Chapter 6 Electricity and Magnetism

An electric iron draws a current of 15 A when connected to a 120-V power source. Its resistance is

B. 8 ohms

When the voltage across a certain resistance is V, the power delivered is P. If the voltage is doubled to V/2, the power delivered becomes

B. P/4.

A transformer used to reduce 240-V ac to 120-V ac has an input current of 15 A. Its output current is

D. 30 A

The electric energy stored in a 6-V battery rated at 20 Ah is

D. 432 kJ.

The voltage needed to produce a current of 8 A in a resistance of 12 ohms is

D. 96 V.

A current-carrying wire is in a magnetic field with the direction of the current parallel to that of the field.

D. The wire has no tendency to move or to turn.

A magnet attracts

D. some substances and repels others.

Direct electric current cannot be used to operate

D. transformers

The volt is the unit of electric

B.potential difference.

Electric Charge

B.occurs only in separate parcels of ±1.6 × 10-19 C.

A kilowatt hour is a unit of

A. energy

The primary coil of a transformer has 200 turns and its secondary coil has 50 turns, and the current in the secondary coil is 40 A. When a 12-V alternating current is applied to the primary coil of the transformer, the secondary voltage will be

A. 3 V

The primary coil of a transformer has 200 turns and its secondary coil has 40 turns. If the current in the secondary coil is 25 A, the current in the primary coil is

A. 5 A

Three 12-V batteries cannot be connected to give a voltage for the set of

A. 6 V

Atoms and molecules are normally

A. electrically neutral.

A current-carrying loop in a magnetic field always tends to turn until the plane of the loop is

A. parallel to the field.

A molecule that has lost an electron becomes a

A. positive ion

Relative to the electric force between two protons, the gravitational force between them is

A. weaker

The watt, the unit of power, is equal to the

A.(volt)(ampere).

Two 60-watt light bulbs are connected in parallel across a 120-V power line. The current in each bulb is

A.0.5 A.

When a current passes through a resistance, the electric energy that is lost

A.becomes heat.

When a magnetized iron bar is strongly heated, its magnetic field

A.becomes weaker.

The nucleus of an atom

A.contains most of the atom's mass.

Ohm's law

A.holds only for metals.

Coulomb's formula F = KQ1Q2/R2 gives the force between two charges in free space. The value of K

A.is the same under all circumstances.

Superconductivity occurs in certain substances

A.only at very low temperatures.

The current in a 20-ohm electric heater operated at 240 V is

B. 12 A

The current in a 24-W, 12-V light bulb is

B. 2 A

The power rating of an electric motor that draws a current of 3 A when operated at 240 V is

B. 720 W

The current with which a transformer is used

B. must be ac

A current-carrying wire is in a magnetic field with the direction of the current perpendicular to that of the field

B.The wire tends to move perpendicular to the field.

The electrons in an atom

B.are some distance away from the nucleus.

An object has a positive electric charge whenever

B.it has a deficiency of electrons.

A compass needle tends to

B.line up parallel to a magnetic field.

Two charges, one positive and the other negative, are initially 2 cm apart and are then pulled away from each other until they are 8 cm apart. The force between them is now smaller by a factor of

C. 16

The resistance of a 30-W, 12-V light bulb is

C. 2.5 ohms.

A magnet does not exert a force on

C. a stationary electric charge.

A current is flowing south along a power line. Neglecting the earth's field, the direction of the magnetic field under the power line is

C. east

A negative electric charge

C. interacts with both positive and negative charges.

Two charges of +Q are 1 cm apart. If one of the charges is replaced by a charge of -Q, the magnitude of the force between them is

C. the same

A charge of +Q attracts a charge of -Q 1 cm away with a force of F. Another charge of +Q is placed next to the first one. The force on the charge of -Q is now

C.2F

Which of the following statements is correct?

C.Moving electrons constitute electric currents.

When an observer moves past a stationary electric charge, she detects

C.both an electric and a magnetic field.

A drawing of the field lines of a magnetic field provides information on

C.both the direction and the strength of the field.

If undisturbed, an electric current in a superconducting wire loop

C.will continue indefinitely.

Two charges repel each other with a force of 10-6 N when they are 10 cm apart. When they are brought closer together until they are 2 cm apart, the force between them becomes

D. 2.5 × 10-5 N.

The primary coil of a transformer is connected to a source of alternating current of voltage V. The voltage of the secondary current is

D. Any of these choices could be correct, depending on the ratio of turns in the transformer coils.

Which of the following statements is not true?

D.A proton in an atom circulates around its nucleus.

Which of the following statements is not true?

D.Protons and electrons have equal masses.

The conventional unit of battery capacity is the ampere-hour, which is actually a unit of

D.electric charge.