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zk/Resistance.md
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@ -4,19 +4,25 @@ tags: [physics, electricity]
# Resistance # Resistance
Resistance is opposition to the flow of current. Different materials have - Resistance is opposition to the flow of [current](./Current.md).
different levels of resistance. For example glass and rubber are excellent
resistors whereas silver and copper offer little resistance to current flow. We - Different materials have different levels of resistance. For example glass and
utilise **resistors** to control the flow of current in a circuit. rubber are excellent resistors whereas silver and copper offer little
resistance to current flow.
- We utilise **resistors** to control the flow of current in a circuit.
## Relationship between _resistance_ and _insulation_ ## Relationship between _resistance_ and _insulation_
Resistance and insulation are not the same thing although they relate to the Resistance and insulation are not the same thing although they relate to the
same natural phenomena. Resistance is a property of an electrical circuit same natural phenomena.
whereas insularity is a property of substances found in the world. Similarly,
insulators are naturally occuring materials that resist the flow of current to Resistance is a property of an electrical circuit whereas insularity is a
different degrees whereas a resistor is a man-made electrical component that is property of substances found in the world.
used to _control_ the flow of current in a circuit.
Similarly, insulators are naturally occuring materials that resist the flow of
current to different degrees whereas a resistor is a man-made electrical
component that is used to _control_ the flow of current in a circuit.
## Factors affecting resistance ## Factors affecting resistance
@ -34,11 +40,8 @@ used to _control_ the flow of current in a circuit.
- The unit of resistance is **ohms** ($\Omega$) - The unit of resistance is **ohms** ($\Omega$)
> One ohm is the resistance of a circuit or circuit element that permits a > One ohm is the resistance of a circuit or circuit element that permits a
> steady current flow of one > steady current flow of one [amp](Current.md#formal-expression) (one
> [amp](Current.md#formal-expression) > coulomb/second) when one [volt](Voltage.md#voltage) is applied to the circuit.
> (one coulomb/second) when one
> [volt](Voltage.md#voltage) is
> applied to the circuit.
### Conductance ### Conductance

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zk/Voltage.md
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@ -2,24 +2,25 @@
tags: [physics, electricity] tags: [physics, electricity]
--- ---
# Voltage
## Difference of potential and the transfer of energy ## Difference of potential and the transfer of energy
We noted in the discussion of We noted in the discussion of [current](Current.md) that current flows when
[current](Current.md) that current there is a difference of potential between two points with negatively charged
flows when there is a difference of potential between two points with negatively atoms at one point and positively charged atoms at the other.
charged atoms at one point and positively charged atoms at the other.
_Difference of potential_ is the same thing as voltage. Voltage is essential to _Difference of potential_ is the same thing as voltage. Voltage is essential to
current because it is the force that enables the current to flow. current because it is the force that enables the current to flow.
Without voltage there can be no current because in their natural state, the Without voltage there can be no current because in their natural state, the
electrons in an atom are in random motion with no direction. To produce a electrons in an atom are **in random motion with no direction. To produce a
current, energy must be imparted to the electrons so that they all flow in the current, energy must be imparted to the electrons so that they all flow in the
same direction. same direction.**
Voltage is the application of this energy. Any Voltage is the application of this energy. Any
[form of energy](Voltage_sources.md) [form of energy](Voltage_sources.md) that dislodges electrons from atoms can be
that dislodges electrons from atoms can be used to produce current. Thus: used to produce current. Thus:
> Voltage is the work required per coulomb to move a charge from one point to > Voltage is the work required per coulomb to move a charge from one point to
> another. > another.
@ -28,11 +29,10 @@ that dislodges electrons from atoms can be used to produce current. Thus:
Given that voltage is the force that generates current, it would be natural to Given that voltage is the force that generates current, it would be natural to
think that voltage only exists when a voltage source (such as a think that voltage only exists when a voltage source (such as a
[battery](Cells_and_batteries.md)) [battery](Cells_and_batteries.md)) is connected to a circuit. This however is
is connected to a circuit. This however is not the case. Even if a 9V battery not the case. Even if a 9V battery isn't connected to anything it still has a
isn't connected to anything it still has a difference of potential of 9-volts difference of potential of 9-volts accross its terminals. Remember voltage is
accross its terminals. Remember voltage is _potential energy_ not just the _potential energy_ not just the actualisation of that energy.
actualisation of that energy.
## Voltage rise and voltage drops ## Voltage rise and voltage drops
@ -58,8 +58,8 @@ resistance.
As they move through the circuit the electrons encounter a **load** which is As they move through the circuit the electrons encounter a **load** which is
what we call resistance to the flow of current. As they run into this, they give what we call resistance to the flow of current. As they run into this, they give
up their energy. The relinquishing of energy happens in the form of a conversion up their energy. The relinquishing of energy happens in the form of a conversion
of electrical energy to heat. The amount lost is equal to the amount of energy of electrical energy to heat (hence why computers heat up). The amount lost is
imparted by the voltage rise. equal to the amount of energy imparted by the voltage rise.
> The voltage drop in a circuit equals the the voltage rise of the circuit > The voltage drop in a circuit equals the the voltage rise of the circuit
> because energy cannot be created or destroyed, only changed to another form. > because energy cannot be created or destroyed, only changed to another form.
@ -95,8 +95,7 @@ obvious enough: they are at the beginning and end of the loop so are equal to
the maximal voltage rise and minimal voltage drop, respectively. the maximal voltage rise and minimal voltage drop, respectively.
We can work out the voltage of the remaining voltage points by inverting We can work out the voltage of the remaining voltage points by inverting
[Ohm's Law](Ohms_Law.md): [Ohm's Law](Ohms_Law.md): $V = I \times R$:
$V = I \times R$:
For the voltage at $V^{B}$: For the voltage at $V^{B}$: