UPCAT Physics — Electromagnetism, Mirrors & OpticsStudy Notes

Electricity is the presence and flow of electric charge, which is fundamental to many devices we use daily. Electric current flows when charges move through a conductor, like water flowing through a pipe. The relationship between voltage, current, and resistance is described by Ohm's Law: V = IR, where V is voltage (measured in volts), I is current (measured in amperes), and R is resistance (measured in ohms). Think of voltage as the 'push' that moves electrons, current as the actual flow of electrons, and resistance as obstacles that slow down this flow. A simple analogy is water flowing through a hose - water pressure is like voltage, the amount of water flowing is like current, and any blockages in the hose represent resistance.

Electric circuits are closed paths that allow current to flow. In series circuits, components are connected one after another like Christmas lights on a string. If one bulb burns out, the whole string goes dark because there's only one path for current. The total resistance in a series circuit equals the sum of all individual resistances: R_total = R1 + R2 + R3 + ... Current remains the same throughout, but voltage is shared among components. In parallel circuits, components branch out from the main path, like the electrical system in your house. Each appliance has its own path to the power source. If one device breaks, others continue working. For parallel circuits, the reciprocal of total resistance equals the sum of reciprocals of individual resistances: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ... Voltage remains the same across each branch, but current divides among the branches.

Magnetism is a force that results from magnetic fields, which can occur naturally in materials like iron or be created by moving electric charges. Every magnet has two poles: north and south. Like poles repel each other, while opposite poles attract. The magnetic field is the invisible area around a magnet where its force can be felt - imagine it like the influence zone around the magnet. Electromagnetism combines electricity and magnetism. When electric current flows through a wire, it creates a magnetic field around the wire. This principle is used in electromagnets, which can be turned on and off by controlling the electric current. Unlike permanent magnets, electromagnets only work when electricity flows through them. This relationship between electricity and magnetism is fundamental to many modern devices.

Plane mirrors are flat reflecting surfaces that create images following specific rules. When you look in a bathroom mirror, the image appears to be the same distance behind the mirror as you are in front of it. This image is virtual (cannot be projected on a screen), erect (right-side up), and the same size as the original object. The image is also laterally inverted - your right hand appears as the left hand of your reflection. The Law of Reflection states that the angle of incidence equals the angle of reflection (θi = θr), where both angles are measured from the normal (an imaginary line perpendicular to the mirror surface). This law explains why you can see yourself in mirrors and why light bounces off surfaces predictably.

Refraction occurs when light changes direction as it passes from one medium to another due to a change in speed. When light travels from air into water, it slows down and bends toward the normal line. When it goes from water back to air, it speeds up and bends away from the normal. This bending is described by Snell's Law: n1 sin θ1 = n2 sin θ2, where n represents the refractive index of each medium and θ represents the angles. The refractive index indicates how much a material slows down light compared to vacuum. Air has a refractive index of about 1, water is 1.333, and diamond is 2.419. The higher the refractive index, the more the material bends light.

Curved mirrors have different properties than plane mirrors. Concave mirrors curve inward (like the inside of a spoon) and can focus light to a point called the focal point. They can create both real images (which can be projected) and virtual images (which cannot be projected), depending on where the object is placed. When the object is far from a concave mirror, the image is real, inverted, and smaller. When the object is close, the image becomes virtual, upright, and magnified - this is why concave mirrors are used for shaving mirrors and makeup mirrors. Convex mirrors curve outward (like the back of a spoon) and always create virtual, upright, and smaller images regardless of object position. They provide a wider field of view, which is why they're used in car side mirrors and security mirrors in stores.

Lenses are transparent materials with curved surfaces that refract light to form images. Converging lenses (convex lenses) are thicker in the middle and bring parallel light rays together to a focal point. They can create both real and virtual images depending on object position, similar to concave mirrors. When the object is far from the lens, the image is real, inverted, and smaller (like in cameras). When the object is close, the image becomes virtual, upright, and magnified (like magnifying glasses). Diverging lenses (concave lenses) are thinner in the middle and spread parallel light rays apart. They always create virtual, upright, and smaller images. Converging lenses are used to correct farsightedness, while diverging lenses correct nearsightedness.