Magnetic Fields, Magnetic Forces, and Electromagnetic Induction Homework
1. A wire 1 m long carries a current of 5 A. The wire is at right angles to a uniform magnetic field. The
force on the wire is 0.2 N. What is the magnetic field strength?
2. Now in number one, the wire is directed at an angle of 30°. What is the magnetic field strength?
3. A 10 cm long wire is at right angles to a uniform magnetic field of 0.06 T. A 4 A current is in the wire.
What magnetic force does it experience? What is the magnetic force if the wire was at an angle of 40°
to the magnetic field?
4. A beam of electrons travels at 3,000,000 m/s through a 0.04 T field. What force does the beam
experience?
5. A square coil with 100 loops and sides of 5 cm is positioned perpendicularly to a uniform 0.06 T
magnetic field. It is quickly and uniformly pulled out of the magnetic field (moving perpendicular to
the field) to a region where the magnetic field suddenly drops to zero. It takes 0.10 sec for the whole
coil to reach the field-free region. Find the change in magnetic flux in the coil. Calculate the emf and
current induced if its resistance is 100 ohms.
6. A 9.2 cm diameter loop of wire containing 10 turns is initially oriented perpendicular to a 1.5 T
magnetic field. It is rotated so that its plane is parallel to the field in 0.20 sec. Find the change in
magnetic flux in the coil. Calculate the emf and current induced if its resistance is 50 ohms.
7. A wire 2 m long moves perpendicularly through a 0.08 T field at a speed of 7 m/s. What emf is
induced? Knowing Ohm’s law and that the wire has a resistance of 0.50 ohms, what current flows?
8. The direction of a 0.045 T field is 60° above the horizontal. A wire 2.5 m long moves horizontally
through this field at 2.4 m/s. What is the vertical component of the magnetic field? What emf is
induced in the wire?
9. An electron (mass = 9.11 x 10-31kg) travels at 2 x 107m/s in a circular path in a 0.010 T magnetic field.
What is the radius of its path? What is its kinetic energy? Ans: 0.011 m; 1.82 x 10-16J
10. A transformer has input voltage and current of 12 V and 3 A. It has an output current of 0.75 A. If
there are 1200 turns on the secondary, how many turns are on the primary? What is the output voltage?
Is this a step-up or step-down transformer?
11. A model electric train requires a low voltage to operate. If the primary coil of its transformer has 400
turns and the secondary has 20 turns, how many volts will power the train when connected to a 120 V
house circuit? Is this a step-up or step-down transformer? What is the input current if the current in the
train is 0.50 A?
Extra Problems
1. A wire 0.10 m long carrying a current of 2 A is at right angles to a magnetic field. The force on the
wire is 0.04 N. What is the strength of the magnetic field? What would its strength be if the wire is at
an angle of 35° to the field? Ans: 0.20 T; 0.35 T
2. A wire 0.50 m long carrying a current of 8 A is at an angle of 15° to a 0.40 T field. What is the
magnetic force exerted on the wire? Ans: 0.41 N
3. A beam of electrons moves at right angles to a 0.60 T field. The electrons have a velocity of 2.5 x 107
m/s. What force acts on the electrons? What force acts if the beam of electrons move at an angle of 45°
to the field? Ans: 2.4 x 10-12 N; 1.7 x 10-12 N
4. A proton moves at right angles to a 0.003 T field directed out of the page. The proton moves from
right to left with a speed of 5 x 106 m/s. What is the magnitude and the direction of the force the
proton experiences? Ans: 2.4 x 10-15 Ν, τοωαρδσ τηε τοπ οφ τηε παγε
5. Α ωιρε 0.5 μ λονγ χυτσ τηρουγη α 0.4 Τ φιελδ ατ α σπεεδ οφ 20 μ/σ. Ωηατ ισ τηε ινδυχεδ εμφ?
Ιφ τηε ωιρε ισ παρτ οφ α χιρχυιτ ωιτη α τοταλ ρεσιστανχε οφ 6 Ω, what is the current in the
circuit? Ans: 4 V; 0.67 A
6. A wire 20 m long is mounted on an airplane flying at an angle of 60° to the earth's magnetic field. It
flies with a speed of 120 m/s. If the induced emf is 0.12 V, what is the strength of the earth's magnetic
field at this point? Ans: 5.77 x 10 -5 T