Ch 13 · 4.5.3 Magnetic Effect of a Current · O Level Physics 5054

Topic 13 of 17 in Electricity and Magnetism — O Level Physics 5054 — 59 lessons.

Introduction

Every wire carrying a current is surrounded by a magnetic field. Coil the wire up and the field becomes strong enough to lift cars in a scrapyard, switch on a car's starter motor or make a loudspeaker play music. (Syllabus section 4.5.3.)

Key ideas — The field around a straight wire

A current in a straight wire produces a magnetic field of concentric circles around the wire, centred on it.

🧑‍🏫 Teacher's explanation

Imagine you are walking around a central pillar in a circular hallway. If the pillar is very powerful, the hallway is narrow and the crowd is dense; as you move further away, the hallway widens and the crowd thins out. In physics, the 'pillar' is the current flowing through the wire, and the 'crowd' is the magnetic field strength.

📖 Definitions

🔍 Worked example — Easy

A straight wire carries a current of 5.0 A downwards. Describe the direction of the magnetic field lines at a point to the left of the wire.

🔍 Worked example — Standard

A student uses a compass to map the field around a wire. If the current is doubled from 2.0 A to 4.0 A, what happens to the strength of the magnetic field at a fixed distance?

🔍 Worked example — Exam-style

A current of 10 A flows through a straight wire. If the distance from the wire is doubled, how does the magnetic field strength change?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Describe the shape and direction of the magnetic field lines produced by this wire. [2] (b) State how the strength of this magnetic field would change if the current is increased. [1] [3 marks]

Key ideas — The field of a solenoid

A solenoid is a long coil of wire. When current flows, its field is like that of a bar magnet:

🧑‍🏫 Teacher's explanation

Imagine a single straight wire. Its magnetic field is a series of concentric circles around it. Now, imagine taking hundreds of these wires, bending them into a coil, and stacking them one on top of another. This is a solenoid. Because the circular fields of each individual loop overlap and reinforce each other, they cancel out the 'circular' component inside the coil and create a straight, powerful field running through the center.

📖 Definitions

🔍 Worked example — Easy

A solenoid has a current of 2.0 A flowing through it. If the current is doubled to 4.0 A, how does the strength of the magnetic field change?

🔍 Worked example — Standard

A student uses a solenoid to create a magnetic field. If the direction of the current is reversed, what happens to the magnetic poles?

🔍 Worked example — Exam-style

A solenoid is used in an electromagnet. To increase the strength of the magnetic field without changing the current, which of the following is the best method: (a) increasing the number of turns, (b) decreasing the current, or (c) using a plastic core? Calculate the factor of increase if the number of turns is tripled.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State how the strength of the magnetic field can be increased. [2] (b) Describe how the direction of the magnetic field can be reversed. [1] [3 marks]

Key ideas — Changing the current

A solenoid's field can also be made stronger by adding more turns and by putting a soft-iron core inside it: this makes an electromagnet.

🧑‍🏫 Teacher's explanation

Imagine a current-carrying wire as a spinning dancer. The faster the dancer spins (higher current), the wider the circle of influence they create. If the dancer suddenly starts spinning in the opposite direction, the entire pattern of influence reverses too. This is exactly how magnetic fields behave: the current creates a circular pattern of magnetic field lines around a straight wire, and a uniform field inside a solenoid.

📖 Definitions

🔍 Worked example — Easy

A straight wire carries a current of 2.0 A downwards. Using the right-hand grip rule, describe the direction of the magnetic field lines at a point to the right of the wire.

🔍 Worked example — Standard

A solenoid has 500 turns of wire. If the current is increased from 1.5 A to 3.0 A, how does the strength of the magnetic field change?

🔍 Worked example — Exam-style

A solenoid is used in a relay. The current in the coil is 0.5 A. If the current is reversed, what happens to the magnetic poles of the solenoid?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State two ways to increase the strength of the magnetic field produced by the solenoid. [2] (b) Explain why a soft-iron core is used inside the solenoid instead of a permanent magnet. [1] [3 marks]

Key ideas — The relay

A relay is a switch worked by an electromagnet.

🧑‍🏫 Teacher's explanation

Imagine you are trying to turn on a massive, heavy industrial lever that requires a huge amount of force. You can't pull it yourself because it's too heavy, so you use a small, easy-to-pull trigger that releases a heavy weight, which then pulls the lever for you. This is exactly what a relay does for electricity.

📖 Definitions

🔍 Worked example — Easy

A relay is used to switch on a heater. The input circuit carries a current of 0.5 A, while the output circuit carries 10 A. If the input current is reduced to 0 A, describe the state of the output circuit.

🔍 Worked example — Standard

In a car ignition system, a relay uses a 2 A current to switch on a starter motor requiring 150 A. Why is it necessary to use a relay instead of connecting the battery directly to the ignition key switch?

🔍 Worked example — Exam-style

A relay uses a solenoid to pull an armature. If the number of turns in the solenoid is doubled while the current remains constant, what happens to the magnetic field strength and the resulting movement of the armature?

⚠️ Common errors

🧠 Quick check 1

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🎯 Exam question

Key ideas — The loudspeaker

Uses: headphones, radios, phones, televisions, public address systems.

🧑‍🏫 Teacher's explanation

Imagine you are holding a magnet near a piece of metal. If you move the magnet, the metal reacts. A loudspeaker works on a similar principle, but instead of moving the magnet, we move the electricity. We use a coil of wire placed inside the magnetic field of a permanent magnet.

📖 Definitions

🔍 Worked example — Easy

A loudspeaker cone vibrates at a frequency of 50 Hz. How many times does the cone complete a full back-and-forth cycle in 2 seconds?

🔍 Worked example — Standard

A coil in a loudspeaker is placed in a magnetic field. If the current in the coil is 0.5 A and the magnetic field strength is 0.4 T, calculate the force F on a 0.02 m length of wire. (Assume F = BIl sinθ where sinθ = 1).

🔍 Worked example — Exam-style

An amplifier provides an alternating current to a loudspeaker coil. If the current frequency is 440 Hz, what is the period T of the vibration?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Explain how the movement of the cone produces sound waves. [2] (b) State what would happen to the sound if the frequency of the alternating current was increased. [1] [3 marks]

Worked example

A vertical wire carries current upwards. Use the right-hand grip rule to find the direction of the field lines when looked at from above.

🔍 Worked example — Easy

A straight vertical wire carries a current of 5.0 A downwards. Describe the direction of the magnetic field lines when viewed from above.

🔍 Worked example — Standard

A solenoid is wound such that the current flows clockwise when viewed from the North pole. What is the direction of the magnetic field lines inside the solenoid?

🔍 Worked example — Exam-style

A student uses a solenoid to create a magnetic field. If the current in the solenoid is doubled, how does the strength of the magnetic field change?

⚠️ Common errors

Common mistakes

Practice

Answers: (1) Concentric circles centred on the wire, closer together near the wire (stronger field); direction given by the right-hand grip rule. (2) (a) the field becomes stronger; (b) the field reverses direction (the poles swap). (3) The small current magnetises the relay coil; it attracts an iron armature which closes the contacts of the second circuit, which carries the large current. (4) The current in the coil, in the field of the permanent magnet, produces a force on the coil; as the current alternates, the force reverses repeatedly, so the coil and cone move back and forth.

Ch 13 · 4.5.3 Magnetic Effect of a Current · O Level Physics 5054

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