Let’s Get Physics-l: Resistance and Ohm’s Law

Resistance, in essence, is the degree at which a substance opposes the flow of an electric current, resulting in the creation of wasted energy. Imagine this,if you will. Detroit Steel, in his infinite wisdom, has decided that his suit is powerful enough to take on Iron Man’s. Tony Stark is a busy man and, what with being an Avenger and owning a global company, doesn’t really have time to battle Detroit Steel for any longer than 5 minutes. To ensure that the battle is concluded post-haste, Iron Man calls in some help. As you can imagine, Detroit Steel didn’t stand a chance and is felled by a single power-packed punch. Back to the Physics bit, if we consider Detroit Steel to be electric current and the Iron Man/ War Machine suits to be the material, we can (just about) see that greater resistances make it harder for current to flow.

 

Resistance
It was at that point that Detroit Steel realised: He really isn’t ready to take on Iron Man

 

When talking about resistance, the work of Georg Ohm simply cannot be ignored (Well, it can but I strongly advise against it). One of his most important contributions to Physics was Ohm’s law which states that: the electric current passing through a conductor is directly proportional to the potential difference across it, so long as temperature remains constant.

With Ohm’s Law in mind, we can use potential difference and current to calculate resistance:

R = V/I

Where:

  • R – Resistance (Ω)
  • V – Potential Difference (V)
  • I – Current (A)

Resistance isn’t quite as straight forward as a constant value across all shapes and sizes of material. Several factors have an effect on resistance. We shall consider these factors in the context of a wire, the instance where it is most prevalent.

The Material of the Wire

As discussed in previous posts, Metals have electrons that can move freely and randomly, allowing current to flow through the metal. The more free electrons a material has, the less resistance it has, metals being a prime example. Fewer free electrons results in a higher resistance, plastics here being a great example.

The Length of the Wire

 

Length
A Khan-do attitude is essential for any Physics student (I’m not even sorry)

 

Electrical resistance increases as the length of the wire increases. This is due to the electrons having to travel a greater distance, a lower drift velocity resulting in a consequent lower current. In relation to the Mighty Ms Marvel, picture the scene. Ms Marvel is going up against a whole bunch of Hydra henchman, charging towards her with stun batons. By embiggening her arm, Ms Marvel is able to take down almost all the Hydra henchmen. Similar to the way that she is able to put up greater resistance to the Hydra henchmen’s charge with a longer arm, longer wires can resist the flow of current better than short wires.

The cross-sectional area (Thickness) of the wire

 

Width
Graphs are a f-ANT-tastic way of of representing the relationship between resistance and properties of the wire (I agree, that was weaker… Doesn’t mean I’m going to stop…)

 

The Thickness of wire affects the flow of movement of electrons. In thinner wires, the “paths” that electrons can take are more limited so it harder for current to flow. Consequently, resistance is higher in thinner wires. In thicker wires, the electrons can travel along more “paths” which allows current to flow, overall, far better. In relation to the Astonishing Ant-man, conjure this scene in the theatre of your mind. Ant-man is “practicing” (mucking about) changing between sizes quickly whilst also seeing how fast he can move at each size. Whilst tiny, he finds that his speed is far greater than when he is at normal size, much like thinner wires have a greater resistance than average wires. Whilst giant, his speed is significantly slower than at human size, much like thicker wires have a lower resistance than average wires.

The temperature of the wire

 

Temperature
Graphical representations are pretty lit. Anyone who tells you otherwise is a liar.

 

Heating stuff up causes its particles to vibrate far more than usual. In wires, this becomes a bit of an issue as the delocalised electrons often collide with these more active particles, limiting the flow of current. As you can imagine, and is seen above, higher temperatures result in higher resistances as there are more collisions and less flow of current. If we consider the above picture, the heat damage each inflicts increases as you progress up the line. A Goblin pumpkin bomb is a little bit irritating, A flame-thrower wielding Crossbones is a sure-fire recipe for trouble and Ghost Rider, well, doesn’t really need to be explained with that outfit. As you increase heat, the damage increases much like increasing wire temperatures result in increased resistances.


Summary:

  • Resistance – The degree at which a material opposes the flow of electrical current
  • Ohm’s Law – The current flowing through a conductor is directly proportional to the potential difference across it, so long as temperature is constant
  • R = V/I

    • R – Resistance (Ω)
    • V – Potential Difference (V)
    • I – Current (A)
  • Material – Higher number of delocalised electrons, lower resistance and vice versa
  • Length – Longer wires, higher resistance and vice versa
  • Cross-sectional Area – Thicker wire, lower resistance and vice versa
  • Temperature – Higher temperature, Higher resistance and lower temperature, lower resistance

There you have it, Resistance and Ohm’s Law with a healthy serving of superheroics (The prospect of including superhero examples was simply irresistable). I hope the addition of images was, if nothing else, mildly amusing and perhaps even a touch helpful. If you have any thoughts or questions please feel free to leave them in the comments and I will assist where I can. Thank you.

 

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