# Homework Help: Simple question about potential

1. Jul 6, 2016

### Clara Chung

1. The problem statement, all variables and given/known data
photo:
http://s613.photobucket.com/user/Yan_Wa_Chung/media/02_zpsbdlufj1e.png.html

2. Relevant equations
potential = kq1q2 / r

3. The attempt at a solution
The answer is D. Which states "The electric potential in P is higher than Q" is correct. But my answer is B. By right hand rule, the current goes from Q to P. The formula of potential has a positive sign. So I think Q has a higher potential as work is needed to move the protons from P to Q. Why am I wrong?

2. Jul 6, 2016

### cnh1995

Your right hand rule analysis correct but P is at a higher potential than Q. The conductor in the magnetic field is acting as a source. You can replace it with a battery. What is the direction of current inside a voltage source when it is delivering power?

3. Jul 6, 2016

### Clara Chung

Do you mean placing a battery in QP, then I get P is the positive terminal and Q is the negative terminal, so P has a higher potential . Thanks for reminding me the source is in the magnetic field.

4. Jul 6, 2016

### cnh1995

Yes. When a voltage source is delivering power, current flows from -ve to +ve terminal inside the source.

5. Jul 6, 2016

### Clara Chung

is positive terminal always have a higher potential even when a voltage source is delivering power?

6. Jul 6, 2016

### cnh1995

Conventionally, +ve terminal is always at a higher potential than the -ve terminal (of the same source), irrespective of the power flow. If the current is flowing from +ve terminal to -ve terminal inside the component, it is absorbing power and if the current is flowing from -ve to +ve terminal inside the component, the component is acting as a source of power. Note that the current here is the conventional current. So, for a power delivering voltage source, current flows from -ve to +ve terminal inside the source and for a power absorbing voltage source, current flows from +ve to -ve terminal inside the source.