7.1 Magnetic flux density

The diagram shows a clockwise current I in a circular coil placed in a uniform magnetic field B with the plane of the coil perpendicular to the magnetic field.

What is the effect on the coil of the interaction between the current and the magnetic field?

• It rotates about the axis with the top moving out of the page.

• It rotates about the axis with the top moving into the page.

• It causes an increase in the diameter of the coil.

• It causes a decrease in the diameter of the coil.

A horizontal copper wire of mass 4.0 × 10⁻³ kg and length 80 mm is placed perpendicular to a horizontal magnetic field of flux density 0.16 T. The magnetic force acting on the wire supports the weight of the wire.

How many electrons are passing a point in the wire in each second?

• 1.9 × 10¹⁸

• 1.9 × 10¹⁹

• 1.9 × 10²⁰

• 1.9 × 10²¹

A horizontal straight wire of length 0.30 m carries a current of 2.0 A perpendicular to a horizontal uniform magnetic field of flux density 5.0 × 10^–2 T. The wire ‘floats’ in equilibrium in the field.

What is the mass of the wire?

• 8.0 × 10^–4 kg

• 3.1 × 10^–3 kg

• 3.0 × 10^–2 kg

• 8.2 × 10^–1 kg

The diagram shows a horizontal conductor of length 50 mm carrying a current of 3.0 A at right angles to a uniform horizontal magnetic field of flux density 0.50 T.

What is the magnitude and direction of the magnetic force on the conductor ?

• 0.075 N vertically upwards

• 0.075 N vertically downwards

• 75 N vertically upwards

• 75 N vertically downwards

The diagram shows a rigidly-clamped straight horizontal current-carrying wire held mid-way between the poles of a magnet on a top-pan balance. The wire is perpendicular to the magnetic field direction.

The balance, which was zeroed before the switch was closed, read 161 g after the switch was closed. When the current is reversed and doubled, what would be the new reading on the balance?

• −322 g

• −161 g

• zero

• 322 g

Four rectangular loops of wire A, B, C and D are each placed in a uniform magnetic field of the same flux density B. The direction of the magnetic field is parallel to the plane of the loops as shown.

When a current of 1 A is passed through each of the loops, magnetic forces act on them. The lengths of the sides of the loops are as shown. Which loop experiences the largest couple?

• A

• B

• C

• D

The diagram shows a vertical square coil whose plane is at right angles to a horizontal uniform magnetic field B. A current, I, is passed through the coil, which is free to rotate about a vertical axis OO'.

Which one of the following statements is correct?

• The forces on the two vertical sides of the coil are equal and opposite.

• A couple acts on the coil.

• No forces act on the horizontal sides of the coil.

• If the coil is turned through a small angle about OO' and released, it will remain in position.

A horizontal straight wire of length 40 mm is in an east-west direction as shown in the diagram. A uniform magnetic field of flux density 50 mT is directed downwards into the plane of the diagram. When a current of 5.0 A passes through the wire from west to east, a horizontal force acts on the wire. Which line, A to D, in the table gives the magnitude and direction of this force?

• magnitude / mN: 2.0

  direction: north

• magnitude / mN: 10.0

  direction: north

• magnitude / mN: 2.0

  direction: south

• magnitude / mN: 10.0

  direction: south

A section of current-carrying wire is placed at right angles to a uniform magnetic field of flux density B. When the current in the wire is I, the magnetic force that acts on this section is F.

What force acts when the same section of wire is placed at right angles to a uniform magnetic field of flux density 2B when the current is 0.25 I?

• 
• 
• F

• 2F

A horizontal straight wire of length 0.30 m carries a current of 2.0 A perpendicular to a horizontal uniform magnetic field of flux density 5.0 × 10^–2 T. The wire ‘floats’ in equilibrium in the field.

What is the mass of the wire?

• 8.0 × 10^–4 kg

• 3.1 × 10^–3 kg

• 3.0 × 10^–2 kg

• 8.2 × 10^–1 kg