Physics Facts to Memorize

Wave definitions

Crests: the high points on a wave

Troughs: the low points on a wave

Amplitude (A): the distance from the midpoint to the crest or trough

Wavelength (λ): the distance between identical parts of the wave.

Frequency (f):the number of waves to pass a position in one second.

Period (T): the time for one wavelength to pass a position

The unit of frequency is called the hertz (Hz).

Whenever a wave changes materials, the wave’s frequency remains the same.

The speed of a wave depends on the material through which the wave moves.

It is the disturbance that moves along the string [or any material], not parts of the string itself.

Equations relating frequency, period, wavelength, wave speed

Frequency and period are inverses of each other: , and . You can calculate the speed of a wave by multiplying the wavelength by the

frequency: .

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Physics Facts to MemorizeYou can calculate the speed of a wave by dividing the wavelength by the period,

.

Transverse/longitudinal waves

Whenever the motion of a material is at right angles to the direction in which the wave travels, the wave is a transverse wave.

Waves on a stringed instrument are transverse; waves in the electromagnetic spectrum, including light, are transverse.

All waves in the electromagnetic spectrum move 300 million m/s in air.

The higher the tension in a string, the faster the wave moves in that string.

When a material vibrates parallel to the direction of the wave, the wave is a longitiudinal wave.

Sound waves are longitudinal waves.

Light can travel through a vacuum; sound cannot.

Interference

Interference occurs when waves arrive at the same point at the same time.

In constructive interference the crest of one wave overlaps the crest of another. The result is a wave of increased amplitude.

In destructive interference the crest of one wave overlaps the trough of another. The result is a wave of reduced amplitude.

Points in a wave are in phase if they would overlap to produce constructive interference.

Points in a wave are out of phase if they would overlap to produce destructive interference.

Standing Waves

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Physics Facts to MemorizeA standing wave is a wave that appears to stay in one place

Nodes are the stationary points on a standing wave.

Antinodes are the positions on a standing waves with the largest amplitudes.

Sound

The pitch of a sound depends on a sound wave’s frequency.

The loudness of a sound depends on the sound wave’s amplitude.

The speed of sound in air is about 300 m/s.

People can easily hear sound with frequencies of tens, hundreds, or thousands of Hz. Most musical notes have frequencies in the hundreds of Hz.

Light

The color of light depends on the light wave’s frequency.

The brightness of light depends on the light wave’s amplitude.

The speed of light in air is about 300 million m/s.

People can see light with frequencies of hundreds of trillions of Hz [hundreds of THz.]

Of light in the visible spectrum, red light has the smallest frequency; violet light has the largest frequency.

Doppler Effect

As a wave source approaches, an observer encounters waves with a higher frequency.

As a wave source moves away, an observer encounters waves with a lower frequency.

This apparent change in frequency due to motion of the source is called the Doppler effect.

The Doppler effect is NOT related to the amplitude of a sound wave.

The frequency of the source does not change. It is only the apparent frequency that changes.

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Physics Facts to Memorize

Diffraction

Any bending of light that is not due to reflection or refraction is called diffraction.

When a wave encounters an obstacle about the same size as a wavelength, diffraction occurs.

Resonance

A forced vibration occurs when an object is made to vibrate by another vibrating object that is nearby.

The natural frequency is the frequency at which an object vibrates when it is disturbed.

Resonance occurs when the frequency of a forced vibration matches an object's natural frequency, and a dramatic increase in amplitude occurs.

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Physics Facts to MemorizeDefinitions of voltage, current, resistance

Voltage is provided by a battery. Voltage is measured in units of volts.

Resistance is provided by a resistor, a lamp, or any electronic device. The units of resistance are ohms (Ω).

Current relates to the amount of charge flowing through a resistor. The units of current are amps.

Ohm’s law states that voltage is equal to current multiplied by resistance: V = IR.

Resistors in series

Resistors are connected in series if they are connected in a single path.

The equivalent resistance of series resistors is the sum of all of the individual resistors.

Series resistors each carry the same current, which is equal to the total current through the circuit.

The voltage across series resistors is different for each, but adds to the voltage of the battery.

In Ohm’s law, use the voltage of the battery with the equivalent resistance of the circuit; or, use the voltage across a single resistor with the resistance of a single resistor.

Resistors in parallel

Resistors are connected in parallel if the path for current divides, then comes immediately back together.

The equivalent resistance of parallel resistors is less than any individual resistor.

The voltage across parallel resistors is the same for each, and equal to the total voltage.

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Physics Facts to MemorizeParallel resistors each carry different currents, which add to the total current in the circuit.

Ammeters and Voltmeters

An ammeter measures current through a resistor. It is connected in series with a resistor.

A voltmeter measures voltage across a resistor. It is connected in parallel with a resistor.

Power and Brightness

The power dissipated by a resistor is equal to the resistor’s voltage squared divided by its

resistance:

Power is measured in units of watts.

The brightness of a light bulb depends on the power dissipated by the bulb.

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Physics Facts to MemorizeHow to know what a graph looks like

When the arrows in the equation point in opposite direction (whether something is squared or not):

When the arrows in the equation point in the same direction and nothing is squared:

● In a straight-line graph, the steepness of the graph depends on the quantity that is held constant.

When the arrows in the equation point in the same direction but one of the variables is squared:

When the vertical-axis variable doesn’t change at all:

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Physics Facts to Memorize

When a wave reaches a boundary between two materials, some wave bounces back into the first material. The return of a wave back into its original material is called reflection.

When a wave that is traveling at an angle changes its speed upon crossing a boundary between two materials, it bends. This bending is called refraction.

Usually waves are partly reflected and partly refracted when they fall on a transparent material.

When light is incident on a rough surface, it is reflected in many directions, producing diffuse reflection.

Reflection: Incident ray: the ray that shines on the mirror

Reflected ray: the ray that reflects from the mirror

Normal: an imaginary line perpendicular to the mirror’s surface

Angle of incidence: the angle between the normal and the incident ray

Angle of reflection: the angle between the normal and the reflected ray

The law of reflection states that the angle of incidence and the angle of reflection are equal to each other.

Plane MirrorsPlane mirrors are flat-shaped.

A virtual image is an image that appears to be in a location where light does not really reach.

Virtual images are right-side-up.

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Physics Facts to MemorizePlane mirrors produce only virtual images.

In a plane mirror, object size equals image size.

In a plane mirror, object distance equals image distance.

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Physics Facts to MemorizeRefraction is caused by changes in the speed of light as it passes from one material to another.

When light rays enter a material in which their speed decreases, the rays bend toward the normal.

When light rays enter a material in which their speed increases, the rays bend away from normal.

The more the change in speed, the more the light beam changes direction.

A “wave front” represents the crest of a wave.

The angle of incidence and angle of refraction are both measured from the normal

Light travels as fast as it can possibly go in air. This speed is 300 million m/s, and is labeled c.

The index of refraction n is defined as , where c is the speed of light in air, and v is the speed of light in a material.

The index of refraction is a property of a material, and can be looked up for many materials.

[Tables below are for use on homework, and will be given on tests – do not

memorize them.]

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Physics Facts to Memorize

[Table above is from the New York Regents exam.]

Total Internal Reflection:Critical angle: The angle of incidence that results in a 90o angle of refraction.

The critical angle only exists when light bends away from the normal.

The critical angle only exists when light speeds up into a new material.

Total internal reflection is the complete reflection of light back into its original material.

Total internal reflection occurs when the angle of incidence is larger than the critical angle.

Lenses

A real image is inverted, meaning it is upside-down compared to the object.

A real image can be focused on a screen.

Converging lens: thicker in the middle, causing parallel rays to meet at the focal point.

Diverging lens: thinner in the middle, causing parallel rays to bend away from the focal point.

The focal length of a lens, whether converging or diverging, is the distance between the center of the lens and its focal point.

The focal lengths on either side are equal, even when the curvatures on the two

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Physics Facts to Memorizesides are not.

A converging lens forms either a real or a virtual image.

When the object is between the focal point and a converging lens, the image is virtual.

When the object is farther from a converging lens than the focal point, the image is real.

A diverging lens always forms a virtual image.

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Physics Facts to MemorizeLens Ray Diagrams

Ray diagrams show the principal rays that can be used to determine the size and location of an image.

An arrow is used to represent the object (which may be anything imaged by the lens).

To locate the position of the image, you only have to know the paths of two rays from a point on the object.

1. A ray parallel to the principal axis will converge to (or diverge from) the focal point

2. A ray through the center of the lens proceeds in a straight line

All light passing through a lens contributes to image formation. We use these particular rays only because their paths through the lens are easy to predict.

The image distance is measured along the principal axis from the center of the lens to the location of the image.

The object distance is measured along the principal axis from the center of the lens to the location of the object.

Mirrors

Converging mirror: shaped like the inside of a spoon, causing parallel rays to meet at the focal point

Diverging mirror: shaped like the outside of a spoon, causing parallel rays to reflect away from the focal point

A converging mirror forms either a real or a virtual image.

When the object is between the focal point and a converging mirror, the image is virtual.

When the object is farther from a converging mirror than the focal point, the image is real.

A diverging mirror always forms a virtual image.

Virtual images formed by mirrors always are located behind the mirror.

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Physics Facts to Memorize

Mirror ray diagrams:

1. A ray parallel to the principal axis will reflect to (or be reflected away from) the focal point

2. A ray through the center of the mirror reflects back toward the center of the mirror.

ALL light reflected off of a mirror contributes to image formation. We use these particular rays only because their paths are easy to predict.

The image distance is measured along the principal axis from the mirror itself to the location of the image.

The object distance is measured along the principal axis from the mirror itself to the location of the object.

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