RQS - 4TH YEAR

A car battery supplies a current of 50 A to the starter motor. Find the charge passing through the starter in half a minute.

• 1500 C

• 3000 C

• 1800 C

• 2000 C

Answer explanation

To find the charge, use the formula Q = I × t. Here, I = 50 A and t = 30 seconds (half a minute). Thus, Q = 50 A × 30 s = 1500 C. Therefore, the correct answer is 1500 C.

There are 5 x 10¹⁶ electrons passing a section of a conductor in 1 minute and 20 seconds. Find the magnitude of the current.

• 1 mA

• 0.01 mA

• 0.1 mA

• 10 mA

Answer explanation

To find the current, use I = Q/t. Here, Q = 5 x 10^16 electrons x 1.6 x 10^-19 C/electron = 8 x 10^-4 C. Time = 80 s. Thus, I = 8 x 10^-4 C / 80 s = 0.01 A = 0.1 mA. Therefore, the correct answer is 0.1 mA.

A copper wire of 3 mm² cross-sectional area carries a current of 5 A. Find the velocity of the electrons in the wire.

• 0.122 mm/s

• 0.134 mm/s

• 0.128 mm/s

• 0.142 mm/s

Answer explanation

To find the electron velocity, use the formula: v = I/(nAe), where I is current, n is electron density (approximately 8.5 x 10^28 m^-3 for copper), and e is charge of an electron (1.6 x 10^-19 C). This gives v ≈ 0.122 mm/s.

A bar magnet has a cross-section of 1 cm by 2 cm. If the flux density inside the magnet is 0.14T, find the total flux on the magnet.

Answer explanation

To find the total flux, use the formula \( \Phi = B \times A \). The area \( A = 1 \text{ cm} \times 2 \text{ cm} = 2 \text{ cm}^2 = 2 \times 10^{-4} \text{ m}^2 \). Thus, \( \Phi = 0.14 \text{ T} \times 2 \times 10^{-4} \text{ m}^2 = 28 \mu Wb \).

A flux of 13.6 mWb penetrates a coil of wire 240 mm in diameter. Find the magnitude of the magnetic flux density in Tesla if the plane of the coil is perpendicular to the field.

• 0.3

• 0.8

• 0.4

• 0.5

Answer explanation

To find the magnetic flux density (B), use the formula B = Φ/A. The area A of the coil is πr², where r = 0.12 m (240 mm diameter). Thus, A = π(0.12)² ≈ 0.0454 m². B = 13.6 mWb / 0.0454 m² ≈ 0.3 T.

According to the convention, the magnetic field emanates

• from north pole to north pole

• from north pole to south pole

• from south pole to soutth pole

• from south pole to north pole

Answer explanation

According to convention, the magnetic field lines are drawn from the north pole to the south pole. This means that the correct answer is 'from north pole to south pole', as it reflects the direction of magnetic field lines.

It is known as the magnetic constant.

• Permeability

• Permittivity

• Resisitivity

• Conductivity

Answer explanation

The magnetic constant, also known as permeability, is a measure of how a material responds to a magnetic field. It is denoted by the symbol μ and is crucial in electromagnetism, distinguishing it from permittivity, resistivity, and conductivity.

As the current through the straight wire increases, the magnetic field:

• Increase

• Decreases

• Remains the same

• Is independent of the current

Answer explanation

As the current through a straight wire increases, the magnetic field around it also increases. This is due to Ampère's law, which states that the magnetic field is directly proportional to the current.

The maximum force happen when the current and magnetic field are:

• Parallel

• Perpendicular

• Equal

• Constant

Answer explanation

The maximum force on a current-carrying conductor in a magnetic field occurs when the current and magnetic field are perpendicular. This is described by the equation F = IBL sin(θ), where θ is the angle between the current and magnetic field.

What is the wavelength of a carrier wave in meters of the radio station that is broadcasting at a frequency of 100 megahertz?

• 1.5 m

• 2.0 m

• 2.5 m

• 3.0 m

Answer explanation

The wavelength (λ) can be calculated using the formula λ = c/f, where c is the speed of light (approximately 3 x 10^8 m/s) and f is the frequency. For 100 MHz (100 x 10^6 Hz), λ = 3 x 10^8 m/s / 100 x 10^6 Hz = 3.0 m.