Circuits
1 Current
- Amount of Charge at a point in time is \(q(t)\)
- Current is \(q'(t)\) or \(\frac{dq}{dt}\)
2 Power box
- The high voltage powerlines goes to your powerbox and is downstepped to 120V via transformer
- Powerline –> Transformer –> Live Wire –> Your home appliance –> Neutral –> Transformer –> Powerline
- THe power provided is AC meaning the current is oscillating back and forth
3 Power adapters, Chargers, LED lights
- Appliances typically use power adapters like your cellphone charger or laptop charger
- These are transformers that downstep that 120V to 30V or so
- LED lights are said to be 12V but how?
- LEDs use LED drivers which also act as transformers that downstep the 120V to 12V
4 Misconceptions
4.1 Ground wire has nothing to do with Grounding Rod
- Ground wire has NOTHING TO DO WITH THE GROUNDING ROD
- Ground wire does NOT mean it’s path terminates at the Ground rod.
- Instead it goes back to the powerbox transformer just like the live and neutral wire. (Otherwise the ground wire would just be an open circuit and worthless)
- Ground wire is basically an alternative neutral and at the powerbox it is BONDED to neutral.
- The Ground wire is designed to short so that it can trigger the circuit breaker.
4.2 All wires are Grounded
- All wires are Grounded via GROUNDING ROD, meaning the live and neutral are also connected to the GROUNDING ROD.
- You can use a multimeter to check GROUNDING ROD should beep when touching any wire.
- GROUNDING ROD provides a reference 0 voltage which is earth !!
- WHAT-IF: What if you didnt have a GROUNDING ROD and measured outlet with a multimeter
- You’d get correct results of 120 V BUT people call that FLOATING GROUND meaning the DIFFERENCE IS 120V aka it could be 1120V on live and 1000V on neutral.
- WHAT-IF: What if you didnt have a GROUNDING ROD and measured outlet with a multimeter
4.3 Current Kills Voltage doesnt
Remember Current is electron passing per second; it’s these electrons that kill.
- Voltage at an outlet is fixed at 120 V.
- Touching the outlet with rubber gloves means you complete the circuit with HIGH RESISTANCE, implying CURRENT PASSING THROUGH YOU IS LOW
- Touching the outlet with wet hands means you complete the circuit with LOW RESISTANCE, implying CURRENT PASSING THROUGH YOU IS HIGH. (this current is what kills)
In the above 2 scenarios, the outlet voltage is high at 120 V but you survive in once case but not the other which shows that current is what is important, NOT Voltage.
BUT REMEMBER, HIGH Voltage means you need higher resistance outfit to survive; the proverb is semi-true in that HIGH VOLTAGE has a higher probability of killing.
5 Circuits
- Voltage has no meaning without a reference point <- reason for grounding circuits
- Voltage only has meaning when you say the voltage across a resistor
This means the potential difference before passing and after passing the resistor is the Voltage.
5.1 Series
- Voltage varies, Total voltage is sum of Voltage across all resistors
- Resistance varies, Total resistance is sum
- Current is same everywhere
5.2 Parallel
- Voltage is same when passing each resistor in parallel
- Inverse(total resistance) = Inverse(sum resistors)
- Current is sum
6 Ohm’s law and Reality
6.1 Real life power sources
How does Ohm’s law play in reality?
- In reality, electronics has a pre-determined static resistance.
- Voltage is supplied by a power adapter which is semi-static since we get to choose the power adapter.
- Voltage and current are linked.
- high voltage power supply, you get a high current.
- low voltage power supply, you get a low current.
- high voltage power supply, you get a high current.
- A power adapter shows :
- INPUT: 100-120V
- OUTPUT: 5V , 1.2A
- This means the house power socket, (typically 120 Volts) is input into the power adapter and reduces the voltage.
- The 1.2A just means the adapter maximum internals can only take 1.2 Amps
- This means if we use the power adapter on a very low resistance electronic device, the power adapter may break.
- This DOES NOT mean the adapter provides current.
power adapter will provide different current to different electronic devices but it will always provide the same voltage.
## Battery doesnt follow ohm’s law
- Ohm’s law only apply to resistors in a circuit
- Battery is classified as a Voltage Source - a fixed voltage in a circuit
- To find this fixed voltage : \(E = E_Cathode - E_Anode\) where \(E=Voltage\)
- Cathode: area of reduction reaction
- Anode: area of oxidation reaction
Chemical chart maps oxidation and reduction reaction to voltages.
Use this to calculate total volts of a battery.
## Does current or voltage kill
It isn’t Volt that kills, it’s current
Standing on the cliff doesn’t kill, it’s the fall
The above quote is an abuse of language, since Current is basically Voltage-in-action.
7 Home wiring
In the above scenario the microwave hot wire is loose and touches the metal case, effectively turning the microwave into a hot wire.
The person touches the hot wire while standing on ground.
But since there is a grounding wire in the microwave that is attached to the metal case, the hot wire will move the current to the ground since (Resistance of Ground > Resistance of Ground + Resistance of Person).
OBSERVE: “Safety Ground wire” is lower resistance than “Person” + “Standing on Ground”, meaning the current will prefer to go through the “Safety Ground wire” back to the power src rather than Going through the Person to the Ground back to the power source (which would electrocute the person).
Here is another scenario. What happens if there is no ground wire but the person is standing on a infinitely insulated surface and touches the hot wire.
SOLUTION: There would be no shock since it is effectively an open circuit.
TAKEAWAY: The earth’s ground completes a circuit.
MISNOMERS: People believe the Earth is a bad conductor but that is false.
