Determining polarity of an AC generators terminals (1 Viewer)

[rand_althor]

Can someone explain how you do this? Here is an example question:

 

[Mr_Kap]

Can someone explain how you do this? Here is an example question:

It is terminal B i believe. Using the right hand palm rule, we face our 4 fingers(not the thumb) facing South (from north to south is direction of magnetic field). Next, we know the direction of motion is clockwise so that means on the left coil the force will be upwards so we make sure that our palm is faced upwards as well. Our thumb points that current is from K to L (current is direction of positive), meaning it travels anticlockwise around the coil, and we look to see where the coil starts from on the right sight of the coil and we see it is STARTING FROM TERMINAL B. Hence Terminal B is positive.

Someone confirm this but I'm pretty confident.

 

[Drsoccerball]

What do you mean polarity ? The direction in AC generators is constantly changing. You cannot label one side as being positive as it is constantly changing. What you can do is know why sides are positive in the diagram but i don't see how you can do that for the connections.

 

[spatula232]

It is terminal B i believe. Using the right hand palm rule, we face our 4 fingers(not the thumb) facing South (from north to south is direction of magnetic field). Next, we know the direction of motion is clockwise so that means on the left coil the force will be upwards so we make sure that our palm is faced upwards as well. Our thumb points that current is from K to L (current is direction of positive), meaning it travels anticlockwise around the coil, and we look to see where the coil starts from on the right sight of the coil and we see it is STARTING FROM TERMINAL B. Hence Terminal B is positive.

Someone confirm this but I'm pretty confident.

Answer from textbook; "Induced current is clockwise around the coil as we view it. The current then emerges from the generator through terminal B. Therefore, terminal B is positive." So B is correct, but I don't think your explanation is.

Apply Lenz's law; that the current is induced in the coil opposing the motion that caused it; therefore you must flip your hand. However, I don't understand why then B would be positive. Since it's positive flow from L-K, positive flow must be attracted towards the negative terminal. Shouldn't that mean that B would then be negative??

You might be right and I've just confused myself :/

 

[spatula232]

What do you mean polarity ? The direction in AC generators is constantly changing. You cannot label one side as being positive as it is constantly changing. What you can do is know why sides are positive in the diagram but i don't see how you can do that for the connections.

It's a generator -- you're able to determine which terminal would be acting as positive or negative at that instant as current is induced in the coil. Basically, which terminal would be acting as + or - once you've connected it to an external circuit.

 

[Mr_Kap]

The current is flowing anti-clockwise, as it's the simple right hand push rule isn't it? You're confusing me now. How could current be flowing clockwise, when the motor is spinning clockwise, AND the magnetic field is to the right?

 

[Zlatman]

The current is flowing anti-clockwise, as it's the simple right hand push rule isn't it? You're confusing me now. How could current be flowing clockwise, when the motor is spinning clockwise, AND the magnetic field is to the right?

It says it's a generator, so we use the left hand push rule, meaning the current is flowing clockwise.

 

[spatula232]

It says it's a generator, so we use the left hand push rule, meaning the current is flowing clockwise.

So which terminal is it then?
Why is B positive?

 

[spatula232]

The current is flowing anti-clockwise, as it's the simple right hand push rule isn't it? You're confusing me now. How could current be flowing clockwise, when the motor is spinning clockwise, AND the magnetic field is to the right?

Because the current is induced in the coil as it's a generator, as Zlatman said. According to Lenz's law, this opposes the motion that caused it, meaning that the current is induced in the opposite direction to which the movement is; i.e. induced current establishes a magnetic field to rotate the rotor anti-clockwise. Thus, the current is going clockwise. As said, you can find this via the left hand palm rule

 

[Zlatman]

So which terminal is it then?
Why is B positive?

Think of the generator like a power pack for the external circuit: the current from a power pack enters the external circuit from the positive terminal then re-enters the power pack through the negative terminal. In this case, the current goes out of the generator through Terminal B (making it the positive terminal) and back in through Terminal A (making it the negative terminal).

 

[Mr_Kap]

Think of the generator like a power pack for the external circuit: the current from a power pack enters the external circuit from the positive terminal then re-enters the power pack through the negative terminal. In this case, the current goes out of the generator through Terminal B (making it the positive terminal) and back in through Terminal A (making it the negative terminal).

Yeh. I didn't read it was a generator, but yeh isn't it just the reverse of the motor, meaning the same terminal is positive.

 

[rand_althor]

Thanks guys! I think I understand now.

 

[Fizzy_Cyst]

Can someone explain how you do this? Here is an example question:

Seems to be addressed, but I will add my way of explaining it anyway!

- Clockwise torque is exerted on the coil, causing coil to spin and hence to experience a change in flux.

- Current will be induced in such a direction to oppose the motion of the coil relative to the magnetic field (i.e., create an anti-clockwise torque)

- Therefore, force downwards on side KL and upwards on side NM

- RHP rule therefore shows that current flows L --> K --> N --> M

- Forget about the external circuit for a second

- All of the 'conventional current' (i.e., positive charges) are flowing L --> K --> N --> M --> Terminal B, making Terminal B positively charged and vice versa for Terminal A (this is where students get confused, as they think that positive charges should move toward the negative terminal inside the coil, this is INCORRECT. They move toward the negative in the EXTERNAL CIRCUIT!)

- Current would therefore flow from Terminal B to Terminal A in the external circuit.

 

[Kaido]

Seems to be addressed, but I will add my way of explaining it anyway!

- Clockwise torque is exerted on the coil, causing coil to spin and hence to experience a change in flux.

- Current will be induced in such a direction to oppose the motion of the coil relative to the magnetic field (i.e., create an anti-clockwise torque)

- Therefore, force downwards on side KL and upwards on side NM

- RHP rule therefore shows that current flows L --> K --> N --> M

- Forget about the external circuit for a second

- All of the 'conventional current' (i.e., positive charges) are flowing L --> K --> N --> M --> Terminal B, making Terminal B positively charged and vice versa for Terminal A (this is where students get confused, as they think that positive charges should move toward the negative terminal inside the coil, this is INCORRECT. They move toward the negative in the EXTERNAL CIRCUIT!)

- Current would therefore flow from Terminal B to Terminal A in the external circuit.

Is 'conventional current' a flow of positive charges?

 

[Fizzy_Cyst]

Is 'conventional current' a flow of positive charges?

It's the direction that positive charges WOULD flow if they were responsible for carrying current.

 

[rand_althor]

- All of the 'conventional current' (i.e., positive charges) are flowing L --> K --> N --> M --> Terminal B, making Terminal B positively charged and vice versa for Terminal A (this is where students get confused, as they think that positive charges should move toward the negative terminal inside the coil, this is INCORRECT. They move toward the negative in the EXTERNAL CIRCUIT!)

That's what I was a bit unsure about. Thanks for clearing it up, I appreciate it.

 

[Mr_Kap]

- All of the 'conventional current' (i.e., positive charges) are flowing L --> K --> N --> M --> Terminal B, making Terminal B positively charged and vice versa for Terminal A (this is where students get confused, as they think that positive charges should move toward the negative terminal inside the coil, this is INCORRECT. They move toward the negative in the EXTERNAL CIRCUIT!)

Can someone explain to me the difference between negative terminal inside the coil, and negative in the external circuit?