Download File

• The wires are separated by distance a and
carry currents I1 and I2 in the same
direction.
• Wire 2, carrying current I2, sets up a
magnetic field B2 at the position of wire 1.
- The direction of the
magnetic field B2 is
perpendicular to the wire.
- F1 = F2 on 1 = I1·(L x B2)
- Angle q between L and B2
is 90.
Force between two parallel current-carrying
straight wires
1. Parallel wires with current flowing in the
same direction, attract each other.
2. Parallel wires with current flowing in the
opposite direction, repel each other.
• When the currents are in opposite
directions, the magnetic forces again equal
in magnitude but are opposite in direction
and the wires repel each other.
Recall:
The Magnetic Field inside a Current Carrying Solenoid
• The magnetic field inside a coil, loop of wire,
or a solenoid, is uniform and proportional to
the current, the number of loops of wire N,
and the length of the solenoid l.
• The is can be expressed with the following
equation:
Where n is the number of loops per unit length:
To determine the direction of the field we
need to either use the first or second right
hand rule.
Direction of magnetic field inside a solenoid:
The magnetic field near
the center of a single coil
carrying a current I.
The magnetic field near the center of
a set of three coils all carrying a
current I.
A solenoid approximates many
current-carrying coils spaced much
more closely than the coil
diameter.
2nd RIGHT-HAND RULE
To determine the direction of the magnetic field
inside and around a solenoid you can use the first
or second right hand rule.
nd
• 2 RIGHT-HAND RULE
• If you grasp the coil
with the right hand and
curl your fingers around
the loops in the
direction of the
conventional current,
your thumb points
towards the North pole
of the electromagnet.
Examples:
• Ex 1: A 0.400 m long
solenoid has 6 720
turns of wire. A current
of 14.5 A flows in the
solenoid. What is the
direction and size of the
magnetic field inside
the solenoid?
(Picture shown
above right)
• Ex 2: An electron inside
the solenoid travels
perpendicular to the
axis of the solenoid with
a speed of 6.50 x105
m/s. What is the
magnitude and
direction of the
magnetic force acting
on the electron?
Electromagnetism and Cathode Ray Tubes
• Instead of having an electric field create a
force on an electron to deflect its path, a
magnetic field can also be used.
• Unlike using charged parallel plates to cause
deflection, the velocity of the electron
doesn’t change the deflection caused by a
magnetic field.
• By increasing its velocity, an electron will
spend less time in the magnetic field, but the
force that it will experience will be greater.
Ex 3: A solenoid placed beneath a cathode ray tube as shown
below produces a magnetic field of 0.011 T on the electron
beam causing it to hit the screen at position 1.
a) The electron beam travels at 4.7 x 107 m/s. What is the acceleration of
the electrons that make up the beam? (9.1 x 1016 m/s2)
b) What direction is the magnetic field caused by the solenoid? (down)
c)What two changes can be made to the current in the solenoid in order
for the beam to now strike the screen at position #2? (reduce current,
change direction)
• Ex 4: The diagrams to the right show two views of an
electron beam in a cathode ray tube. When the
accelerating voltage is 100.0 V, the deflection of the
beam is zero. Which way will the beam be deflected if
the accelerating voltage is changed to 400.0 V? Explain
your reasoning using the principles of physics
Practice:
1. Protons travelling at 5.2 X 105 m/ s enter at right angles to a
magnetic field. The field is produced by a 0.36 m long
solenoid. A current of 6.3 A flows through the 520 turns of
wire of the solenoid. What is the magnetic field in the
solenoid?
2. What is the strength of the magnetic field in the center of a
solenoid 15 cm in length if the solenoid has 5.0 x 102 turns of
wire and carries a current of 5.0 A?
3. The magnetic field at the center of a solenoid of length 0.25
m is 1.2 x 10-2 T. The current in the windings is 7.5 A. How
many windings does the solenoid have? (1.3 x 103)