Lesson Notes By Weeks and Term v5 - Grade 12
Revision and examination preparation – Week 8 focus
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Revision and examination preparation – Week 8 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Physical Sciences
Class: Grade 12
Term: Term 4
Week: 8
Theme: General lesson support
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This week focuses on consolidating your understanding of key concepts and refining your exam preparation strategies for Physical Sciences. This is a crucial stage in your Grade 12 year as we move closer to the final exams. Mastering these concepts is not only essential for academic success but also provides a strong foundation for future studies in science, engineering, and related fields. In a country like South Africa, a solid grasp of physical sciences opens doors to opportunities in various sectors, including energy, telecommunications, mining, and environmental management, all of which are vital for national development.
This week's revision will cover: Momentum and Impulse, Electrostatics, Electromagnetic Induction, Waves, Sound and Light, and Photoelectric Effect. Let's delve into each area.
A. Momentum and Impulse: Momentum (p): The product of an object's mass (m) and velocity (v). Mathematically, p = mv. Momentum is a vector quantity, meaning it has both magnitude and direction. The SI unit is kg.m/s.
Impulse (J): The change in momentum of an object. It is also equal to the net force acting on an object multiplied by the time interval for which the force acts. Mathematically, J = Δp = F net Δt. Impulse is also a vector quantity, with the same direction as the net force. The SI unit is N.s.
Law of Conservation of Momentum: In a closed system (no external forces), the total momentum before a collision is equal to the total momentum after the collision. This is a fundamental principle. Mathematically, Σp initial = Σp final *.
Elastic Collision: A collision in which kinetic energy is conserved.
Inelastic Collision: A collision in which kinetic energy is not conserved (usually converted to heat or sound).
Collisions in Two Dimensions: Momentum is conserved in both the x and y directions independently. Requires vector analysis.
Example 1 (Momentum): A taxi with a mass of 1500 kg is traveling east at 20 m/s. A passenger with a mass of 70 kg is sitting inside. What is the total momentum of the taxi and the passenger?
Solution: Calculate the momentum of the taxi: p taxi = m taxi v taxi = (1500 kg)(20 m/s) = 30000 kg.m/s east Calculate the momentum of the passenger: p passenger = m passenger v passenger = (70 kg)(20 m/s) = 1400 kg.m/s east Calculate the total momentum: p total = p taxi + p passenger = 30000 kg.m/s + 1400 kg.m/s = 31400 kg.m/s east Example 2 (Impulse): A soccer ball with a mass of 0.45 kg is moving at 15 m/s towards a player. The player kicks the ball, and it now moves at 22 m/s in the opposite direction. If the contact time between the player's foot and the ball is 0.08 s, what is the average force exerted by the player on the ball?
Solution: Calculate the change in momentum: Δp = m(v f - v i ) = (0.45 kg)(-22 m/s - 15 m/s) = (0.45 kg)(-37 m/s) = -16.65 kg.m/s (The negative sign indicates the change in direction)
Calculate the impulse: J = Δp = -16.65 kg.m/s Calculate the average force: F net = J/Δt = -16.65 kg.m/s / 0.08 s = -208.13 N (The negative sign indicates the force is in the opposite direction of the initial motion)
B. Electrostatics: Electric Charge: A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. Measured in Coulombs (C).
Coulomb's Law: The force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. Mathematically, F = kQ 1 Q 2 /r 2 *, where k = 8.99 x 10 9 N.m 2 /C 2 (Coulomb's constant).
Electric Field (E): The force per unit positive charge experienced at a point in space. A vector quantity. E = F/q. Units are N/
C. For a point charge: E = kQ/r 2 *.
Electric Potential Difference (V): The work done per unit charge to move a charge between two points in an electric field. Measured in Volts (V). V = W/q.
Also related to electric field: V = Ed where d is the distance between the points in a uniform field.
Example 3 (Coulomb's Law): Two point charges, Q 1 = +4 μC and Q 2 = -6 μC, are separated by a distance of 0.3 m. Calculate the magnitude and direction of the force between them.
Solution: Convert microcoulombs to Coulombs: Q 1 = 4 x 10 -6 C, Q 2 = -6 x 10 -6 C Apply Coulomb's Law: F = kQ 1 Q 2 /r 2 = (8.99 x 10 9 N.m 2 /C 2 )(4 x 10 -6 C)(-6 x 10 -6 C) / (0.3 m) 2 = -2.397 N Magnitude: The magnitude of the force is 2.397
N. Direction: Since the charges are of opposite signs, the force is attractive.
C. Electromagnetic Induction: Faraday's Law: The induced electromotive force (EMF) in any closed circuit is equal to the negative of the time rate of change of the magnetic flux through the circuit. Mathematically, ε = -N(ΔΦ/Δt), where ε is the induced EMF, N is the number of turns in the coil, and ΔΦ/Δt is the rate of change of magnetic flux. Magnetic Flux (Φ): The measure of the quantity of magnetism, taken as the product of the magnetic field and the area the field passes through; measured in Weber (Wb). Φ = BAcosθ, where B is the magnetic field strength, A is the area, and θ is the angle between the magnetic field and the normal to the area.
Lenz's Law: The direction of the induced EMF (and current) is such that it opposes the change in magnetic flux that produced it.
Generators: Convert mechanical energy into electrical energy using electromagnetic induction.
Transformers: Used to increase or decrease AC voltage. The voltage ratio is proportional to the turns ratio: V p /V s = N p /N s *, where V p and V s are the primary and secondary voltages, and N p and N s are the number of turns in the primary and secondary coils, respectively.