Current and Power

[fullthrottle]

Hi All

Need a refresher on ohms law

Can anyone explain to me why in a DC circuit, such as the winch cabling for example, that both the positive and negative cables are the same gauge?

For example, the winch may draw 300Amps from a 12volt battery, thus it is using 3600watts of power to drive the winch.

Is the same gauge cable from the batt positive, to motor and motor to batt negative required to support the amount of electrons generated by the battery that need to pass from the positive terminal, through the motor and back to the batt negative terminal again?

So there is no difference in flow on both sides of the circuit?

I was thinking that as the electrons travel through the wire and in particular through the motor, a great deal of these become consumed and transferred into power, heat etc. So the amount of electrons from the motor back to the battery has been substantially reduced.

EDIT: Sorry, I think electrons (being negatively charged) travel negative to positive. But current flow is the opposite???

[Mehrts]

To throw a curly one in here, the electrons actually travel from the negative plates to the positive ones in the battery

This is due to the electrons having a negative charge therefore the positively charged plates attract them while the negatively charged plates repel.

Just like a magnet!

Anyways, in answer to your question... Current is the major factor when dealing with cable size. There is exactly the same current going through the positive and negative cables when a circuit is complete (winch in operation).

So, long story short, yes the negative cable "should" be the same size as the positive!

[fullthrottle]

Thanks for that. Crystal clear now.

[L-andy]

Current (amps) is constant throughout a circuit so same size cable needed on negative. Current is basically rate of flow of electrons, like litres/min.

Voltage changes (drops) across the load. Voltage is basically electrical pressure like PSI.

Winch cable - lots of flow (250-500amps or more)but low pressure (12v) so lots of copper but not much insulation needed.

HT lead very little flow (maybe 0.1amps) but lots of pressure (20000 - 50000volts) so hardly any copper but very thick insulation.

Power (watts) is volts x amps

Cheers

Andy

[fullthrottle]

Voltage is basically electrical pressure like PSI.

Winch cable - lots of flow (250-500amps or more)but low pressure (12v) so lots of copper but not much insulation needed.

HT lead very little flow (maybe 0.1amps) but lots of pressure (20000 - 50000volts) so hardly any copper but very thick insulation.

Ahhh, great way to think of it. Cheers Andy.

[Petemcc]

voltage = current * resistance (v=I*R) (1)

Power = voltage * current (P=I*V) (2)

therefore substituting (1) to the place the V in (2) you get

P=I*I*R (P=Isquared*R)

When you have current flow through a cable (or anything else with resistance) you get heating losses (ie power = current flowing * current flowing * resistance of the cable) when you have too much current the heat gets too much and you melt the cable. The bigger the cable the lower the resistance therefore to create less heat you need to have a bigger cable which will handle the current drawn from your motor.

and a little more....

Electrons move around a circuit (current is the number of electrons moving or flow) they each have a certain amount of voltage (12V DC for example).
your cable has a resistance which is ohms/meter. Each item in your circuit has a set resistance (cable , motor , switch etc with the motor having a much much higher resistance than the other components). the electrons will dish out their voltage to all items of resistance as they move around the circuit and will end up with zero when they get back to the start. They give the most volts to the item with the biggest resistance.


to take that a little further a cable has resistance which is ohms per meter. the total resistance (ohms per meter * number of meters) will have a voltage drop associated with it (V=I*R). if you cable is really long the voltage drop will be too much and you will need to go to a bigger cable with less resistance. (normally about 5% voltage drop across the cable is acceptable). This is the other reason you might go for a bigger cable even though they current might not be that high. but this shouldn't be a problem in your car.

Hope that make sense. Cant be bothered proof reading

[SamLogan]

i was taught when i was in school if you think of current as dump trucks and voltage as their payload the trucks always have to do the loop and then drop their payload off at the destination (the object being powered). When you have something that is being powered there is a voltage drop across it and there is not a current drop.

Sam