UPCAT Physics — Electromagnetism, Mirrors & OpticsFlash Cards

State Ohm's Law and solve: Find the current when voltage is 12V and resistance is 4Ω.

Ohm's Law: V = IR where V = voltage (volts), I = current (amperes), R = resistance (ohms). Solution: I = V/R = 12V ÷ 4Ω = 3A. Answer: 3 amperes

Why do we use fuses in electrical circuits?

Fuses protect circuits from overcurrent by melting when current exceeds safe limits. This breaks the circuit, preventing damage to appliances and fire hazards. Example: A 15A fuse will melt if current exceeds 15 amperes, protecting household wiring.

Calculate total resistance in series: Three resistors of 2Ω, 3Ω, and 5Ω connected in series.

Series formula: R_total = R₁ + R₂ + R₃ + ... Step 1: R_total = 2Ω + 3Ω + 5Ω = 10Ω. Answer: 10 ohms

Calculate total resistance in parallel: Two resistors of 6Ω and 3Ω connected in parallel.

Parallel formula: 1/R_total = 1/R₁ + 1/R₂. Step 1: 1/R_total = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2. Step 2: R_total = 2Ω. Answer: 2 ohms

What is the difference between AC and DC current?

DC (Direct Current): electrons flow in one direction continuously. Used in batteries, phones. AC (Alternating Current): electrons rapidly change direction back and forth. Used in household electricity (60Hz in Philippines). AC is easier to transform voltage levels for transmission.

How does an electromagnet work and give an example?

An electromagnet uses electric current through wire coils around iron core to create magnetic field. Current on = magnetic, current off = no magnetism. Example: MRI machines, electric motors, doorbell buzzers. Stronger current = stronger magnetic field.

State the Law of Reflection and calculate: Light hits mirror at 30° to normal.

Law of Reflection: θᵢ = θᵣ (angle of incidence equals angle of reflection). Given: θᵢ = 30°. Therefore: θᵣ = 30°. The reflected ray makes 30° with the normal.

What type of image does a plane mirror always produce?

Plane mirrors always produce virtual, erect, and same-size images. The image appears the same distance behind the mirror as the object is in front. Example: bathroom mirror shows your face upright, same size, but you can't project it on a screen (virtual).

Apply Snell's Law: Light travels from air (n=1.0) to water (n=1.33) at 45° incidence angle.

Snell's Law: n₁sin θ₁ = n₂sin θ₂. Step 1: (1.0)(sin 45°) = (1.33)(sin θ₂). Step 2: 0.707 = 1.33 sin θ₂. Step 3: sin θ₂ = 0.707/1.33 = 0.531. Step 4: θ₂ = 32.1°. Light bends toward normal in denser medium.

Why does light bend when entering water?

Light bends due to refraction - change in speed when entering different medium. Light slows down in denser materials (water) compared to air. This speed change causes direction change. Bends toward normal when entering denser medium, away from normal when entering less dense medium.

What type of images do convex mirrors always produce?

Convex mirrors always produce virtual, erect, and smaller images regardless of object position. Used as side mirrors in vehicles because they show wider field of view. Image is always between mirror and focal point, appearing diminished but covering more area.

For concave mirror: Object at center of curvature, describe the image formed.

When object is at center of curvature of concave mirror: Image is real, inverted, and same size as object. Image forms at center of curvature (same location as object). Example: used in telescopes for precise image formation.

What happens when object is placed at the focal point of a concave mirror?

When object is exactly at focal point of concave mirror, no real image is formed. Light rays reflect parallel to each other and never converge. This principle is used in flashlights and car headlights - bulb at focus produces parallel beam.

Distinguish between converging and diverging lenses.

Converging (convex) lenses: thickest at center, bring parallel rays to focus point. Used in magnifying glasses, cameras. Diverging (concave) lenses: thinnest at center, spread parallel rays apart. Used to correct nearsightedness. Shape determines function.

Calculate power consumed: An appliance uses 5A current at 220V for 2 hours.

Power formula: P = VI. Energy = P × t. Step 1: P = 220V × 5A = 1100W = 1.1kW. Step 2: Energy = 1.1kW × 2h = 2.2kWh. Step 3: Cost = 2.2kWh × rate per kWh. Answer: Power = 1100W, Energy = 2.2kWh

Why are transformers used in electrical power transmission?

Transformers change AC voltage levels efficiently. High voltage reduces current for same power (P=VI), minimizing power loss in transmission lines (P_loss = I²R). Step-up transformers increase voltage for transmission, step-down transformers reduce voltage for safe household use (220V in Philippines).

What creates static electricity and give two examples?

Static electricity forms when electrons transfer between materials through friction, creating charge buildup. Examples: 1) Rubbing balloon on hair makes balloon negatively charged, attracts hair. 2) Walking on carpet transfers electrons, causing shock when touching metal doorknob.

Calculate equivalent resistance: 4Ω and 12Ω resistors in parallel, then in series with 6Ω.

Step 1: Parallel combination: 1/R₁₂ = 1/4 + 1/12 = 3/12 + 1/12 = 4/12 = 1/3, so R₁₂ = 3Ω. Step 2: Series with 6Ω: R_total = 3Ω + 6Ω = 9Ω. Answer: 9 ohms total resistance

How do electric motors work using electromagnetic principles?

Electric motors use electromagnetic induction. Current through wire coil in magnetic field experiences force (motor effect). Rotating coils continuously change current direction, maintaining rotation. Example: electric fans, washing machines. Converts electrical energy to mechanical motion through magnetism.

Why do we see objects and how does vision work optically?

We see objects because they reflect light into our eyes. Eye's cornea and lens focus light on retina (like camera). Convex lens in eye refracts light to form inverted image on retina. Brain interprets signals as upright image. Eyeglasses correct focusing problems by adding converging or diverging power.