1 of 25

�Magnetic Force on a Current-Carrying Conductor�Charges moving in a wire�

Physics 1 (Garcia) SJSU

27-Feb-22

2 of 25

Learning objectives

Solve problems involving forces on a current carrying wire in a magnetic field and torque on a current carrying loop of wire in a magnetic field.

3 of 25

Success Criteria

• Describe the effects of a magnetic force on a current-carrying conductor.

• Calculate the magnetic force on a current-carrying conductor.

4 of 25

Clifton Bluhm

(+)

(-)

5 of 25

Clifton Bluhm

(+)

(-)

6 of 25

Clifton Bluhm

AC

7 of 25

Charges moving in a wire

Up to this point we have focused our attention on PARTICLES or CHARGES only. The charges could be moving together in a wire. Thus, if the wire had a CURRENT (moving charges), it too will experience a force when placed in a magnetic field.

You simply used the RIGHT HAND ONLY and the thumb will represent the direction of the CURRENT instead of the velocity.

8 of 25

Charges moving in a wire

At this point it is VERY important that you understand that the MAGNETIC FIELD is being produced by some EXTERNAL AGENT

9 of 25

Example

A 36-m length wire carries a current of 22A running from right to left. Calculate the magnitude and direction of the magnetic force acting on the wire if it is placed in a magnetic field with a magnitude of 0.50 x10^-4 T and directed up the page.

0.0396 N

B = +y

I = -x

F =

+y

+x

+z

-z, into the page

10 of 25

Lab simulation

https://javalab.org/en/dc_motor_en/

11 of 25

North

South

120 v

-120 v

AC Generator

12 of 25

North

South

AC Generator

120 v

-120 v

13 of 25

North

South

AC Generator

120 v

-120 v

14 of 25

North

South

AC Generator

120 v

-120 v

15 of 25

North

South

http://www.lightbulbmarket.com/files/1974975/uploaded/Philips%20Incandescent%20A-19%20Light%20Bulb.gif

AC Generator

120 v

-120 v

16 of 25

North

South

AC Generator

120 v

-120 v

17 of 25

North

South

AC Generator

120 v

-120 v

18 of 25

North

South

AC Generator

120 v

-120 v

19 of 25

North

South

AC Generator

Clifton Bluhm

120 v

-120 v

20 of 25

21 of 25

Clifton Bluhm

22 of 25

WHY does the wire move?

The real question is WHY does the wire move? It is easy to say the EXTERNAL field moved it. But how can an external magnetic field FORCE the wire to move in a certain direction?

THE WIRE ITSELF MUST BE MAGNETIC!!! In other words the wire has its own INTERNAL MAGNETIC FIELD that is attracted or repulsed by the EXTERNAL FIELD.

As it turns out, the wire’s OWN internal magnetic field makes concentric circles round the wire.

23 of 25

A current carrying wire’s INTERNAL magnetic field

To figure out the DIRECTION of this INTERNAL field you use the right hand rule. You point your thumb in the direction of the current then CURL your fingers. Your fingers will point in the direction of the magnetic field

24 of 25

Recall: The MAGNITUDE of the internal field

The magnetic field, B, is directly proportional

to the current, I, and inversely proportional

to the circumference.

25 of 25

Example

A long, straight wires carries a current of 5.00 A. At one instant, a proton, 4 mm from the wire travels at 1500 m/s parallel to the wire and in the same direction as the current. Find the magnitude and direction of the magnetic force acting on the proton due to the field caused by the current carrying wire.

5A

4mm

+

v

2.51 x 10^{- 4} T

6.02 x 10^{- 20} N

X X X

B = +z

v = +y

F =

-x