Category: Pure Physics

Movement of electrons in discharge ball

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An electrostatic generator is used to produce sparks as shown below.

The belt carries negative charges to the dome, making it negatively charged. Before a spark is produced, the discharge ball becomes positively charged.

Describe and explain the movement of electrons in the discharge ball and in the conducting rod as the ball becomes charged.

The electrons in the discharge ball and in the conducting rod moves downwards through the conducting wire, and to the ground.
Electrostatic charging by induction occurs in the ball and rod as they are placed near to the negatively charged dome. Like charges repel, causing the negatively charged electrons in the ball and rod to move away from the dome, escaping through the conducting wire to the ground.

Movement of air particles in sound waves

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A sound wave in air is illustrated below.

Points A, B, C and D are all the centers of regions of compression in the sound wave.
Describe what happens as one complete sound wave moves past a point.

The particles in the air are disturbed by the sound waves and vibrates to and fro, in a direction parallel to the direction of the sound waves.
An air particle in the sound wave will vibrate from its rest position to the left, until it reaches its maximum displacement to the left, then vibrate towards the right, move past its rest position, until it reaches its maximum displacement to the right. Then it vibrates to the left, until it returns back to its original rest position to complete one wave motion of the sound wave.

Differences between boiling and evaporation

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BoilingEvaporation
Occurs at a fixed temperatureOccurs at any temperature below boiling point
Quick processSlow process
Takes place throughout the liquidTakes place at the surface of the liquid
Bubbles formNo bubbles
Temperature remains constantTemperature may vary
Thermal energy supplied by energy sourceThermal energy supplied by surroundings

Advantages and Disadvantages of a Thermocouple Thermometer

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AdvantagesDisadvantages
Can measure a wide range of temperature, from -100 deg C to 2500 deg C Low sensitivity to small temperature changes
Can measure temperature at a pointRelatively low accuracy, degree of error is 1 deg C to 2 deg C
Rugged, not easily damagedProne to corrosion, due to two metals of different reactivity being in contact
Respond quickly to temperature changesLow voltage output, which makes the output easily distorted by interference from nearby electronic devices.

What is “inertia”?

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Inertia refers to the reluctance of the object to change its state of rest or motion.

The bigger the mass of a body, the greater its inertia.

(ie, the bigger the mass, the more difficult it is to make a stationary object move, or to make a moving object change its speed or direction, due to its inertia)

Describe an experiment to determine the centre of gravity of an irregularly shaped thin plastic sheet.

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a) Drill 3 small holes on the sheet, as near to the edge as possible, as far away from one another as possible.

b) Hang a plumb line from one of the holes.

c) When the sheet is stationary, trace the vertical line made by the plumb line using a marker.

d) Hang the plumb line from each of the other two holes and repeat the tracing of the lines.

e) The intersection point of the 3 traced lines is the centre of gravity of the plastic sheet.

State Newton’s Laws of Motion

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Newton’s First Law of Motion (or Law of Inertia):
Every object will continue in its state of rest or uniform motion in a straight line unless a resultant force acts on it to change its state.

Newton’s Second Law of Motion:
When a resultant force acts on an object of constant mass, the object will accelerate and move in the direction of the resultant force. The product of the mass and acceleration of the object is equal to the resultant force.

Newton’s Third Law of Motion:
For every action, there is an equal and opposite reaction, and these forces act on mutually opposite bodies.

A metal sphere, mass 2kg, and a wooden sphere, mass 1kg, of the same size, are dropped at the same time from the same height of 5m, on Earth. Which sphere would hit the ground first? Explain.

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Both spheres would hit the ground at the same time.
The spheres are dropped near to ground, so the effect of air resistance on the spheres’ resultant force is negligible.
The resultant force acting on the sphere will be its downward weight.
The ratio of the weight to mass will be 10 for each sphere, so both spheres will free-fall towards the ground at a constant acceleration of 10m/s2.