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### Course: Physics library>Unit 18

Lesson 6: Discovery of resistors

Why does the deflection change when an electromagnet is attached to the circuit? Does it matter what side of the electromagnet we test? Created by Brit Cruise.

## Want to join the conversation?

• I've heard the terms "in series" and "in parallel", can someone help me understand the difference between the two and what they mean?
• In simplest terms series means only one path for current to travel on, and parallel means more than one path for current to travel on.
• what is difference between resistance and resistivity?
(1 vote)
• In simple, you could understood resistance by using a simple analogy, which would also lead to understand the Law of Resistivity and the Unit of Resistivity. So, Suppose you are moving through a narrow path, and a lot of guys are coming on the opposite direction, which is letting you to slow down. But, Imagine if the path was too wide you would easily go through them. But, If its too long, the rush of people will slow you down along with a lot of Distance. Similar, in a movement of Charges through a wire :-
You are the Charge, which is moving by the potential difference created by the batter. But, there are some bad guys who are resisting you to slow down(these guys can also be good sometime) which is resistance. And, if you somehow manage to squeeze through them, it would take time but, if the path was wider, you could easily move in a fast speed.
There fore, we have reached the Equation,

`` Resistance = Potential Difference / CurrentR = V/I``

Scientifically Resistance is the opposition that a substance offers to the flow of electric current. The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega (Ω). When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm.
And as i have said above, through a long path, you would have to go slowly, since there are along way to travel and so many guys to conquer, but if the way was wider, its pretty easier to escape fast
`` So, we have reached our second equation, Resistance is directly proportional to Length of wire and inversely proportional to the area of cross section.R ∝ l/aSo, we use a constant of proportionality ρ (Which is unit of Resistivity) to  remove the proportionality sign so, R = ρ * l/aSo, ρ = R*a/l = Ω* m^2 / m => ΩmSo, Si  unit of resistivity is Ωm.``

Scientifically , its a measure of the resisting power of a specified material to the flow of an electric current.
• Hot acid again in ?
(1 vote)
• What is the electromagnet in this video?

What did Mr. Cruise put into the cup of water at the beginning of the video, and why did it affect the compass to point in a specific direction?

What is deflection?
(1 vote)
• The electromagnet in the video is the coil of wire around the compas.

I am not exactly sure what was put into the cup but it looks like a stack of coins and I assume that the liquid poured in is something like vinegar and possibly salt, this makes a simple wet cell battery. The different coins are of different metals and the vinegar and salt make an electrolyte for the battery.

Deflection is something being changed from its default position direction.
• What are the advantages of parallel Combination of resistors to Series Combination?
• We know in a electric heater high resistance wire is used as H is proportional to I^2rt.why a low resistance wire not used so that heat produced would increase withI^2
• In fact, it is used, which is why heaters have much lower resistance than other appliances.
If you want to know why it's not lower, the efficiency of the heater is approximately equal to R/(R+r), where r is the internal resistance of the grid, so if a lower resistance wire is used, the efficiency is much lower. In addition, if the resistance is too low, the current can be higher than the maximum permitted and can damage the wires.
(1 vote)
• So connecting the battery has a larger affect than connecting the electromagnet?
(1 vote)
• The battery was still connected, there was just an additional load (the electromagnet) added to the circuit. The wires were partially not shown in the video, but if you follow the connections assuming that each wire completes the circuit, it makes more sense.