Poland Szkoła podstawowa, klasa VIII Physics

Chapters: 4

1. Electricity

Electric charge and electrostatics · Current, voltage and resistance · Electrical energy and circuits · Experiments: electricity

Static Electricity – Rubbing two different materials moves electrons from one to the other. The one that gains electrons becomes negative; the one that loses them becomes positive. Charge is never made or destroyed, only moved. Like charges repel and unlike charges attract. A charged object can pull a neutral one by induction. Earthing lets extra charge flow safely away.
Ohm's Law – At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = I × R. R is the resistance, measured in ohms (Ω); 1 Ω = 1 V/1 A. The V–I graph of an ohmic conductor is a straight line through the origin.
Electric Current, Potential Difference and Electric Circuits – Electric current is the rate of flow of charge, I = Q/t, measured in amperes with an ammeter joined in series. Potential difference is the work done to move a unit charge between two points, V = W/Q, measured in volts with a voltmeter joined in parallel. Charge flows only in a closed circuit.

Magnets and the compass · Magnetic effect of current

Magnetism: Magnets and the Compass – A magnet pulls iron, nickel and cobalt. Its pull is strongest at its two ends, the north (N) and south (S) poles. Like poles repel and unlike poles attract. Around a magnet is a magnetic field, drawn as lines from N to S; a compass needle lines up with it. Inside a magnet, tiny atomic magnets all point the same way. The Earth is a giant magnet, so a compass points north. An electric current in a coil makes an electromagnet.
Magnetic Effect of Electric Current – A wire carrying electric current makes a magnetic field around itself. The field forms circles around a straight wire, becomes nearly straight at the centre of a loop, and is uniform inside a solenoid, which then acts like a bar magnet.

Periodic motion · Mechanical waves and sound

Simple Harmonic Motion (SHM) – A motion that repeats after a fixed time is periodic. If the object goes to and fro about a middle point and the force pulling it back is proportional to its distance from the middle (F = −kx), the motion is simple harmonic. Its position is x = A sin(ωt + φ), with ω = 2π/T = 2πf. Speed is largest in the middle, acceleration is largest at the ends, and total energy ½kA² stays constant.
Sound – Sound is made by vibrating objects. It travels through a medium (air, water, solids) as a longitudinal wave: particles move back and forth, making crowded parts (compressions) and spread-out parts (rarefactions). Frequency (Hz) sets the pitch, amplitude sets the loudness, and speed v = f × λ. Sound cannot travel in vacuum. Humans hear 20 Hz to 20,000 Hz; below is infrasound, above is ultrasound. Reflected sound gives echoes, used and controlled in buildings.

Light travel and reflection · Refraction and lenses · Colour and electromagnetic waves · Experiments: optics

Reflection of Light: Plane and Spherical Mirrors – Light bounces off a shiny surface so that the angle of incidence equals the angle of reflection. A plane mirror gives a virtual, upright, same-size image as far behind as the object is in front. A concave mirror can give real or virtual images depending on where the object is; a convex mirror always gives a virtual, upright, smaller image. The mirror formula 1/v + 1/u = 1/f and magnification m = −v/u (with the New Cartesian sign convention) let you find the image without drawing.
Spherical Lenses: Images, Lens Formula and Power – A convex lens is thick in the middle and bends parallel light to meet at its focus; a concave lens is thin in the middle and spreads light out as if from its focus. With two simple rays (one parallel to the axis, one through the optical centre) you can find the image for any object position. A convex lens gives real, inverted images when the object is beyond F₁ and a virtual, erect, enlarged image inside F₁; a concave lens always gives a virtual, erect, diminished image. The lens formula 1/v − 1/u = 1/f, magnification m = v/u and power P = 1/f (in metres, unit dioptre) let you solve lens numericals.