Physics O Levels Definitions & Concepts

State the Principle of Moments

For an object in equilibrium, the sum of the clockwise moments about a pivot is equal to the sum of the anti-clockwise moments about the same pivot.

Describe what happens when a gas is being heated.

When a gas is heated, it gains thermal energy, which is converted to kinetic energy. Particles move faster.

State the Principle of Conservation of Energy

Energy cannot be created or destroyed. It can only be converted from one form to another. The total amount of energy (in a closed system) remains constant.

Describe what happens to the pressure of a gas being heated.

When a gas is heated, it gains kinetic energy and moves faster. The particles collide with the walls more often and with more force. This increases the force exerted on the walls, pressure increases.

Describe how a gas exerts a pressure on the walls of a container

Particles of a gas can move freely in all directions, at high speeds. When they collide with the walls of the container, they exert a force. The sum of all the forces, averaged over area gives the pressure.

Describe conduction of heat in a solid

(molecular vibrations)

When one end of a solid is heated, the particles vibrate more vigorously. The particles collide with the neighbouring particles, transferring the energy from particle to particle, from hot to cold end.

Describe conduction of heat in a metal (free electron diffusion)

When a metal is heated, free electrons gain kinetic energy and move faster. As the electrons move through the metal, they collide with atoms, transferring the energy to them. Metals are good conductors of heat due to these free electrons.

Describe convection in a beaker of water, being heated below

Water at the bottom is heated, expands, becomes less dense and rises. Cooler, denser water at the top sinks to replace this water and is in turn heated. This process repeats, setting up convection currents

How does the colour and texture of a substance affect radiation?

Shiny / White smooth surfaces are poor absorbers/emitters of radiation

Black / Dark dull surfaces are good absorbers/emitters of radiation

Why is there no change of temperature during state change?

The energy absorbed/released is used to break/form intermolecular bonds. There is an increase/decrease in internal potential energy. No change in KE, thus no change in temperature.

Define specific heat capacity

Specific heat capacity is the amount of thermal energy required to increase the temperature of 1kg of a substance by 1K

Define specific latent heat of

fusion

Specific latent heat of fusion is the amount of thermal energy required to change the state of 1kg of a substance from solid to liquid and vice versa.

What is a transverse wave? Give an example

A transverse wave travels in a direction which is perpendicular to the direction of vibration of particles. Example: Water, Light, any EM wave

What is a longitudinal wave? Give an example

A longitudinal wave travels in a direction which is parallel to the direction of vibration of particles. Example: Sound / Ultrasound waves

State 2 properties of electromagnetic waves

1. Travels at 3 x 10⁸ m/s in vacuum

2. Transverse wave

Define critical angle

Critical angle is the angle of incidence in the optically denser medium, for which the angle of refraction in the optically less dense medium is 90°

What wave property is loudness related to? What about pitch?

Loudness is related to amplitude. Louder sound = Larger amplitude.

Pitch is related to frequency. Higher pitch = high frequency

How do static electric charges behave? (for like and unlike charges)

Like charges repel, unlike charges attract

What are the 2 factors affecting the resistance of a wire? How?

1. Length. Double length, double R

2. Cross-sectional area. Double thickness, half R

What are the colours of the wires in a

3-pin plug?

Live - Brown

Neutral - Blue

Earth - Green/Yellow

Why must the switch be connected on the live wire?

So that the appliance will be isolated from the live wire when it is switched off. Otherwise the appliance will still be charged to "live" when it is off.

Why must the fuse be connected on the live wire?

So that the appliance will be isolated from the live wire when the fuse melts. This makes it safe for the user to check for any electrical fault in the appliance.

What is the purpose of the earth wire? Where is it connected?

The earth wire is connected to the metal casing, to prevent the user from getting an electric shock, when the live wire breaks and touches the metal casing.

Name 4 magnetic materials

Iron, Cobalt, Nickel, Steel

Name 2 methods to magnetise an iron nail. How to determine its poles?

1. Stroking - if N-pole touches left end first, left end becomes an N-pole

2. Electrical method using d.c., by coiling a wire around the iron nail. Use right hand grip rule

Name 3 methods to demagnetise a permanent magnet

1. Hammering

2. Heating

3. Electrical method using a.c. - pull magnet out of coil slowly with a.c. in east-west direction

State Faraday's Law of EM Induction

The magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux in the circuit.

State Lenz's law

The direction of the induced e.m.f. and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it

Why is electricity transmitted at high voltages?

So that the current is low, and there will be less heat loss in the cable by P = I²R

State the magnitudes represented by the following prefixes:

nano (n),

micro (),

milli (m),

centi (c),

deci (d),

kilo (k),

mega (M),

giga (G)

nano (n) - 10⁻⁹

micro (µ) - 10⁻⁶

milli (m) - 10⁻³

centi (c) - 10⁻²

deci (d) - 10⁻¹

kilo (k) - 10³

mega (M) - 10⁶

giga (G) - 10⁹

State what is meant by scalar and vector quantities

A scalar quantity is a physical quantity with magnitude only.

A vector quantity is a physical quantity with magnitude and direction.

Give examples of scalar quantities

Speed, Distance, Time, Energy

Give examples of vector quantities

Velocity, Displacement, Force, Acceleration

How can we calculate the distance travelled given a velocity-time graph?

Area under velocity-time graph

What is the acceleration of an object being dropped on earth?

10 m/s²

What is the effect of balanced forces acting on an object?

It may remain stationary, or it may continue to move at a constant speed in the same direction.

In which direction is the resultant force, when we add up two non-parallel vectors?

The resultant force will be in between the two initial vectors (use parallelogram method)

Define centre of gravity

CG is the point through which the entire weight of the object seems to act.

Two factors which affect stability?

1. Position of CG - the lower the CG, the more stable the object.

2. Base area - the wider the base, the more stable the object

What are the two fixed points (temperatures) used for calibrating a centigrade thermometer?

Ice point - temperature of pure melting ice

Steam point - temperature of steam above pure boiling water

What does the internal energy of a body comprise?

Potential energy - relating to the intermolecular bonds. During melting/boiling, intermolecular bonds are broken, PE is increased.

Kinetic energy - related to the temperature of the body. Higher temperature, KE is increased.

Law of reflection

Angle of incidence (i) = Angle of reflection (r)

(NOTE: angles must be measured from the normal)

Two conditions for total internal reflection?

1. Angle of incidence greater than the critical angle.

2. The ray of light must be travelling from an optically denser to optically less dense medium.

Define focal length for a converging lens

Focal length is the distance between the optical centre and the principal focus (F).

Know how to draw the ray diagrams for an object placed:

-beyond 2f,

-at 2f,

-between f and 2f

-at f

-less than f

Know the properties of the image formed at each of these positions

-beyond 2f - image is real, inverted and diminished

-at 2f - image is real, inverted and same size as object

-between f and 2f - image is real, inverted and magnified

-at f - image is formed at infinity (no image formed)

-less than f - image is virtual, upright and magnified

Define ultrasound and state one use of ultrasound

Ultrasound are sound waves with frequency greater than 20 kHz.

e.g. quality control and pre-natal scanning (to view image of unborn foetus)

Define electric field

Region in which an electric charge experiences an electric force

Draw the electric field lines around a positive and negative charge

-Electric field lines go from the positive to negative charge

-Field lines do not cross each other

-Field lines closest together nearer the charges, indicating a stronger electric field.

4 steps to charge an isolated metal sphere?

1. Bring charged object close to, but not touching, the metal sphere

2. Earth the metal sphere.

3. Remove earth connection.

4. Remove charged object.

Define electromotive force (e.m.f.)

Work done by a source in driving unit charge around a complete circuit

Difference between conventional current and electron flow

Conventional current - flows from positive (+) to negative (-) terminal. Referred to as the current I.

Electron flow - opposite to conventional current. Flows from the negative (-) to positive (+) terminal.

State Ohm's Law

Ohm's law states that for a metallic conductor, the current flowing between two points is directly proportional to the voltage across the two points.

Sketch the I-V graphs for an ohmic conductor, and non-ohmic conductors such as a semi-conductor diode and a filament lamp.

For ohmic conductor, I-V graph is straight line passing through the origin.

For non-ohmic conductor, please check your notes. (Can't attach an image here)

How can thermistors be used as an input transducer?

Thermistor varies resistance depending on temperature.

Higher temperature, less resistance, more current.

Lower temperature, more resistance, less current.

How can light-dependent resistor (LDR) be used as an input transducer?

LDR varies resistance depending on temperature.

Typically:

More light, less resistance, more current.

Less light, more resistance, less current

State the hazard when using a cable with a larger current flowing through then is specified.

Larger current -> More heat generated -> Possibility of insulation melting -> Sparks due to short circuit -> Fire?

What are soft and hard magnetic materials. Give examples and uses of each.

A soft magnetic material is magnetised and demagnetised easily. E.g. Soft iron. Used as electromagnets

A hard magnetic material retains its magnetism for a long time. E.g. Steel. Used as permanent magnets

Describe the application of the magnetic effect of a current in a circuit breaker

When the current becomes too large, the electromagnet/coil becomes STRONGER (it was already an electromagnet, as there was a current passing through). This creates enough force to attract the soft iron latch, which causes the circuit to be open. The circuit remains open until the reset button is pressed.

Factors affecting the strength of an electromagnet

1. Number of coils for the electromagnet

2. Strength/size of current

3. Use of soft iron core

Purpose of a split-ring commutator in a d.c. motor

To allow the coil to rotate continuously, by reversing the direction of the current every 180 degree rotation. By reversing the current, the direction of the force, by Faraday's LHR, is also reversed, allowing the coil to rotate continuously.