CXC - Caribbean Examinations Council Physics: Electricity and Magnetism 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?

Accepted Answer

A straight line passing through the origin with a constant positive gradient.

Answer

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?

Accepted Answer

Increasing the number of turns in the coil around the nail.

Answer

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?

Accepted Answer

5.0 A

Answer

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?

Accepted Answer

2 Ω

Answer

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:

Accepted Answer

fewer turns than the primary coil.

Answer

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).

CXC - Caribbean Examinations Council Practice Test

CXC - Caribbean Examinations Council Practice Test

CXC - Caribbean Examinations Council Physics: Electricity and Magnetism Questions and Answers