CXC - Caribbean Examinations Council Physics: Electricity and Magnetism Questions and Answers — Questions and Answers
Question 1: Which of the following I-V graphs correctly represents the behavior of an ohmic conductor, such as a fixed resistor at constant temperature?
- A curved line starting from the origin with a decreasing gradient.
- A straight line passing through the origin with a constant positive gradient. (Correct answer)
- A horizontal line indicating constant current regardless of voltage.
- A straight line that does not pass through the origin.
Correct answer: A straight line passing through the origin with a constant positive gradient.
According to Ohm's Law (V=IR), for an ohmic conductor at constant temperature, the current (I) is directly proportional to the potential difference (V) across it. This direct proportionality is represented graphically as a straight line passing through the origin. The gradient of an I-V graph (I on the y-axis, V on the x-axis) is equal to 1/R, which is constant for an ohmic conductor.
Question 2: A student builds an electromagnet by wrapping a copper wire around an iron nail and connecting it to a battery. Which of the following changes would be MOST effective in increasing the strength of the electromagnet?
- Replacing the iron nail with a wooden dowel.
- Decreasing the number of turns in the coil.
- Using a battery with a lower voltage.
- Increasing the number of turns in the coil around the nail. (Correct answer)
Correct answer: Increasing the number of turns in the coil around the nail.
The strength of an electromagnet is directly proportional to two main factors: the current flowing through the coil and the number of turns (or windings) in the coil. Therefore, increasing the number of turns will increase the magnetic field strength. Replacing the iron core with wood (a non-magnetic material) would weaken it, while decreasing the turns or the voltage (which reduces the current) would also weaken the electromagnet.
Question 3: A household appliance is rated at 1200 W and is designed to operate on a 240 V supply. What is the correct current drawn by the appliance when it is operating normally?
- 0.2 A
- 288,000 A
- 5.0 A (Correct answer)
- 20 A
Correct answer: 5.0 A
The relationship between power (P), voltage (V), and current (I) is given by the formula P = V × I. To find the current, we can rearrange the formula to I = P / V. Substituting the given values: I = 1200 W / 240 V = 5.0 A.
Question 4: Two resistors, one with a resistance of 6 Ω and the other with 3 Ω, are connected in parallel to a 12 V battery. What is the total equivalent resistance of the circuit?
- 9 Ω
- 0.5 Ω
- 2 Ω (Correct answer)
- 18 Ω
Correct answer: 2 Ω
For resistors connected in parallel, the reciprocal of the total resistance (1/R_T) is the sum of the reciprocals of the individual resistances (1/R_1 + 1/R_2 + ...). So, 1/R_T = 1/6 Ω + 1/3 Ω. To add these fractions, find a common denominator: 1/R_T = 1/6 + 2/6 = 3/6 Ω⁻¹. Simplifying gives 1/R_T = 1/2 Ω⁻¹. Therefore, the total resistance R_T is the reciprocal of this value, which is 2 Ω. An important check is that the total resistance in a parallel circuit is always less than the smallest individual resistance.
Question 5: A step-down transformer is used to convert a high input voltage to a lower output voltage. This is achieved because the secondary coil has:
- fewer turns than the primary coil. (Correct answer)
- more turns than the primary coil.
- the same number of turns as the primary coil.
- a thicker wire than the primary coil.
Correct answer: fewer turns than the primary coil.
A step-down transformer works on the principle of electromagnetic induction. Its purpose is to reduce voltage. The ratio of the voltages between the primary (Vp) and secondary (Vs) coils is equal to the ratio of the number of turns in each coil (Np and Ns). For a step-down transformer, the output voltage (Vs) must be lower than the input voltage (Vp), which means the number of turns in the secondary coil (Ns) must be fewer than the number of turns in the primary coil (Np).
Question 6: Which statement BEST describes the process of charging a neutral metal sphere by induction using a negatively charged rod?
- The rod touches the sphere, transferring negative charges to it.
- The rod is brought near the sphere, and protons are attracted from the ground onto the sphere.
- The rod repels electrons in the sphere, the sphere is grounded allowing electrons to leave, and the ground is removed before the rod, leaving a net positive charge. (Correct answer)
- The rod attracts electrons from the sphere, causing it to become positively charged without needing a ground connection.
Correct answer: The rod repels electrons in the sphere, the sphere is grounded allowing electrons to leave, and the ground is removed before the rod, leaving a net positive charge.
Charging by induction involves redistributing charge without direct contact. When a negatively charged rod is brought near a neutral metal sphere, it repels the free electrons in the sphere to the far side. If the sphere is then grounded (e.g., by touching it), these repelled electrons have a path to flow away from the sphere into the ground. The ground connection must be removed while the charged rod is still in place to trap the net positive charge (left by the departed electrons). Finally, when the rod is removed, the sphere is left with an overall positive charge.
CXC - Caribbean Examinations Council Physics: Electricity and Magnetism Questions and Answers
Which of the following I-V graphs correctly represents the behavior of an ohmic conductor, such as a fixed resistor at constant temperature?