Electromagnets

April

Electricity vs. Magnetism

Electricity Magnetism+ and - North and SouthElectric field, E caused by electric charges, stationary or in motion

Magnetic field, B caused by electric charges in motion

Like charges repel; unlike charges attract

Like poles repel; unlike poles attract

Electric charges can be isolated

Magnetic poles cannot be isolated

Magnetic field lines

N S

Draw magnetic field lines around a magnet.

N S

Outside the magnet: mfl move from the north pole to the south pole.Inside the magnet: mfl move from the south pole to the north pole.

Electromagnets

• Electric current produces magnetic field. An electromagnet is a current-carrying coil of wire with many loops

The Right-Hand Rule

If the wire is grasped in the right hand with the thumb in the direction of the current, the fingers will curl in the direction of magnetic field.

electromagnets

A current-carrying coil of wire with many

loops is an electromagnet.

I

Magnetic Field due to the electric current

Magnetic Field due to the electric current

Magnetic Field inside a Solenoid

A coil of several closely spaced loops is called a solenoid.

Demonstration

• Electromagnetism• Materials list: D-cell battery, insulated wire, nail,

compass• Use the compass to determine whether the nail

is magnetized.• Next, flip the battery so that the direction of the

current is reversed.• Again bring the compass toward the same part

of the nail.

symbols of magnetic field

electromagnets

A current-carrying coil of wire with many

loops is an electromagnet.

Right-hand rule practice

Electric Field

Magnetic force (Fmagnetic)

• A charge moving through a magnetic field experiences a force.

• Fmagnetic = qvB

q is the magnitude of charge. (C) v is the speed of charge. (m/s) B is the strength of magnetic field. Tesla (T)

1 T is the magnetic field strength in which a charge of 1 C experiences 1 N of force when it moves at the speed of 1 m/s.

direction of magnetic force

• To determine the direction of the force, use the right-hand rule.

• Place your fingers in the direction of B with your thumb pointing in the direction of v.

• The magnetic force on a positive charge is directed out of the palm of your hand.

Right-hand rule practice

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A charge moving through a magnetic field follows a circular path.

practice

• An electron moving north at 4.5 x 104 m/s enters a 1.0 mT magnetic filed pointed upward.

• a. what is the magnitude and direction of the force exerted on the electron?

• b. what would the force be if the particle were a proton?

• c. what would the force be if the particle were a neutron?

answers

• a. 7.2 x 10-18 N west• b. 7.2 x 10-18 N east• c. 0.0 N

Practice

• A proton moves perpendicularly to a magnetic field that has a magnitude of 4.20 x 10-2 T. What is the speed of the particle if the magnitude of the magnetic force on it is 2.40 x 10-14 N?

• 3.57 x 106 m/s

Force on a current-carrying conductor perpendicular to a magnetic field is proportional to magnetic field, current and length of conductor.F magnetic = BIl

B is the strength of magnetic field. Tesla (T)I is the current. (A)l is the length of the wire (m)

Practice

• A 4.5 m wire carries a current of 12.5 A from north to south. If the magnetic force on the wire due to a uniform magnetic field is 1.1 x 103 N downward, what is the magnitude and direction of the magnetic field?

• 2.0 x 101 T, to the west

Homework

• Pages 782 and 783• Questions 26, 30, 33, 34 and 41.