Chapter 21 - Magnetic Forces Flashcards
F = qvBsinx
What is this formula?
What does x represent?
Force experienced by particle in a magnetic field
x is the angle between the velocity and magnetic field
F = ILBsinx
What is this formula?
What does x represent?
The force on a current carrying wire placed in a magnetic field
x is the angle between the wire/current and the magnetic field
B = µI/2πr
What is the formula?
Magnetic field produced by a long straight wire
B = NµI/2R
What is the formula?
Magnetic field produced at the center of a circular coil
B = unI
What is the formula?
What does n represent?
Magnetic field produced at the center of a solenoid
n represents the turns per unit length of solenoid
n = N/L
N is total turns and L is length of solenoid
Explain the rule of RHR#3 and what equation it is used for
It is used to find the direction of the magnetic field around current carrying wire. Relates to B = uI/2pir
1) Point thumb in direction of current, curl fingers around the wire, your fingers show the direction of the magnetic field
Explain the rule of RHR#1 and what equation it is used for
It is used to find the direction of the magnetic force acting on moving charged particle in magnetic field. Relates to F = qvBsinx
1) Point thumb in direction of velocity of positive charge and fingers in direction of magnetic field
2) Palm shows direction of magnetic force on positive charge, for negative charge the force is in the opposite directon
Explain the rule of RHR#2 and what equation it is used for
It is used to find the direction of the magnetic force on a wire in a magnetic field. Relates to F = ILBsinx
1) Point thumb in direction of current, fingers in direction of magnetic field, palm shows direction of force on wire
What do dots and crosses represent?
The crosses (like dashes) are into the page or away from you and dots (like wedges) out of the page or towards you
A long straight wire
carries a current of 8.0 A
and a circular loop of
wire carries a current of
2.0 A and has a radius of
0.030 m.
Find the magnitude and
direction of the magnetic
field at the center of the
loop C
Given that B1 points upwards and out of page and B2 points inward and into page
Sooooo
Long straight wire-circular coil
B = uI1/2pir - uI2/2R
= 1.1x10^-5
The radius of a coil of wire with N turns
is R = 0.22 m.
A current Icoil = 2.0 A flows clockwise
in the coil, as shown. A long, straight
wire carrying a current Iwire = 31 A
toward the left is located 0.05 m from
the edge of the coil. The magnetic field
at the center of the coil O is zero Tesla.
Determine N, the number of turns
Really not too hard!!
STEP 1
Find the magnetic field in the wire. r is distance so that would be 0.22 + 0.05
STEP 2
Find magnetic field in the coil
Now becuase one field goes into page and other out, they cancel and their sum is 0 so you can set them equal to e/o
Sub in all known values and you should get N = 4.02
Number of turns must be integer though so round to nearest whole integer, 4
When is sin at a max and min?
Max: 90
Min: 0 or 180
A 150 V battery is connected across two parallel metal plates of area 28.5 cm2 and separation 8.20 mm. A
beam of alpha particles (charge +2e, mass 27 6.64 10 kg − × is accelerated from rest through a potential
difference of 1.75 kV and enters the region between the plates perpendicular to the electric field. What
magnitude and direction of magnetic field are needed so that the alpha particles emerge undeflected from
between the plates?
Undeflected means particle goes straight and in order for this to happen, magnetic field and electric field must cancel
Kinetic energy is equal to work done, and positive in this case because it’s moving WITH the field
qV= 1/2mv^2 isolate for v and you should get 4.11 x 10^5
Electric field between plates E = V/d
= 18300 V/m
To cancel (becuase it’s perpendicualr you do not need the sine)
qE = qvB
B = E/v
= 0.0445T
What are the conditions to experience magnetic field?
Charge is moving
Velocity of charge must have component perpendicular to direction of magnetic field
[10] (4) A horizontal wire is hung from the ceiling of a room by two massless strings. The wire has a length of
0.40 m and a mass of 0.14 kg. A uniform magnetic field of magnitude 0.08 T is directed from the ceiling to the floor.
When a current of I = 46 A exists in the wire, the wire swings upward and, at equilibrium, makes an angle ϕ with
respect to the vertical, as the drawing shows. Find
[5] (4a) The angle ϕ and
[5] (4b) The tension T in each of the two strings
a) Vertical component and horizontal component
2Tcosx = mg
2Tcosx = 1.3734
2Tsinx = ILB (sin 90 is 1 so you can disregard this, current horizontal and mag field is vertical so angle is 90)
2Tsinx = 1.472
2Tcosx = 1.3734
tanx = 1.0718
x = 46.5
b) Solving for T,
2Tcosx = 1.3734
T = 1.3734/2cos46.5
T = 1N
An electron in the beam of a TV picture tube is accelerated through a potential difference of 2.00 kV. It then
passes into a magnetic field perpendicular to its path, where it moves in a circular arc of diameter 0.360 m.
What is the magnitude of this field?
Kinetic energy from acceleration
qV = 1/2mv^2
solve for v
2.65 x 10^-7
Forces balance
mv^2/r = qvB
solve for B
8.38 x 10^-4
What does no deflection mean?
Undeflected means particle goes straight and in order for this to happen, magnetic field and electric field must cancel
electric force = magnetic force
A long wire carrying 4.50 A of current makes two 90° bends, as shown in Figure 20.64. The bent part of the
wire passes through a uniform 0.240 T magnetic field directed as shown in the figure and confined to a
limited region of space. Find the magnitude and direction of the force that the magnetic field exerts on the
wire.
Photo in word
Label three segments a,b, c
length of A is x and length of c is 0.06-x
Fa = ILB
= 4.5x(0.240)
Fc = 4.5(0.6-x)(0.240)
The direction of both these forces are downwards so you add them
Fa + Fc and you get
(4.50A)(0.6)(0.240) = 0.648N
Fb = 4.50 x 0.3 x 0.240 = 0.324
Then to get F you do Pythagorean theorem and get
0.724N
tanx = Fac/fb = 0.648/0.324
x = 63.4
How to figure out number of turns given length of wire and radius?
N = L/2pir
Length divided by circumferance of each length
Two closed loops A and C are close to a long wire carrying a current I.
(See Figure) Find the direction (clockwise or counterclockwise) of the
current induced in each of these loops if I is steadily increasing.
With current use RHR#3 to determine if the field is coming into or out of the page with each of the shapes. Out of page for C and into page for A.
Since current is increasing, flux is too and it has to oppose this increasing flux by orienting field the other way
So now for C you are going into page and A out of. So based on this you can find the direction of the current using RHR#3
C = counterclockwise
A = clockwise
