7.2 Magnetic effects of currents and the motor effect — Practise

1. The field around a straight wire

• The field lines are circles around the wire. The direction of the field depends on the direction of the current.
• The field is strongest close to the wire and gets weaker as the distance from the wire increases.

2. How a solenoid increases the field

• A solenoid is a coil with many turns of wire, so the current passes around the same space many times.
• The magnetic field of each turn adds to the fields of the other turns, so the total field inside the coil is much stronger.

3. The iron core and switching off

• Iron is easily magnetised by the field of the coil. The magnetised core adds its own field, so the electromagnet is much stronger.
• When the current stops, the field of the coil goes and the iron loses its magnetism, so the electromagnet can be switched off.

4. How the deflection changes with distance

• The deflection of the needle gets smaller as the compass is moved further from the wire.
• The magnetic field of the wire is weaker further away, so it turns the needle less.

5. The field inside a solenoid

• The field inside the solenoid is strong, because the fields of all the turns add together.
• The field inside is uniform. The field lines are straight, equally spaced and parallel to the axis of the coil.

6. The two factors

• The size of the current through the wire: a larger current makes a stronger field.
• The distance from the wire: the field is weaker further from the wire.

7. An advantage and a use

• An electromagnet can be switched on and off, and its strength can be changed by changing the current.
• A scrapyard crane uses an electromagnet to lift steel scrap and then releases it by switching the current off.

8. Fleming's left-hand rule

• The first finger points in the direction of the magnetic field, from north to south.
• The second finger points in the direction of the current.
• The thumb points in the direction of the force on the conductor. The thumb and the two fingers are at right angles to each other.

9. Force on the wire

Answer: $F = BIl = 0.50 \times 1.5 \times 0.20 = 0.15\,\text{N}$.

10. Magnetic flux density

Answer: $B = \dfrac{F}{Il} = \dfrac{0.36}{2.0 \times 0.30} = 0.60\,\text{T}$.

11. How the motor rotates

• The current flows in one direction along one side of the coil and in the opposite direction along the other side.
• Both sides sit in the same magnetic field, so the forces on them act in opposite directions.
• The two opposite forces form a turning effect, so the coil rotates.

12. Current in the wire

Answer: $I = \dfrac{F}{Bl} = \dfrac{0.80}{0.40 \times 0.50} = 4.0\,\text{A}$.

13. Choosing the method

• Use F = BIl, the force on a current-carrying conductor in a magnetic field.
• I need the magnetic flux density B, the current I and the length l of the wire in the field.