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Resistance - GCSE level
Understanding what resistance is
12
Physics
Not Applicable
07/29/2005

Term
 What is the unit that resistance is measured in?
Definition
 Ohms - greek capital letter W (omega) [image]
Term
 What is resistance?
Definition
 A measure of an object's reluctance to allow charge to pass through it.If you put the same voltage across two objects the one that allows the smallest current to flow will have the biggest resistance.
Term
 Name the four factors that affect the resistance of a wire.
Definition
 The material it is made ofThe temperatureThe length of the wireThe cross sectional area of the wire.
Term
 How does the length of the wire affect its resistance?
Definition
 The longer the wire that has a particular potential difference (voltage) across it, the smaller the voltage drop across each centimetre of that wire will be. The less volts per centimetre of the wire (in other words the less the potential gradient) the less current flows.The less current flows the larger the resistance.Therefore the longer the wire the higher the resistance. This relationship is one of direct proportionality (a straight line graph of resistance against length could be plotted! It would go through the origin).
Term
 How does the temperature of the wire affect its resistance?
Definition
 The higher the temperature of the wire the more the ions in the metal lattice vibrate. This makes it more difficult for the electrons to move through the wire without interacting with the ions so the movement of charge is impeded and resistance rises.[image]
Term
 What is the structure of a metal like?
Definition
 [image] Metal atoms are bonded together by metallic bonding. A 3-D lattice of metal ions is surrounded by a 'sea' of valence electrons that can move quite freely around the lattice structure.
Term
 What happens when a potential difference is applied across a wire?
Definition
 Even when no potential difference is applied to a wire the electrons will be moving about. But no current flows as the charges are in 'random movement'. That means that as many move to the left as to the right, as many up as down etc. and they cancel each other's effect out! When a potential difference is applied across the wire the charges tend to move more in one direction than in any other - there is a general direction of flow of charge which we call a current.The bigger the potential difference applied the bigger the current flow.
Term
 How does the material of the wire affect its resistance?
Definition
 Some atoms hold onto their outmost electrons tighter than others. The more free the electrons are to move the easier it is to make a current flow. If an atom only has one electron in its outermost shell that electron is free to move - either in a flow of current or to form a bond. If the outer shell is nearly full then the atom holds on tightly to the electrons, hoping to gain more to make a complete shell.Therefore the resistance of the material depends on its electronic configuration - the more loosely held the valence electrons are the better conductor it will be.
Term
 How does the cross sectional area of the wire affect its resistance?
Definition
 The large the cross section of the wire the more electrons experience the potential gradient from the applied voltage.The more charges that flow, the more 'total charge' moves in a given time - this makes a bigger current.Therefore the bigger the cross section the bigger the current.The bigger the current the smaller the resistance.The relationship is an inverse relationship. A graph of resistance against cross sectional area produces a curve NOT a straight line.You would have to plot the resistance against the reciprocal of the cross sectional are to get a straight line through the origin.[image]
Term
 What erroneous explanation of why resistance increases with length will you come across on the WWW?
Definition
 The misleading explanation that electrons in a long wire have further to travel and therefore slow down (get tired!)
Term
 What erroneous explanation of why resistance decreases with cross sectional area will you come across on the WWW?
Definition
 The misleading explanation that electrons in a wide wire have more space to move in and therefore move faster!This is nonsense!!A wider wire still has the same density so the atoms are still as close to each other! An electron in a thin or thick wire will move at the same average speed for a given potential gradient.... but there are MORE moving in the wire of greater diameter. Don't get fooled by sites that suggest otherwise!
Term
 What is the equation that links together all of the four factors that affect resistance?
Definition
 [image] Resistivity takes into account both the material and the temperature!
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