite to that predicted for a positive charge.
Worked Example 1: An electron (charge $q = -1.6 \times 10^{-19} C$) moves at a speed of $3 \times 10^6 \text{ m/s}$ perpendicularly to a uniform magnetic field of $0.5 \text{ T}$. Calculate the magnitude of the magnetic force on the electron.
Solution: Given: Charge, $q = -1.6 \times 10^{-19} C$ (For magnitude, we use $1.6 \times 10^{-19} C$) Velocity, $v = 3 \times 10^6 \text{ m/s}$ Magnetic field, $B = 0.5 \text{ T}$ Angle, $\theta = 90^\circ$ (perpendicular), so $\sin\theta = \sin 90^\circ = 1$. Using the formula $F = qvB \sin\theta$: $F = (1.6 \times 10^{-19} C) \times (3 \times 10^6 \text{ m/s}) \times (0.5 \text{ T}) \times 1$ $F = (1.6 \times 3 \times 0.5) \times 10^{(-19+6)} \text{ N}$ $F = (1.6 \times 1.5) \times 10^{-13} \text{ N}$ $F = 2.4 \times 10^{-13} \text{ N}$ The magnitude of the magnetic force on the electron is $2.4 \times 10^{-13} \text{ N}$. 2.
8. Relation Between Magnetic Force and Motion of a Charge in a Magnetic Field Case 1: Charge moving perpendicular to the magnetic field ($\theta = 90^\circ$). $F = qvB$. The magnetic force is maximum. The force is always perpendicular to both the velocity and the magnetic field. This causes the charged particle to move in a circular path. This principle is utilized in devices like mass spectrometers and cyclotrons.
Case 2: Charge moving parallel or anti-parallel to the magnetic field ($\theta = 0^\circ$ or $\theta = 180^\circ$). $F = qvB \sin 0^\circ = 0$ or $F = qvB \sin 180^\circ = 0$. In this case, the magnetic force on the charge is zero. The particle continues to move in a straight line with constant velocity.
Case 3: Charge moving at an angle to the magnetic field ($0^\circ < \theta < 180^\circ$). The velocity component parallel to the field ($v \cos\theta$) causes no force, while the component perpendicular to the field ($v \sin\theta$) experiences a force, causing circular motion. The combination results in a helical (spiral) path. This phenomenon is observed in the Earth's Van Allen radiation belts, where charged particles from the sun are trapped along magnetic field lines. of the soft iron bar. Stroke outwards simultaneously in opposite directions, lifting the magnets high after each stroke. The ends of the soft iron bar will acquire polarity opposite to the poles that touched them at the start. The centre will become the opposite pole to the ends. b.
Electrical Method (using a Solenoid): Place the soft iron bar inside a solenoid. Pass a direct current (DC) through the solenoid for a few minutes. The soft iron bar becomes magnetized. To make a permanent magnet, use steel and demagnetize with alternating current (AC) or by heating. Soft iron makes a temporary magnet, easily losing its magnetism. 2.
4. Making an Electromagnet An electromagnet is a temporary magnet formed by passing electric current through a coil of wire wound around a soft iron core.
Construction: Wind insulated copper wire around a soft iron core (e.g., a large nail). Connect the ends of the wire to a DC power supply.
Working Principle: When current flows, the soft iron core becomes strongly magnetized. When the current is switched off, the core largely loses its magnetism.
Factors Affecting Strength: Number of turns in the coil (more turns, stronger field). Magnitude of current (higher current, stronger field). Nature of the core material (soft iron core significantly enhances strength compared to air core). 2.
5. Working Principles of an Electric Bell and a Telephone Earpiece a.
Electric Bell: Components: An electromagnet, a soft iron armature, a hammer, a gong, a contact screw, and a spring.
Working: When the switch is pressed, current flows through the electromagnet. The electromagnet becomes magnetized and attracts the soft iron armature. The hammer attached to the armature strikes the gong, producing sound. As the armature moves towards the electromagnet, it breaks contact with the contact screw. This interrupts the circuit, causing the electromagnet to lose its magnetism. The spring pulls the armature back, restoring contact with the screw. The circuit is completed again, and the process repeats rapidly, causing continuous ringing. b.
Telephone Earpiece: Components: A permanent magnet, a soft iron diaphragm, and coils of wire wound around the poles of the permanent magnet.
Working: Voice sound waves are converted into varying electrical current signals at the transmitting end. This varying current flows through the coils in the earpiece. The varying current creates a varying magnetic field that either strengthens or weakens the magnetic field of the permanent magnet. This causes the soft iron diaphragm to vibrate back and forth at the same frequency as the original sound waves. These vibrations produce sound waves that are heard by the listener. 2.
6. Locating the Earth's Magnetic North-South Direction The Earth acts like a giant bar magnet, generating its own magnetic field.
Magnetic Poles: The Earth's magnetic North pole is geographically near the South Pole, and its magnetic South pole is geographically near the North Pole. This is why the North pole of a compass needle points approximately towards the Earth's geographic North.
Declination: The angle between the geographic meridian (true North) and the magnetic meridian (magnetic North) at a particular location.
Dip (or Inclination): The angle a freely suspended magnet makes with the horizontal at a particular location. It is 0° at the magnetic equator and 90° at the magnetic poles.
Procedure: Suspend a bar magnet freely by a thread from its centre of gravity or use a plotting compass placed on a flat, non-magnetic surface. Allow it to settle. The direction indicated by the North pole of the compass (or the North-seeking pole of the suspended magnet) is the Earth's magnetic North. Note that this is not precisely the geographic North. 2.
7. Magnetic Force on a Moving Charge When a charged particle moves through a magnetic field, it experiences a force, provided its velocity is not parallel or anti-parallel to the magnetic field. This is known as the Lorentz force.
Formula: The magnitude of the magnetic force (F) on a charge (q) moving with velocity (v) in a magnetic field (B) is given by: $F = qvB \sin\theta$ Where: $F$ is the magnetic force (in Newtons, N) * $q$ Teacher Activities: Introduction and Brainstorming: Begin by asking students what they know about magnets and their uses in Nigeria. Guide a discussion to elicit initial ideas about magnetic fields.
Demonstration: Plotting Magnetic Fields: Bar Magnet: Demonstrate placing a bar magnet on paper, sprinkling iron filings, and gently tapping to reveal field patterns. Alternatively, use a plotting compass to trace lines. Discuss characteristics like poles and direction (N to S).
Straight Conductor: Set up an experiment with a wire passing through cardboard, connected to a power source. Use plotting compasses or iron filings to show concentric circles when current flows. Apply and explain the Right-Hand Grip Rule.
Solenoid: Set up a solenoid, pass current, and demonstrate its field pattern using iron filings or a compass. Apply and explain the Right-Hand Solenoid Rule to determine polarity.
Practical Session: Making Magnets and Electromagnets: Making a Magnet: Guide students to perform the single-touch and double-touch stroking methods using a strong magnet and a soft iron nail/bar. Discuss the effectiveness and polarity.
Making an Electromagnet: Provide materials (insulated copper wire, large iron nail, battery/power supply, switch). Guide students to construct a simple electromagnet. Demonstrate its ability to pick up paper clips and discuss factors affecting its strength (number of turns, current, core material).
Explanation and Discussion: Electric Bell and Telephone Earpiece: Use diagrams or actual working models (if available) to explain the step-by-step operation of an electric bell and a telephone earpiece, emphasizing the role of the electromagnet and permanent magnets.
Practical: Locating Earth's Magnetic North-South: Provide plotting compasses or suspended bar magnets. Instruct students to use them to find the magnetic North-South direction in the classroom or school compound. Discuss the difference between magnetic and geographic North.
Theoretical Explanation: Force on Moving Charge: Introduce the concept of magnetic force on a moving charge. Explain the formula $F = qvB \sin\theta$ and break down each variable. Demonstrate Fleming's Left-Hand Rule using visual aids and practical examples (e.g., electron in a CRT, force on current-carrying wire).
Problem Solving: Work through the provided guided practice examples step-by-step on the board, encouraging student participation. Explain the implications of the angle $\theta$ on the force and the resulting motion (circular, straight, helical).
Student Activities: Observation and Sketching: Observe the teacher's demonstrations of plotting magnetic fields and sketch the patterns in their notebooks.
Hands-on Experimentation: Participate in plotting field lines around a bar magnet using iron filings and/or plotting compasses. Practically make a temporary magnet from a soft iron bar using the stroking methods. Construct a simple electromagnet and test its strength.
Active Listening and Questioning: Pay attention during explanations of the electric bell, telephone earpiece, and Earth's magnetic field, asking clarifying questions.
Practical Application: Use a plotting compass or suspended magnet to identify the Earth's magnetic North-South direction.
Rule Application: Practice applying the Right-Hand Grip Rule, Right-Hand Solenoid Rule, and Fleming's Left-Hand Rule to determine directions of field, polarity, and force.
Problem Solving: Attempt guided practice problems individually or in pairs, applying the formula for magnetic force and interpreting the results.
Discussion: Engage in class discussions about the principles and real-world applications of magnetic fields.
Electricity Generation in Nigeria: Large hydroelectric power plants like Kainji Dam and thermal power stations (e.g., Egbin Power Plc) utilize massive generators based on the principle of electromagnetic induction. These generators employ strong magnetic fields and rotating coils to produce the electricity that powers homes, businesses, and industries across Nigeria. Understanding magnetic fields helps to appreciate how our national power grid functions. Telecommunications and Entertainment Industry: Magnetic fields are integral to the functionality of loudspeakers and microphones. In Nigeria's vibrant music and film industries, and everyday use of phones and radios, sound is converted to electrical signals (microphones) and back to sound (loudspeakers) using principles of electromagnetism. The varying magnetic field created by an alternating current causes a diaphragm to vibrate, producing sound, enabling clear communication and immersive entertainment. Industrial Sorting and Lifting in Local Industries: In recycling plants, mining operations (e.g., iron ore separation), and scrap metal yards in Nigerian cities like Lagos or Aba, powerful electromagnets are used to lift heavy ferrous metals. This magnetic separation efficiently sorts materials, reduces manual labour, and aids in resource recovery, contributing to local waste management and industrial processes.