19. A negative charge moves away from a current carrying...

19. A negative charge moves away from a current carrying wire. What is the direction of the magnetic force on the charge? A. Left. B. Right. C. To the bottom of the page. D. To the top of the page. E. There is no magnetic force on the charge. 20. A vertical wire carries an electric current into the page. An electron approaches the current from east. What is the direction of the magnetic force on the electron? A. East. B. West. C. North. D. South. E. Into the page.

Transcript

00:01 So in this question we are dealing with currents which create magnetic fields and with the motion of an electron in that corresponding magnetic field.

00:11 So let's take a look at the first question, question 19.

00:14 We have a current carrying wire, so a wire which carries a current i, and this current points upwards in the vertical direction on the screen.

00:28 So we know that a current flowing through wire creates a magnetic field.

00:35 And we can use a rule to determine the direction of the magnetic field.

00:39 And the rule is the following.

00:41 If a current carrying conductor is held by the right hand, keeping the thumb straight, and if the direction of the electric current is in the direction of the thumb, then the direction of wrapping of all the other fingers will show the direction of the direction of the direction the magnetic field.

01:00 So in this case, if you place your thumb of the right hand in this direction, you will see that the direction of wrapping the other four fingers will be around the magnetic field like this.

01:17 So i will use a different color actually.

01:19 So this direction of wrapping the other fingers actually give you the direction of the magnetic field b.

01:27 So in other words, on this side of the screen on the right -hand side, the magnetic field is pointing into the page, and on the left side of the wire, the magnetic field is pointing out of the page.

01:41 Now, let's take a look at the electron.

01:43 So we have a negative charge sitting on the right -hand side of the screen, and this electric charge moves with velocity v to the right.

01:55 Okay? so we have the magnetic field, which points into the page and we have the velocity which points to the right now we know that when a charge when a charge particle is moving in a magnetic field that charge experience a force which is called lawrence force and that force uh fl is given by the charge and then the product between velocity and the magnetic field so in this case fl is minus e so the charge of that particle v b.

02:36 Okay, so let's see the direction of vb.

02:40 So in order to determine the direction of vnb, we have to use fleming's left -hand rule.

02:50 So in fleming's left -hand rule, the first finger, the index points along the magnetic field, and the second finger, the middle finger, points along the velocity of the particle.

03:04 So in this case, the index which needs to point along the magnetic field goes into the page and the middle finger points to the right.

03:16 So along the velocity v.

03:20 Now the thumb will show you the direction of the force.

03:24 So the thumb points upwards if you're orienting your hand properly.

03:30 So this means that vb points upwards.

03:34 Now notice that we have a minus sign in the expression for the force since this is a negative charge.

03:41 So this means that the actual force will be pointing in the opposite direction, which is downwards.

03:47 So the final answer to this question is downwards or to the bottom of the page.

03:54 Good.

03:54 Now let's take a look at the second question...

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