1. Day 3 Magnetic Forces on Current-Carrying wires
Magnetism
Day 3 Magnetic Forces on Current-Carrying wires

2. RHR-1 PRACTICE Sign + Magnetic Field East Velocity North Force Sign
Down
Velocity
West
Force
South
Sign -
Magnetic Field
North
Velocity
Force
West
Sign +
Magnetic Field
East
Velocity
West
Force

3. When current flows through a wire, the individual charges are forced in a certain direction, and thus the wire experiences a force.
Since d = rt,
L = vDt
Wire of length L I
Fmag
Remember, I consists of + charges.

5. THE DC MOTOR Split-Ring Commutator Field Magnets (SRC)
(Permanent Magnets)
Loop of current-carrying wire
Brushes
Current Source

6. The purpose of each part
Source: Provides current to the loop.
Loop: Provides a path for the current to flow,
and feels the FORCE that turns the motor.
Split-Ring Commutator (SRC):
Allows current to flow into the loop and out of the loop.
Switches POLARITY every 180o (or half-turn)
Brushes: Keep in contact with the SRC (which is
turning) without causing the wires to the
source to get wound up, kinked, or coiled.
Field Magnets: Create a B field that forces the
wire to turn.

7. DC Motors: How they work

8. The picture below shows a cross-section of the wire loop (showing a cut at the location of the pink dashed line).

9. Find the direction of the force on the loop of wire at point ….B C D B C D
When does the wire loop experience ….
#1) Maximum Force?
#2) Zero Force?
What major event happens every half-turn?

10. Some practice problems
Which direction will the orange dot move? I S N
Which direction is current flowing? Into A or out of A? N S A A

11. What would happen if …. #1) The loop of wire was perpendicular to the
B-field when the battery was hooked up?
#2) The magnets were removed from a DC
Motor?
#3) The Split-Ring Commutator wasn’t split,
but was a solid ring?
#4) The wire loop had many LOOPS
(WINDINGS) before coming back to the
commutator?
#5) The current was AC (Alternating Current)
instead of DC (Direct Current)
#6) The brushes hit the “split” in the split-ring