Unit 4 concepts

Question 1

How is AC power produced?

Answer 1

AC power is produced in an electrical generator which consists of a coil of wire that is rotated in a magnetic field.

Question 2

What are the advantages of AC vs. DC power?

Answer 2

Voltages in an AC power system can be changed using transformers, and AC plants can be located far away from the cities they supply.

Question 3

What energy sources are used to produce AC power?

Answer 3

Steam produced in a fossil fuel burning plant or nuclear power plant, hydroelectric power, and wind power.

Question 4

What is the role of transformers in the electric power grid?

Answer 4

Transformers are used to step‐up the voltages coming from the power plant for
transmission and then stepped‐down for distribution to homes.

Question 5

What is a Ground Fault Circuit Interrupter (GFCI)?

Answer 5

A GFCI is an electrical safety device that detects small amounts of current flowing in the ground circuit. Its purpose is to prevent electric shocks.

Question 6

Why is AC power transmitted at high voltages?

Answer 6

The power transmitted P is the product of current and voltage. If large currents are
transmitted this requires that the transmission lines use large diameter conductors to minimize losses. Power is more efficiently transmitted at high voltage and low currents.

Question 7

When you pay your electricity bill, do you pay for power used or energy?

Answer 7

Your electricity bill is the cost for the total amount of electrical energy used over a certain period.

Question 8

Why is it advantageous to replace conventional incandescent lights with compact fluorescent lights?

Answer 8

Incandescent bulbs produce a lot of heat compared to the light energy. Compact fluorescent lights produce less heat and are more efficient and produce the same light output for considerably less electrical power.

Question 9

What are the two types of magnets?

Answer 9

The two types of magnets are permanent magnets and electromagnets (currents in wires).

Question 10

What is the general direction and shape of magnetic fields?

Answer 10

Magnets always have a north and a south pole and the magnetic field points from the North Pole to the South Pole. Magnetic field lines always form closed loops.

Question 11

What contributions to magnetism were made by

(a) Oersted, and
(b) Ampere?

Answer 11

(a) Oersted first observed that a compass needle was deflected by a current in a wire.
(b) Ampere discovered the mathematical formula that relates the current in a wire to the magnetic field that it produces.

Question 12

What type of materials are permanent magnets made from?

Answer 12

Permanent magnets are formed from a class of materials known as ferromagnetic.

Question 13

(a) What is the shape of the earth’s magnetic field?

(b) Why does a compass point north?

Answer 13

(a) The earth’s magnetic field points in the direction from the south geographic pole to the north geographic pole. Thus in the northern hemisphere the earth’s magnetic field points downward and at an angle to the ground.

(b) The north pole of the compass points to the
earths geographic north because the geographic north pole of the earth is the south magnetic pole.

Question 14

What are the van Allen radiation belts?

Answer 14

The van Allen radiation belts are two regions surrounding the earth containing a large
accumulation of charged particles. These particles come to the earth from the sun and get trapped by the earth’s magnetic field.

Question 15

What is the effect of a magnetic field on a charged particle?

Answer 15

A magnetic field produces a deflection on a moving charged particle. Charged particles revolve around magnetic field lines in spiral orbits.

Question 16

What role do magnetic forces play in an electric motor?

Answer 16

An electric motor consists of a coil of wire located inside a permanent magnet. When
current flows in the coil the magnetic forces on the current produce a net torque that cause the coil to rotate – this rotation is the mechanical action of a motor.

Question 17

What is the shape of the magnetic field produced by a current in a long straight wire?

Answer 17

The magnetic field lines of the current in a long straight wire are a series of concentric circles surrounding the wire. The magnetic field is stronger near the wire.

Question 18

Why do magnets “stick” to certain materials which are not magnets?

Answer 18

Permanent magnets can stick to materials that have properties that allow them to be magnetized when a permanent magnet is brought close to them. These materials have
internal microscopic magnets that are usually not aligned to form a permanent magnetic field. However, when a magnetic field is present, the microscopic magnets are temporarily
aligned and thus exhibit overall magnetic properties. Thus magnets stick to objects made of
steel (like refrigerators). Some materials like copper, aluminum and non‐conductors are not
magnetized when a permanent magnet is brought close to them.

Question 19

What is the ultimate source of magnetism in electromagnets and permanent magnets?

Answer 19

The ultimate source of the magnetic field is current. When these currents flow in wires and coils they are called electromagnets. The origin of permanent magnetism is atomic currents due to electron spin and orbital motion in certain materials.

Question 20

Are neutral particles affected by magnetic fields?

Answer 20

Only moving charged particles can experience magnetic forces. Neutral particles are not affected by magnetic fields whether they are moving or at rest.

Question 21

What is electromagnetic induction? Who discovered it?

Answer 21

Electromagnetic induction in a phenomena in which current in induced in a coil if the magnetic field that penetrates through the coil changes in any manner.

Question 22

Why do transformers not work with DC?

Answer 22

Transformers are an application of the principle of electromagnetic induction, and therefore only work if the current in the primary changes in time.

Question 23

What was Maxwell’s contribution to the laws of electricity and magnetism?

Answer 23

Maxwell theorized that a changing electric field would generate a magnetic field.