ESAT Physics practice questions

A polythene rod held by an insulating handle is rubbed with a dry woollen cloth. Afterwards the rod has a negative charge.

Which of the following statements is/are correct?

1. Electrons have been transferred from the cloth to the rod.

2. The cloth is left with a positive charge.

3. If the charged rod is then touched against an earthed metal water pipe, electrons flow from the earth onto the rod.

A small float on a pond bobs up and down as water waves pass it, making 12 complete oscillations in 4.0 s. The horizontal distance between a crest and the neighbouring trough is 0.25 m.

What is the speed of the waves?

A car is travelling at 30 m s⁻¹ when the driver sees a hazard. The driver's reaction time is 0.60 s, during which the car continues at constant speed. The brakes then give the car a constant deceleration of 5.0 m s⁻² until it stops.

What is the total distance the car travels from the moment the driver sees the hazard until it stops?

Four types of electromagnetic radiation are X-rays, microwaves, visible light and infrared.

Which option lists them in order of decreasing wavelength (longest wavelength first)?

A steady current of 3.2 mA flows in a wire for 5.0 s. The magnitude of the charge on an electron is $1.6\times {10}^{-19}$ C.

How many electrons pass a given point in the wire in this time?

Which of the following statements is/are correct?

1. When a pan of water is heated from below, the warmed water near the bottom rises because it has become less dense than the cooler water above it.

2. At the same temperature, a matt black surface emits thermal radiation at a lower rate than a shiny silver surface of the same area.

3. If the air in the gap between the two panes of a double-glazed window were replaced by a vacuum, energy could still be transferred across the gap by thermal radiation.

A ship's sonar sends a short pulse of ultrasound of frequency 50 kHz vertically downwards. The echo from the seabed is detected 0.24 s after the pulse is sent. The speed of sound in seawater is 1500 m s⁻¹.

What is the depth of the sea below the ship, and what is the wavelength of the ultrasound in seawater?

A ball of mass 0.20 kg travelling horizontally at 15 m s⁻¹ strikes a vertical wall and rebounds horizontally along the same line at 10 m s⁻¹. The ball is in contact with the wall for 0.050 s.

What is the size of the average force exerted by the wall on the ball?

A 6.0 Ω resistor and a 12 Ω resistor are connected in parallel with each other. This pair is connected in series with a 4.0 Ω resistor across the terminals of a 12 V battery. Ignore the resistance of the battery and the connecting wires.

What is the power dissipated in the 12 Ω resistor?

Thorium-232, ${}_{90}^{232}\mathrm{T}\mathrm{h}$, decays through a chain of alpha and beta-minus decays to the stable nuclide lead-208, ${}_{82}^{208}\mathrm{P}\mathrm{b}$.

How many alpha decays and how many beta-minus decays occur in the chain?

An electric kettle has a power input of 2.1 kW and transfers 90% of its input energy to the water inside it. The kettle contains 0.60 kg of water at 25 °C. The specific heat capacity of water is 4200 J kg⁻¹ °C⁻¹.

How long does it take to bring the water to 100 °C? Ignore any water that evaporates.

A submarine has a small flat window of area 0.020 m². The submarine is at rest with the window 40 m below the surface of the sea. Treat the water pressure as the same over the whole window.

The density of seawater is 1000 kg m⁻³, g = 10 N kg⁻¹ and atmospheric pressure is 100 kPa. The air inside the submarine is kept at atmospheric pressure.

What is the size of the resultant force on the window from the water outside and the air inside?

A straight horizontal wire runs north–south. A 0.25 m length of the wire lies in a uniform magnetic field of flux density 0.30 T that is directed vertically downwards. A current of 4.0 A flows through the wire towards the north.

What are the size and direction of the force on this length of wire?

A ray of light travelling in glass meets a flat glass–air boundary at an angle of incidence of 30° and passes out into the air.

Which of the following statements about the light as it crosses the boundary is/are correct?

1. Its frequency increases.

2. Its wavelength increases.

3. It bends away from the normal.

Two identical light springs each have spring constant 200 N m⁻¹ and obey Hooke's law.

In arrangement P, the springs are joined end to end, one below the other, and hung from a fixed support. A 6.0 N weight hangs from the bottom of the lower spring.

In arrangement Q, the springs hang side by side from the support, and the same 6.0 N weight hangs from a light bar fixed to both lower ends, so that the springs share the load equally.

What is the value of $\frac{\text{total elastic energy stored in P}}{\text{total elastic energy stored in Q}}$?

The table gives the velocity of a trolley moving along a straight track at certain times. Between consecutive times in the table, the velocity changes at a steady rate. A negative velocity means the trolley is moving in the opposite direction to its initial motion.

What are the average speed and the average velocity of the trolley over the 10 s?

An ideal (100% efficient) transformer has 1200 turns on its primary coil, which is connected to a 240 V ac supply. The secondary coil supplies a 12 V lamp, which works normally with a current of 5.0 A.

How many turns are on the secondary coil, and what is the current in the primary coil?

A 0.20 kg block of ice at −10 °C is to be turned completely into water at 20 °C.

Specific heat capacity of ice = 2000 J kg⁻¹ °C⁻¹

Specific heat capacity of water = 4200 J kg⁻¹ °C⁻¹

Specific latent heat of fusion of ice = 340 000 J kg⁻¹

How much energy must be supplied, assuming none is lost to the surroundings?

A fixed resistor of resistance 1.0 kΩ and an NTC thermistor are connected in series to a 6.0 V supply. A voltmeter with a very high resistance is connected across the fixed resistor.

At 20 °C the resistance of the thermistor is 2.0 kΩ. When the thermistor is warmed to 60 °C, its resistance changes by a factor of 4.

How does the voltmeter reading change as the thermistor is warmed from 20 °C to 60 °C?

An electric motor is connected to a 100 V supply and draws a current of 5.0 A. It lifts a 50 kg load vertically upwards at a constant speed of 0.40 m s⁻¹. Take g = 10 N kg⁻¹.

What is the efficiency of the motor?

A radioactive source is placed 2.0 cm from a detector in air. Different absorbers are placed between the source and the detector, and the count rate is measured each time. All the count rates have been corrected for background radiation.

Which type or types of radiation does the source emit?

A simple ac generator has a coil that rotates at a steady rate in a uniform magnetic field. When the coil makes 50 revolutions per second, the output voltage has a peak value of 6.0 V and a frequency of 50 Hz.

The coil is replaced by one of the same shape and size but with half as many turns, and it is rotated at 100 revolutions per second in the same field.

What are the new peak voltage and frequency?

Trolley X, of mass 3.0 kg, moves to the right at 4.0 m s⁻¹ along a straight horizontal track. It collides with trolley Y, of mass 1.0 kg, which is moving to the left at 6.0 m s⁻¹. The trolleys stick together. Ignore friction.

How much kinetic energy is transferred to other energy stores in the collision?

A radioactive isotope X decays with a half-life of $1.3\times {10}^9$ years to a stable isotope Y. A mineral sample contained X but no Y when it formed, and no atoms of X or Y have entered or left the sample since.

Today the sample contains 3 atoms of Y for every atom of X.

What will the ratio of Y atoms to X atoms be $2.6\times {10}^9$ years from now?

A power station transmits 40 MW of electrical power to a town through cables with a total resistance of 5.0 Ω. At present the power is transmitted at 200 kV.

If the same 40 MW were transmitted at 400 kV through the same cables, what would the power lost as heat in the cables be?

An air bubble has a volume of 1.0 cm³ when it is 20 m below the surface of a lake. It rises slowly, and its temperature and the mass of air inside it stay constant.

Atmospheric pressure is 100 kPa, the density of the water is 1000 kg m⁻³ and g = 10 N kg⁻¹.

At what depth below the surface will the volume of the bubble be 2.0 cm³?

A block of mass 2.0 kg rests on a horizontal table. It is attached to a light, inextensible string that passes over a light, frictionless pulley at the edge of the table and supports a hanging mass of 3.0 kg.

When the system is released from rest, the block slides along the table and a constant frictional force of 5.0 N acts on it. Take g = 10 N kg⁻¹.

What is the tension in the string while the masses are moving?