Digging up the Past

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Resistivity ($\rho$) is a property of a material that describes the extent to which it opposes the flow of current.

$\rho =\frac{RA}{l}$

Where $R$ is resistance, $A$ is the cross sectional area and $l$ is the length of the wire.

CORE PRACTICAL 2: Determine the electrical resistivity of a material.

Diagram:

Method:

Set up equipment as shown in the diagram.

Analysis:

$I=nqvA$

Where $n$ is the number density of charge carriers, $q$ is the charge of each charge carrier, $v$ is the mean drift velocity of the charge carriers and $A$ is the cross sectional area of the wire.

Referring to the above equation, as $n$ increases, so does the current. As current increases, resistance decreases, since $R=\frac{V}{I}$. Furthermore, as resistance decreases, so does resistivity (see equation for resistivity above).

$n$ varies a lot between different materials, and so there is a big variation in the resistivities of different materials.

The potential along a uniform current-carrying wire increases uniformly with the distance along it.

A potential divider circuit is a circuit with a voltage source and several resistors in series, and is used to get a fraction of a source voltage.

As the resistance of a component increases, so does the potential difference dropped across it (since $V=IR$).

potential divider circuit with two series resistors

$V_{1}=\frac{R_{1}}{R_{1}+R_{2}}V$

$V_{2}=\frac{R_{2}}{R_{1}+R_{2}}V$

A light dependent resistor (LDR) has a higher resistance when it is darker, and a lower resistance when it is brighter.

An NTC (negative temperature coefficient) thermistor has a higher resistance at lower temperatures, and a lower resistance at higher temperatures.

An LDR or a thermistor can be used in a potential divider to give an output voltage that depends on brightness or temperature.

Diffraction is the spreading out of a wave as it passes through a gap or past an object.

Maximum diffraction occurs when the size of the gap is approximately equal to the wavelength of the wave.

A diffraction grating is a slide containing many equally spaced slits very close together. When light passes through a diffraction grating, it forms a diffraction pattern.

$n\lambda =d\sin\theta$

Where $n$ is the order of the maximum, $\lambda$ is the wavelength, $d$ is the distance between the slits and $\theta$ is the angle to the normal made by the maximum.

$diffraction pattern on screen$

CORE PRACTICAL 8: Determine the wavelength of light from a laser or other light source using a diffraction grating.

Diagram:

Method:

Set up equipment as shown in the diagram.

Analysis:

The de Broglie hypothesis states that all matter exhibits wave-like behaviour. The wavelength ($\lambda$) of any particle can be found using the following formula:

$\lambda =\frac{h}{p}$, or $\lambda =\frac{h}{mv}$

Where $p$ is momentum, $m$ is mass, $v$ is velocity and $h$ is Planck's constant.