Download Magnetic Fields - Fanatical Physicists

Edexcel A2 Physics
Unit 4
Bev
Magnetic Fields
Revision Booklet
Rules, Rules, Rules!
BIL
Contents
Review Questions.................................................................................................................................... 3
Wire Rule (Right Hand) ........................................................................................................................... 4
Coil Rule (Right Hand) ............................................................................................................................. 4
Motor Rule (Fleming’s Left Hand Rule) ................................................................................................... 5
Motor Rule for Charged Particles ....................................................................................................... 5
Motor Rule for Current-Carrying Wires .............................................................................................. 7
Generator Rule (Fleming’s Right Hand Rule) .......................................................................................... 9
Faraday’s Law & Lenz’s Law ................................................................................................................ 9
2|Page
Review Questions
1. An iron bar is placed between two magnets as shown below, which makes the iron a temoprary
magnet. Sketch the magnetic field (the lines of flux) between the permanent magnets and the nereast
end of the iron bar.
S
N
2. Lines of flux by convetion point from the magnetic _____ pole to the magnetic _____ pole.
3. What pole of the bar magnet shown on the right is nearest the compass?
a) North
b) South
Magnet
?
4. In the case below, the poles of the left magnets are _____ poles.
5. Attraction or repulsion of charges depends on their signs: positive or negative. Attraction of
magnets depends on their magnetic _____: _____ or _____. Opposite poles _______; like poles _____.
6. You have an electromagnet that is not quite strong enough. What three changes could you make to
increase the strength of the magnetic?
1. _________________________________________________________________________
2. _________________________________________________________________________
3. _________________________________________________________________________
3|Page
Wire Rule (Right Hand)
A straight current-carrying wire creates a radial magnetic field around it. To determine its orientation,
point your thumb in the direction of the conventional current (from + to – along the length of the
wire). Your fingers will curl around the wire in the same direction as the magnetic field.
-
+
Coil Rule (Right Hand)
A current-carrying wire coiled around a core will create a magnetic
field resembling that of a bar magnet, with distinct north and south
poles at the ends of the coil. To determine which end of the coil is
which, curl your right fingers around the coil so that they follow the
direction of conventional current. Your thumb will point to the coil’s
north pole.
S
N
Use the Wire Rule and Coil Rule to answer the following questions.
7. A wire is shown with a radial magnetic field emanating
from it. If the right end is positive, does the magnetic flux
along the front side of the wire point upwards or
downwards?
_________________________________________________
8. A wire is shown to the left in cross-section with its radial field around
it. If the conventional current flows out of the page (as shown by the dot
in the centre of the wire), is the field clockwise or anticlockwise?
__________________________________________________________
+
9. A coil wrapped around a core is shown to the right, with the
left end of the coil positive and the wire wrapped around it as
indicated. Which end of the coil is the north pole?
_________________________________________________
10. A coil is wrapped around a core as shown on the diagram on the left, with its
lines of flux. Which end is positive?
__________________________________________________________________
4|Page
Motor Rule (Fleming’s Left Hand Rule)
A charged particle moving perpendicularly through a magnetic field will experience a force that is
perpendicular to both its velocity and the field. The same effect can also push a current-carrying wire.
This is called a deflecting force.
What does the thumb stand for?
____________________________________________
What does the first finger stand for?
____________________________________________
What does the second finger stand for?
____________________________________________
Motor Rule for Charged Particles
For a charged particle moving through a magnetic field:
I’m Bev!
𝐹 = 𝐵𝑒𝑣. 𝑠𝑖𝑛𝜃
Where θ is the angle between the velocity vector and the magnetic field vector.
Since sin90° = 1, the force, F, is greatest when the particle moves perpendicular to
the magnetic field lines. But since sin0° = 0, the force disappears if the particle
moves parallel to the magnetic field.
11. A positively charged particle is moving
uniform magnetic field with a velocity, v, directed out of the
experiences a force as shown on the diagram on the right.
direction of the magnetic field, B?
through
page
What is
a
and
the
____________________________________________________________
12. A wire is immeresed in a magnetic field that points into the page. Which direction must the
convetional current flow through the wire to cause the wire to be deflected to the left as shown in the
diagram?
_____________________________________________________
5|Page
13. What is the direction of the force on
the current-carrying wire in the magnetic
field shown in the diagram? Conventional
current flow is into the page.
__________________________________________________________________________________
14. The current flowing in the wire shown to
the left creates a downward force on the
wire. Show the direction of the conventional
current flow. (Use a dot for out of the page
and an X for into the page).
15. A proton in the solar wind travels perpendicularly at 3.00x105ms-1 across the Earth’s magnetic field
lines. The field at that location is 0.0000100T.
Proton Charge = +1.6x10-19C
Proton Mass = 1.67x10-27kg
Use the motor rule equation for a particle to determine the magnitude of the force on the particle.
__________________________________________________________________________________
__________________________________________________________________________________
_______________________________________________________________________________
The motor rule predicts that this force will act as a centripetal force, causing the proton to have circular
motion. Calculate the radius of the circle it begins to make.
__________________________________________________________________________________
__________________________________________________________________________________
_______________________________________________________________________________
16. A neutron and an electron are shot between the poles of a U magnet which generates a uniform
magnetic field across their paths of magnitude 0.600T.
One of the particles flies straight through the magnet’s field lines without any deflection. Is that
particle the neutron or the electron? Briefly explain how you know this.
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
6|Page
The other particle is deflected by a force of 4.00x10-15N. Determine the speed at which the particle
was moving through the magnetic field.
__________________________________________________________________________________
__________________________________________________________________________________
_______________________________________________________________________________
Motor Rule for Current-Carrying Wires
For a current-carrying wire in a magnetic field:
𝐹 = 𝐵𝐼𝐿. 𝑠𝑖𝑛𝜃
I’m BIL!
Where θ is the angle between the velocity vector and the magnetic field vector.
Since sin90° = 1, the force, F, is greatest when the current flows perpendicular to
the magnetic field lines. But since sin0° = 0, the force disappears if the current
flows parallel to the magnetic field.
17. A simple motor consists of a straight wire 0.500m long lying at 90° across a uniform magnetic field
of 0.300T. The motor will be used to accelerate a 5.00 gram mass at 3.00ms -2. What is the smallest
current that could be used?
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
18. A conductor of mass 40.0 grams per
metre is hung by two wires in a magnetic
field of strength 3.60T directed out of the
page as shown.
Which source, A or B, should be switched
on so that the magnetic field will deflect
the wire upward, helping balance the
conductor’s weight? Explain your answer.
A
B
_________________________________________________________________________________
__________________________________________________________________________________
How much current from the source would be needed so that the conductor would levitate, reducing
the tension in the wires to zero?
_________________________________________________________________________________
__________________________________________________________________________________
7|Page
8|Page
Generator Rule (Fleming’s Right Hand Rule)
We have seen that a charge moving across magnetic field lines will experience a deflecting force. We
can use this property to create or “induce” electric current when wires and coils are mechanically
moved through magnetic fields. The charges in the wires and coils are then deflected so that they flow
down the wire – we say that an electric current has been induced. This is the principle behind the
electric generator, as first discovered by Michael Faraday.
If you mechanically spin a coil of wire in a magnetic field (such as the armature of a motor), current
will then be produced and you have a generator. Since a
generator is a motor in reverse, you reverse the motor
rule to use it for generators.
What does the thumb stand for?
____________________________________________
What does the first finger stand for?
____________________________________________
What does the second finger stand for?
____________________________________________
Faraday’s Law & Lenz’s Law
The induced voltage that creates an induced current is directly proportional to the strength of the
magnetic field, the length of wire moving through the field, and the speed at which the wire moves
through the field. The induced voltage is sometimes called the electromotive force or emf and given
the symbol, ε.
𝜀 = 𝐵𝐿𝑣
The induced voltage can also be defined by Faraday’s Law:
The magnitude of an induced emf is proportional
to the rate of change of flux linkage.
9|Page
∆(𝑁Ф)
𝜀= −
𝑡
The negative sign is a result of Lenz’s Law:
The direction of an induced emf is such as to
oppose the change creating it.
19. A wire is immersed in a magnetic field that points
out of the page. Which way must the wire be pushed to
induce a conventional current that flows down the page
as shown in the diagram on the right?
_____________________________________________
_____________________________________________
20. In which diagram below would electricity be produced? The diagrams show magnetic fields as
arrows and a wire in cross-section as a circle with an arrow indicating its movement. _____________
A.
B.
21. In the diagram you selected above, which way does the induced conventional current flow? Add
a dot to your chosen diagram to represent the induced current flowing out of the page and a cross to
represent the induced current flowing into the page.
22. A simple generator consists of a 2.00m long copper wire that is suspended in a perpendicular
magnetic field and moved at right angles to the lines of flux. The resistance of the wire is 3.00Ω and
the current induced in the wire is 0.0600A. If the wire moves at 0.150ms-1, what is the magnitude of
the magnetic field?
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
10 | P a g e
23. A rod is perpendicular to a magnetic field of 0.450T and is moved at right angles to the flux at the
rate of 30.0cms-1. If the emf induced across the rod is 0.0200V, what is the length of the rod?
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
THE END
11 | P a g e