To find out the factors that affect the resistance of a wire. Also to find out the relationship between resistance and length, and resistance and cross-sectional area. Theory The current flowing through a metal wire is proportional to the potential difference across it providing the temperature of the wire remains constant. Resistance (R) = Pd across the wire (V) Current through the wire (I) If a conductor obeys ohm’s law, the current will increase in proportion to the potential difference.

If you double the voltage, the current will also double. If the graph ofthe cu nt against potential difference is not a straight line, or does not pass through zero, then ohm’s law does not apply. The amount of resistance of a wire depends on many different factors. Some of these factors are: . Length . Thickness . Material type . Temperature Resistance is the opposition to a flow of electric current in a wire. Longer wires have more resistance than shorter wires, this is because there are more particles for the electrons to get passed from one end of the wire to the other because there is a greater distance to travel. Energy is lost each time the electron hits an atom.

The friction that occurs between the electrons colliding into each other as well as the atoms in the metal creates heat and heat is a major factor that effects the resistance of a wire. The heat increases, the particles that make up the wire vibrate more, which means the electrons lose more energy. This is because it is harder for the electrons to flow through the wire without collisions. If the particles are cooler they vibrate less which makes it easier for the electrons to flow through so they don’t lose as much energy , resulting in a stronger current. The thinner the wire the higher the resistance.

This is because all the electrons try to push through the wire at the same time and collide into one another which also creates heat. If the wire is thick then there is much more room for the electrons to flow through then there is in a thin wire. Different materials have different levels of resistance. For example, Nichrome wire has more resistance than copper wire of the same size. There are two reasons as to why current flows more readily through copper then nichrome: . Copper contains more sufficient electrons (charge carriers) . More mobility of the electrons (charge carriers) in the copper

Current passes easily through a piece of copper connecting wire, but it doesn’t pass so easily through the thin nichrome wire of an electrical fire element. This wire has much more resistance so energy has to be spent to force electrons through it and heat comes off as a result. If the temperature changes, so does the resistance. If a metal is warmed, its resistance goes up. The resistivity of a material is numerically equal to the resistance of a specimen of length 1m and cross sectional area 1m squared when a current flows perpendicularly to that area. Resistivity is quoted in m or in cm; in the latter case it relates to a specimen 1cm long and 1cm squared in cross-section.

Conductors like copper have very low values of resistivity while the plastics generally have very high values. When the concept of density was introduced in Ch. 10 it gave us the ability to compare different materials in a fair way. Although different samples of aluminium and iron could be lighter or heavier than each other there is no doubt that aluminium has a lower density than iron. To get the idea of density we needed to fix a certain volume (a cubic metre or cubic centimetre) and use this standard-sized sample for all materials.

Therefore mass is a property of any individual object but density is a property of the material of which it is made. In a similar way we have resistance as a property of any object but we need to be able to compare the materials of which different objects are made. As with the density case there is a fixed standard size and shape so that fair comparisons are made. There are factors of size and shape that decide the resistance of an object. So consider a thin rectangular-section piece of material with current flowing along its length.

If two such lengths were connected end to end they would be in series and therefore have twice the resistance of each one separately. Therefore the resistance R must be directly proportional to the length since when is doubled, so is R Similarly, if two pieces were placed side by side so that the current could flow through each of them they would be parallel giving a combined resistance of half that of each one separately. Clearly the resistance must be inversely proportional to the area of cross- section, A, since when A is doubled the resistance is halved: Combining these two results gives: or: where P is the constant of proportionality and will have the units of resistance X length.

This constant, P, is called the resistivity of the material and from the equation we see that if = 1m and A= 1m , p = R. Prediction I predict that the thicker the wire the less the resistance. I think this because it is a lot easier for the electrical ions to pass through a wire with a wider cross-sectional area, because there is more space for them to pass through resulting in less collisions. When there is a wire with a small cross-sectional area there will be more resistance.

This is because there is less space for the ions to pass through so they collide into each other not passing through as easy creating friction and heat, this also creates more resistance. The longer the wire the more the resistance. I think this because there is a greater distance for the electrons to flow through resulting in more particles to bump into along the way, and the electrons lose more energy. If the wire was shorter it wouldn’t take as long for the electrons to flow through so they don’t lose as much energy from the collisions that are of a shorter period of time.