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Electric current is the rate of flow of electric charge, and its SI unit is the ampere, defined as one coulomb per second. Volt measures potential difference, ohm measures resistance, and watt measures power.
Acceleration is the rate of change of velocity. Uniform velocity means the velocity does not change, so the rate of change is zero and acceleration is zero.
Nitrogen makes up roughly 78 percent of the atmosphere by volume, making it the most abundant gas. Oxygen is second at about 21 percent, while carbon dioxide and argon are present only in small amounts.
Table salt is sodium chloride, formed from sodium and chlorine ions in a 1:1 ratio, giving the formula NaCl. The other formulas represent potassium chloride, calcium carbonate, and sodium hydroxide.
The iris contains muscles that adjust the size of the pupil, thereby regulating how much light enters the eye. The retina detects light, the cornea refracts it, and the optic nerve carries signals to the brain.
The boiling point of a liquid depends on pressure. Water boils at 100 degrees Celsius only at one standard atmosphere (about 101.3 kPa); at lower pressures it boils at lower temperatures.
An inclined plane is a flat surface set at an angle that reduces the force needed to lift a load by increasing the distance over which the force acts. A pulley, lever, and wedge are distinct simple machines.
Sound is a mechanical wave and needs a material medium to propagate. A vacuum contains no particles to vibrate, so sound cannot pass through it, whereas solids, liquids, and gases all transmit sound.
Kinetic energy is given by one-half times mass times velocity squared. Substituting 5 kg and 4 m/s gives 0.5 x 5 x 16 = 40 joules.
Pressure equals force divided by area. Dividing 60 N by 0.3 square metres gives 200 pascals.
A chemical change produces a new substance with different properties. Rusting of iron forms hydrated iron oxide, a new compound, whereas melting, dissolving, and evaporation only change physical state or dispersion.
The pH scale runs from 0 to 14, with 7 representing a neutral solution where hydrogen ion and hydroxide ion concentrations are equal. Values below 7 are acidic and above 7 are basic.
A convex mirror always forms a diminished, erect virtual image and has a wider field of view, which helps drivers see more of the road behind. Concave mirrors can magnify and invert images depending on distance.
Ohm's law states that potential difference equals current times resistance. Multiplying 0.5 A by 12 ohms gives 6 volts.
Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in the tissues. White blood cells defend the body, platelets aid clotting, and plasma cells produce antibodies.
At the highest point the upward velocity has been fully reduced to zero, but gravity continues to act, so the acceleration remains g directed downward throughout the flight.
Renewable sources are naturally replenished on a human timescale. Wind is continuously generated by atmospheric processes, whereas coal, petroleum, and natural gas are finite fossil fuels.
Sublimation is the direct conversion of a solid to a gas, as seen with dry ice or camphor. Condensation is gas to liquid, fusion is solid to liquid, and deposition is gas to solid.
In a series circuit there is only one path for charge, so the same current flows through every component. The voltage divides among the resistors, and resistance and power can differ for each.
Mitochondria carry out aerobic respiration and produce most of the cell's ATP, the energy currency, which is why they are called the powerhouse of the cell.
Average speed is total distance divided by total time. The total distance is 240 km and total time is 3 hours, giving 80 km/h.
When light passes from a rarer to a denser medium it slows down and bends towards the normal, as described by Snell's law. Its frequency stays constant while its wavelength and speed change.
For a convex lens, when the object lies within the focal length, the refracted rays diverge and the image formed is virtual, erect, and magnified, on the same side as the object. This is the principle of a simple magnifier.
Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. The identity and mass of the gas do not affect this count.
Work done against gravity is weight times height, giving 200 x 5 = 1000 J. Power is work divided by time, so 1000 J divided by 4 s equals 250 W.
Force is measured in newtons, where one newton is the force that gives a 1 kg mass an acceleration of 1 m/s². The other units measure energy, power and pressure respectively.
Root hairs are fine extensions of root epidermal cells that greatly increase the surface area for absorbing water and minerals. Leaves mainly carry out photosynthesis and gas exchange.
A vector has both magnitude and direction; displacement specifies how far and in which direction an object has moved. Mass, temperature and speed have magnitude only.
Mercury orbits nearest to the Sun, at an average distance of about 58 million kilometres. Venus, Earth and Mars lie farther out in that order.
Ultraviolet light from the Sun converts a cholesterol derivative in the skin into vitamin D. The other vitamins listed must be obtained mainly from the diet.
A barometer measures atmospheric pressure, commonly using a column of mercury or an aneroid capsule. The other instruments measure humidity, temperature and current.
For free fall from rest, speed equals g multiplied by time, so 10 × 3 = 30 m/s. The mass of the stone does not affect the acceleration due to gravity.
Charge equals current multiplied by time in seconds, so 2 × 300 = 600 C. The time must first be converted from minutes to seconds.
A plane mirror forms an image as far behind the mirror as the object is in front, so the image is 30 cm behind it. The distance between object and image is therefore 30 + 30 = 60 cm.
Oxides of non-metals such as sulphur are acidic and form an acid with water; sulphur dioxide gives sulphurous acid. Oxides of metals are generally basic.
For an ideal lever the mechanical advantage equals effort arm divided by load arm, so 80 ÷ 20 = 4. This means the effort needed is one quarter of the load.
A pH of 3 lies well below 7 and close to the lower end of the scale, indicating a strongly acidic solution. Values near 7 are neutral and above 7 are basic.
During photosynthesis, green plants use carbon dioxide and water in the presence of light to make glucose and release oxygen. Respiration, by contrast, consumes oxygen.
For two resistors in parallel the reciprocal of the equivalent resistance is the sum of the reciprocals, so 1/R = 1/6 + 1/3 = 1/2, giving R = 2 ohms.
Heat absorbed equals mass × specific heat capacity × temperature rise, so 0.5 × 400 × 20 = 4000 J. The specific heat capacity links heat, mass and temperature change.
Ionic compounds contain free-moving ions when molten or dissolved, so they conduct electricity. In the solid state the ions are held in a lattice and cannot move.
The two displacements are perpendicular, so the resultant is found from the Pythagorean relation: the square root of (6² + 8²) equals 10 km. Displacement is a vector, so direction matters.
For a uniform rod the mass is evenly distributed, so the centre of gravity is at its geometric middle, the 50 cm mark. The rod would balance at this point.
Centripetal force equals mass × speed squared ÷ radius, so 4 × 9 ÷ 2 = 18 N. The force is directed towards the centre of the circular path.
Boyle's law states that at constant temperature the pressure of a fixed mass of gas is inversely proportional to its volume. Halving the volume therefore doubles the pressure.
Using 1/v − 1/u = 1/f with u = −30 cm and f = +20 cm gives 1/v = 1/20 − 1/30 = 1/60, so v = 60 cm. The positive sign shows a real image on the far side of the lens.
Zinc displaces hydrogen from the acid, releasing hydrogen gas, which burns with a characteristic pop. Evolution of a new gas is strong evidence of a chemical change.
For a floating body the fraction submerged equals the ratio of the body's density to the liquid's density. Three quarters are submerged, so the density is 0.75 × 1200 = 900 kg/m³.
Energy in kilowatt-hours equals power in kilowatts × time in hours × number of days, so 1 × 3 × 30 = 90 kWh. Converting watts to kilowatts is the key step.
For uniformly accelerated motion starting from rest, the distance covered is given by s = ut + ½at². With u = 0, a = 4 m/s² and t = 5 s, this gives s = ½ × 4 × 25 = 50 m.