4.5.3 Magnetic effect of a current (3)
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1.
A solenoid has 100 turns of wire and carries a current of 2A. Describe how the magnitude and direction of the magnetic field inside the solenoid would be affected if the current is increased to 4A. Explain your answer in terms of the relationship between current and magnetic field.
If the current in the solenoid is increased from 2A to 4A, the magnitude of the magnetic field inside the solenoid will increase proportionally. The magnetic field strength is directly proportional to the current. Therefore, doubling the current will double the magnetic field strength. The direction of the magnetic field inside the solenoid will remain the same. The magnetic field lines will still be parallel to the axis of the solenoid, as determined by the right-hand rule (where the thumb points in the direction of the current and the fingers curl in the direction of the magnetic field).
2.
A coil of wire with N turns carries a current I. The magnetic field at the centre of the coil is B. Describe how the magnitude and direction of the magnetic field at the centre of the coil would be affected if the number of turns (N) is doubled and if the current (I) is halved. Explain your answers.
Doubling the number of turns (N): The magnitude of the magnetic field produced by a coil is directly proportional to the number of turns. Therefore, if the number of turns is doubled, the magnitude of the magnetic field at the centre of the coil will also double. The magnetic field strength is proportional to the number of turns. The direction of the magnetic field remains the same – it is determined by the right-hand rule (or Fleming's left-hand rule), which dictates the direction of the magnetic field based on the direction of the current and the direction of the curls of the fingers.
Halving the current (I): The magnitude of the magnetic field produced by a coil is also directly proportional to the current. Therefore, if the current is halved, the magnitude of the magnetic field at the centre of the coil will also be halved. The magnetic field strength is proportional to the current. The direction of the magnetic field remains the same – it is determined by the right-hand rule (or Fleming's left-hand rule), which dictates the direction of the magnetic field based on the direction of the current and the direction of the curls of the fingers.
3.
A straight wire carries a current. Describe the pattern and direction of the magnetic field produced around the wire. Explain how the right-hand rule can be used to determine the direction of the magnetic field.
The magnetic field produced by a straight current-carrying wire forms concentric circles around the wire. The direction of the magnetic field can be determined using the right-hand rule. Point your thumb in the direction of the conventional current (positive charge flow) in the wire. Your fingers will curl in the direction of the magnetic field lines.
The magnetic field is strongest close to the wire and decreases with distance from the wire. The field lines are perpendicular to the wire.
| Diagram of a straight wire with magnetic field lines. Thumb points in current direction, fingers curl in field direction. |