Science: Physics Essentials

A lamp has a resistance of 4 Ω and a potential difference of 12 V across it. What current flows through it?

• A8 A
• B0.33 A
• C48 A
• D3 A✓

Answer: From V = IR, I = V ÷ R = 12 ÷ 4 = 3 A. Multiplying gives 48, and 0.33 comes from dividing the wrong way round.

A 2 kg ball is moving at 3 m/s. What is its kinetic energy?

• A18 J
• B3 J
• C6 J
• D9 J✓

Answer: Kinetic energy = ½ × mass × speed² = 0.5 × 2 × 3² = 0.5 × 2 × 9 = 9 J. Forgetting to square the speed gives 6 J, and forgetting the half gives 18 J.

Which of these is a renewable energy resource?

• ANuclear fuel
• BNatural gas
• CWind✓
• DCoal

Answer: Wind is replenished naturally and will not run out, so it is renewable. Coal and natural gas are fossil fuels, and nuclear fuel (uranium) is also a finite resource.

A motor transfers 1,000 J of electrical energy and does 600 J of useful work. What is its efficiency?

• A60%✓
• B40%
• C6%
• D167%

Answer: Efficiency = useful energy out ÷ total energy in = 600 ÷ 1,000 = 0.6, or 60%. 40% is the wasted fraction, and an efficiency above 100% is impossible.

Which statement about the three states of matter is correct?

• AParticles in a liquid are held in fixed positions
• BParticles in a solid are far apart and move freely
• CParticles in a gas are far apart and move randomly at high speed✓
• DParticles in a gas are packed closely together in a regular pattern

Answer: In a gas the particles are widely spaced and move rapidly and randomly. Solids have particles in fixed positions vibrating in place, and liquids have particles close together but able to move past each other.

A car accelerates from rest to 20 m/s in 5 seconds. What is its acceleration?

• A15 m/s²
• B100 m/s²
• C0.25 m/s²
• D4 m/s²✓

Answer: Acceleration = change in velocity ÷ time = (20 − 0) ÷ 5 = 4 m/s². Multiplying gives 100, dividing the wrong way gives 0.25, and 15 is 20 − 5.

A force of 60 N pushes a box 5 m along the floor. How much work is done?

• A65 J
• B300 J✓
• C3,000 J
• D12 J

Answer: Work done = force × distance = 60 × 5 = 300 J. Dividing gives 12, adding gives 65, and 3,000 misplaces a zero.

Which statement about series and parallel circuits is correct?

• AIn a series circuit the same current flows through every component✓
• BIn a parallel circuit the voltage is shared between the branches
• CIn a series circuit each component receives the full supply voltage
• DIn a parallel circuit the current is the same in every branch

Answer: Components in series carry the same current and share the voltage; components in parallel each receive the full voltage and the current splits between the branches.

Which of these is a vector quantity?

• AMass
• BTemperature
• CSpeed
• DVelocity✓

Answer: A vector has both size and direction: velocity is speed in a stated direction. Speed, mass and temperature have size only, so they are scalars.

A kettle rated at 3,000 W is used for 2 minutes. How much energy does it transfer?

• A6,000 J
• B1,500 J
• C360 J
• D360,000 J✓

Answer: Energy = power × time, with time in seconds: 2 minutes = 120 s, so 3,000 × 120 = 360,000 J (360 kJ). Using 2 as the time gives 6,000 J.

Which of these is a non-renewable energy resource?

• ACoal✓
• BHydroelectric
• CTidal
• DSolar

Answer: Coal is a fossil fuel that took millions of years to form and cannot be replaced once burned. Solar, tidal and hydroelectric energy are renewable because their sources are continually replenished.

Sound cannot travel through a vacuum. Why not?

• ASound is a vibration that needs particles to pass it on, and a vacuum has none✓
• BSound is absorbed by the walls of the container that holds the vacuum before it can cross the space
• CSound travels too slowly to cross a vacuum
• DSound is a form of light with a much longer wavelength, and a vacuum blocks light of that wavelength

Answer: Sound is a mechanical wave carried by particles vibrating and colliding; with no particles there is nothing to carry it. Light, by contrast, is electromagnetic and crosses a vacuum easily.