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Define the SI unit of electric current.
Current is charge flow per unit time, \(I=\frac{Q}{t}\). One ampere is the current when one coulomb passes a cross-section each second.
One ampere = one coulomb per second.
A myopic eye has a far point of \(1.2\,\mathrm{m}\). Which type of lens restores distant vision?
A concave lens diverges the nearly parallel rays from a distant object so that they appear to come from the person's far point. The eye can then focus them on the retina. Its focal length and power must be negative.
A concave (diverging) lens.
A spherical mirror has radius of curvature \(20\,\mathrm{cm}\). Determine its focal-length magnitude.
For a spherical mirror, \(f=\frac{R}{2}\).
The sign would be negative for a concave mirror and positive for a convex mirror.
Focal-length magnitude = \(10\,\mathrm{cm}\).
How does variation improve the chance that a species survives?
Individuals in a population differ. If climate, food, predators or disease changes, some variants may possess characters that confer better survival and reproduction. Those variants pass their alleles to the next generation, preventing the entire population from being eliminated and enabling adaptation over time.
Variation ensures that some individuals may survive and reproduce under changed conditions.
Why can variation benefit a species even when it does not benefit every individual?
A particular variation may be neutral or disadvantageous in present conditions. If the environment changes, however, some variants may survive better than others. Those individuals reproduce and keep the population from being eliminated. Variation therefore provides population-level insurance, although no single variant is useful in every situation.
Variation increases the chance that some members survive environmental change.
Describe transmission of a signal across a synapse.
An electrical impulse reaching the axon terminal causes neurotransmitter to be released into the synaptic cleft. The chemical diffuses across the gap and binds to receptors on the next neuron's dendrite or cell body. Ion movement then starts a new electrical impulse. Because transmitter is released only from the presynaptic ending, transmission is normally one-way.
Neurotransmitter crosses the synaptic cleft and initiates a new impulse in the next neuron.
Which observations are used to decide whether an object is living?
Visible movement can suggest life but is not sufficient. The reliable criterion is the continuous occurrence of organised metabolic and maintenance processes: nutrition, respiration, transport, excretion, growth, response and cellular repair. Some of these continue even when no external movement is visible.
Living things carry out cellular metabolism and life-maintenance processes.
Show the electron-dot arrangement of an eight-atom sulphur molecule, \(\ce{S8}\), whose atoms form a ring.
Each sulphur atom has six valence electrons. In the \(\ce{S8}\) molecule, every sulphur shares one electron pair with each of its two neighbouring sulphur atoms. The eight S atoms therefore form a closed puckered ring containing eight S-S single bonds. Each S retains two lone pairs and completes its octet.
A closed \(\ce{S8}\) ring with one S-S bond on every side and two lone pairs on each sulphur atom.
Define the terms malleable and ductile.
A material is malleable if it can be hammered or rolled into thin sheets without breaking. It is ductile if it can be drawn into thin wires. Gold and aluminium are malleable; copper is ductile.
Malleable: can form sheets. Ductile: can be drawn into wires.
Explain why curd, pickle and other sour foods should not be stored in brass or copper vessels.
Sour foods contain acids. These acids can react with copper or brass and form soluble metal salts.
The salts may contaminate the food and can be harmful if consumed. The reaction also attacks the vessel. Therefore, sour food should be stored in glass, ceramic or food-grade containers that do not react with acids.
The acids in sour food react with copper or brass and may form harmful metal salts.