Homework Help: Magnetic Forces

1. Mar 14, 2009

superjen

Three charges, Q1, Q2, and Q3 are located in a straight line. The position of Q2 is 0.290 m to the right of Q1. Q3 is located 0.147 m to the right of Q2. The force on Q2 due to its interaction with Q3 is directed to the (Give ALL correct answers, i.e., B, AC, BCD...)

A) left if the two charges are positive.
B) right if the two charges have opposite signs.
C) left if the two charges have opposite signs.
D) left if the two charges are negative.
E) right if the two charges are negative.

i guessed BC , i just dont see how the other ones can apply. anyone care to help me? Thanks

2. Mar 15, 2009

Redbelly98

Staff Emeritus
Have you drawn a figure showing Q2 and Q3? (Q1 does not matter here.)

What do you know about the direction of the force for charges of the same or opposite sign?

3. Jul 8, 2009

chloechloe

I guessed.....

a. true
b. false - if 2 is positive and 3 is negative, the force on 2 from 3 is to the right (charge from positive goes out and is attracted to negative) vice versa if 2 is negative and 3 is positive.
c. false
d. true
e. false

4. Jul 8, 2009

queenofbabes

How can both b and c be false?

5. Jul 8, 2009

chloechloe

because I don't understand what the question is asking. to me, b and c are asking the same thing.
B) right if the two charges have opposite signs. 2 could be + or - and 3 could be + or -
C) left if the two charges have opposite signs. 2 could be + or - and 3 could be + or -

if 2 is + and 3 is -, force is to left. If 2 is - and 3 is +, then force is to the right.

6. Jul 8, 2009

queenofbabes

Charges of opposite signs attract!

7. Jul 8, 2009

chloechloe

If 2 is + and 3 is -, what is force on 2?

If 3 is + and 2 is -, what is force on 2?

I know opposites attract, but what does that mean for force on 2?

thanks.

8. Jul 8, 2009

queenofbabes

If two things attract, do they move towards each other or away from each other?

Thus is the force on 2 towards 3 or away from 3?

9. Jul 8, 2009

chloechloe

If they attract they move towards each other, so regardless of which one is positive and which is negative, force will be directed to the right.

right?

I get confused because it is explained to us that convention is charge goes out of a positive <-----+-----> and into a negative ------->-<--------

So, instead of thinking that in attraction they move towards each other, I was thinking that the positive is attracted to the negative, so that the force goes towards the negative if the signs are opposite.

thank you for explaining, this class makes me feel so stupid!

10. Jul 9, 2009

Redbelly98

Staff Emeritus
Yes, correct.

You have some terminology mixed up. It is the electric field that points away from positive and towards negative charges.

Electric field tells you what the direction of the force would be for a positive test charge. As you know, the force on a positive test charge would be away from another positive charge (repelling), or towards a negative charge (attracting). This is the same direction as the electric field, but that only applies to a positive test charge.

The force on the positive charge does goes toward the negative one. And, the force on the negative charge goes toward the positive charge.