AQA GCSE Physics 4.7 · Year 11

Electromagnetism & Solenoids

Magnetic field around wire and solenoid · Right-hand rule · Factors affecting field strength · Electromagnets and their applications

Learning Objectives

🔵 Describe the magnetic field pattern around a straight current-carrying wire and identify the direction using the right-hand rule

🌀 Describe the magnetic field pattern inside and around a solenoid and explain how it resembles a bar magnet

⚡ Explain the factors that affect the strength of the magnetic field produced by a wire and a solenoid

🔧 Explain how an electromagnet works and describe its advantages over a permanent magnet

📐 Apply the right-hand grip rule to determine the polarity of a solenoid and the field direction around a wire

🏭 Describe real-world applications of electromagnets including relays, electric bells, and MRI machines

⚡ Magnetic Fields from Electric Currents

In 1820, Hans Christian Ørsted discovered that a compass needle was deflected when placed near a wire carrying an electric current. This was the first evidence that electricity and magnetism are linked — a relationship we now call electromagnetism.

Whenever an electric current flows through a conductor, it creates a magnetic field in the space surrounding that conductor. This field exists in three dimensions around the wire, and its shape depends on the geometry of the conductor.

Magnetic Field: A region of space where a magnetic force can be experienced by a magnet or moving charge. It is represented by field lines that show the direction a free north pole would move.

For a straight wire, the magnetic field forms concentric circles centred on the wire. The field lines are circular loops that surround the wire. The closer you are to the wire, the more tightly packed the field lines are — indicating a stronger field. As distance from the wire increases, field lines spread apart and the field weakens.

The direction of the field circles depends on the direction of conventional current (from + to −). This is summarised by the right-hand rule for a straight wire:

Right-Hand Rule (Straight Wire): Point your right thumb in the direction of conventional current flow. Your curled fingers show the direction of the magnetic field lines (circling around the wire).

If the current flows upward through a wire, looking from above, the magnetic field circles anticlockwise. If the current flows downward, the field circles clockwise. On diagrams: a dot (⊙) represents current coming out of the page, and a cross (⊗) represents current going into the page.

Symbol	Meaning	Field direction (viewed from front)
⊙ (dot)	Current out of page	Anticlockwise circles
⊗ (cross)	Current into page	Clockwise circles

🌀 The Solenoid and Its Magnetic Field

A solenoid is a long coil of wire with many turns wound closely together. When current flows through a solenoid, each turn of wire produces its own circular magnetic field. These individual fields add together (superpose) to produce a much stronger, more uniform magnetic field.

Solenoid: A cylindrical coil of wire. When carrying a current it produces a magnetic field very similar in shape to that of a bar magnet — with a north pole at one end and a south pole at the other.

Inside the solenoid, the field lines run parallel and are evenly spaced, showing a uniform (constant strength) magnetic field pointing along the axis of the coil. Outside the solenoid, the field lines curve from the north pole back round to the south pole — exactly like a bar magnet.

This is why a solenoid behaves like a bar magnet: it has two poles, it attracts iron and steel, and a free solenoid will align itself with Earth's magnetic field just like a compass needle.

Right-Hand Grip Rule (Solenoid): Wrap your right hand around the solenoid so your fingers point in the direction of conventional current flow around the coil. Your thumb points towards the North pole of the solenoid.

Alternatively, look at the end of the solenoid: if the current flows anticlockwise, that end is the North pole. If the current flows clockwise, that end is the South pole. (Think: N looks like anticlockwise arrows, S like clockwise arrows.)

The polarity of the solenoid can be reversed simply by reversing the direction of the current. This is one of the key advantages of an electromagnet over a permanent magnet.

📈 Factors Affecting Magnetic Field Strength

The strength of the magnetic field produced by a current-carrying conductor depends on several factors. Understanding these allows engineers to design electromagnets for specific purposes.

For a straight wire:

Current (I): Increasing the current increases the magnetic field strength. The field is directly proportional to current.
Distance from wire: The field strength decreases as distance from the wire increases.

For a solenoid / electromagnet:

Current (I): A larger current produces a stronger magnetic field.
Number of turns (N): More turns of wire per unit length increase the field strength. More turns means more individual magnetic fields adding together.
Iron core: Placing a soft iron core inside the solenoid dramatically increases the field strength. Iron is a magnetically soft material — it becomes magnetised in the presence of a field and greatly amplifies it.
Length of solenoid: A shorter solenoid with the same number of turns (i.e. more tightly wound) produces a stronger field.

Factors increasing solenoid field strength:
↑ Current I (A) | ↑ Turns per unit length N | Add iron core

The iron core is described as magnetically soft because it is easily magnetised and demagnetised. This is essential for electromagnets — the core loses its magnetism when the current is switched off.

Hard magnetic materials (like steel) retain their magnetism and are used in permanent magnets, not electromagnet cores.

🔧 Electromagnets and Their Advantages

An electromagnet is a solenoid with a soft iron core. It only acts as a magnet when current flows through the coil. This controllability makes electromagnets far more versatile than permanent magnets.

Electromagnet: A solenoid (coil of wire) wound around a soft iron core. The magnetic field is produced only when current flows and can be switched on and off.

Advantages of electromagnets over permanent magnets:

Can be switched on and off by controlling the current
Strength can be varied by changing the current or number of turns
The polarity can be reversed by reversing the current direction
Can be made extremely powerful for industrial use

Applications of electromagnets:

Electric bell: An electromagnet repeatedly attracts and releases a metal arm (armature) that strikes a bell. When attracted, the circuit breaks, the magnet switches off, the spring pulls the arm back, the circuit reconnects, and the cycle repeats.
Relay: A small current in one circuit controls a large current in another. The small current activates an electromagnet that closes a switch (armature) in the high-current circuit. Used in car starter motors, industrial machinery, and safety systems.
Scrapyard electromagnet: Powerful cranes with electromagnets lift and release large masses of iron and steel. The load is released simply by switching off the current — impossible with a permanent magnet.
MRI (Magnetic Resonance Imaging): Hospitals use extremely powerful superconducting electromagnets to produce the strong, uniform magnetic fields needed to image soft tissues inside the body.
Maglev trains: Electromagnets in both the track and train repel each other to levitate the train, eliminating friction and enabling very high speeds.

A relay allows a low-voltage, low-current control circuit (e.g. a sensor or switch) to safely operate a high-voltage, high-current circuit without direct electrical connection between the two circuits.

📊 Summary: Key Relationships

Here is a concise overview of the key relationships and rules in electromagnetism:

Concept	Rule / Relationship
Field around straight wire	Concentric circles; stronger closer to wire; direction from right-hand rule
Field in solenoid	Uniform inside; like bar magnet outside; stronger with more turns, higher current, iron core
Direction (wire)	Thumb → current; fingers → field direction (right-hand rule)
Polarity (solenoid)	Thumb → North pole; fingers wrap in direction of current (right-hand grip rule)
Reversing polarity	Reverse current direction
Increasing strength	Increase I, increase N (turns), add soft iron core
Switching off	Remove current → iron core demagnetises instantly (magnetically soft)

Electromagnet strength ∝ Current (I) × Number of turns (N)

Example 1: A vertical wire carries a conventional current flowing upward. Describe the magnetic field pattern around the wire and state the direction of the field at a point to the right of the wire, when viewed from above.

1 Identify the field shape: A straight current-carrying wire produces a magnetic field in the form of concentric circles centred on the wire. The field exists in a plane perpendicular to the current direction.

2 Apply the right-hand rule: Point your right thumb upward (in the direction of conventional current). Your curled fingers show the direction the field circles go — when viewed from above (looking down), the circles go anticlockwise.

3 Determine field direction at the specific point: At a point to the right of the wire, the anticlockwise circles mean the field is pointing away from you (into the page, if the current is upward and we look from the right side) — or more precisely, at the rightmost point of an anticlockwise circle, the field direction is pointing toward the bottom of the page (southward) when viewed from above.

4 Describe field strength variation: The field lines are closer together near the wire and spread further apart as distance increases, indicating the field is strongest closest to the wire and decreases with distance.

The field forms anticlockwise concentric circles (viewed from above). At a point to the right of the wire, the field points downward/southward (in the plane of the page, away from the viewer). Field strength decreases with distance from the wire.

Example 2: A solenoid has 200 turns, a length of 10 cm, and carries a current of 2 A. The current is then increased to 6 A and the number of turns is doubled to 400 while keeping the length the same. By what factor does the magnetic field strength increase?

1 Identify the relevant relationship: The magnetic field strength (B) inside a solenoid is proportional to both the current (I) and the number of turns per unit length (N/L):
B ∝ I × (N ÷ L)
Since L is constant, B ∝ I × N

2 Calculate initial proportional value:
B₁ ∝ I₁ × N₁ = 2 × 200 = 400 (arbitrary units)

3 Calculate new proportional value:
B₂ ∝ I₂ × N₂ = 6 × 400 = 2400 (arbitrary units)

4 Calculate the factor of increase:
Factor = B₂ ÷ B₁ = 2400 ÷ 400 = 6

The magnetic field strength increases by a factor of 6. (Current tripled × turns doubled = 3 × 2 = 6)

Example 3: A student builds a solenoid and looks at the left-hand end of the coil. She observes that conventional current flows anticlockwise around the coil at that end. (a) Which magnetic pole is at the left-hand end? (b) What happens to the polarity if she reverses the battery connections?

1 Apply the right-hand grip rule to find the pole: Wrap your right hand around the coil so your fingers curl in the direction of the conventional current. If the current is flowing anticlockwise when viewed from the left end, then the right thumb (when the fingers curl anticlockwise as viewed from left) points to the left — toward the observer.

2 Alternative memory method: Looking at the end of a solenoid:
• Anticlockwise current → North pole (think: the letter N has anticlockwise-like strokes)
• Clockwise current → South pole
Since the left end shows anticlockwise current → left end is the North pole.

3 Effect of reversing the battery: Reversing the battery reverses the direction of current flow throughout the solenoid. The current at the left end would now flow clockwise, making it a South pole. The right end would become the North pole.

(a) The left-hand end is the North pole (anticlockwise current = North pole).
(b) Reversing the battery reverses the current, making the left end the South pole and reversing the polarity of the entire solenoid.

Example 4: Explain how a relay works and why it is useful to use an electromagnet rather than a direct switch in this application. Give one real-world example of where a relay is used.

1 Structure of a relay: A relay consists of two separate electrical circuits. The first is a low-current control circuit containing an electromagnet (a solenoid with an iron core). The second is a high-current operating circuit containing the device to be switched (e.g. a motor), connected via a set of contacts (a switch called the armature).

2 How it operates: When a small current flows in the control circuit, the electromagnet becomes magnetised and attracts the soft iron armature. The armature pivots and closes the contacts in the high-current circuit, completing it and allowing the high current to flow. When the control current is switched off, the electromagnet demagnetises, a return spring pulls the armature back, and the high-current circuit is broken.

3 Why an electromagnet is essential: The electromagnet can be switched on and off using a very small, safe current. A direct mechanical switch in the high-current circuit would be impractical — it could be dangerous, require large physical force, or be impossible to automate. The electromagnet provides electrical isolation between the two circuits, improving safety.

4 Real-world example: In a car, a small current from the ignition key switch activates a relay that allows a very large current (100+ A) to flow to the starter motor. This protects the ignition switch and wiring from the large current.

A relay uses an electromagnet activated by a small control current to switch a larger current in a separate circuit. The electromagnet is essential because it provides safe electrical isolation between circuits and can be controlled remotely or automatically. Application: car starter motor relay.

Question 1: A horizontal wire carries a conventional current flowing to the right. What is the direction of the magnetic field directly above the wire?

Question 2: Which of the following changes would NOT increase the strength of an electromagnet?

Question 3: A student looks at the right-hand end of a solenoid and sees that conventional current flows clockwise around the coil. What is the polarity of the right-hand end?

Question 4: A solenoid originally has a current of 3 A and 150 turns. The current is doubled to 6 A and the number of turns is tripled to 450. By what factor does the magnetic field strength increase? Enter a whole number.

Question 5: Why is soft iron used as the core of an electromagnet rather than steel?

Challenge 1 (6 marks): A scrapyard electromagnet consists of a solenoid with 500 turns wound over a length of 25 cm, carrying a current of 8 A, with a soft iron core.

(a) State two ways the engineer could double the strength of this electromagnet without changing the current. [2 marks]

(b) Explain why the electromagnet uses a soft iron core rather than a steel core, referring to the magnetic properties of each material. [2 marks]

(c) The polarity of the electromagnet needs to be reversed to reposition a load. Describe how this could be achieved and explain what happens to the field inside the coil. [2 marks]

Challenge 2 (5 marks): A student investigates how the number of turns affects the strength of an electromagnet by measuring how many steel paperclips can be picked up.

Results: 50 turns → 3 clips; 100 turns → 6 clips; 150 turns → 9 clips; 200 turns → 12 clips.

(a) State the pattern shown by this data and the conclusion about the relationship between number of turns and field strength. [2 marks]

(b) Suggest two control variables the student must keep constant for this to be a fair test. [2 marks]

(c) Suggest one limitation of using paperclips as a measure of magnetic field strength. [1 mark]

Challenge 3 (6 marks): A relay is used in the circuit of a greenhouse heating system. A temperature sensor outputs a small current of 50 mA when the temperature drops below 5°C. This activates a relay that switches on a 240 V, 2 kW electric heater.

(a) Calculate the current drawn by the heater at full power. Use the equation P = IV. Show your working. [3 marks]

(b) Explain why it would be dangerous to use the 50 mA sensor current to directly switch the heater, and how the relay solves this problem. [3 marks]

Challenge 4 — Extended Response (6 marks): Compare the magnetic field of a solenoid to that of a bar magnet. In your answer, discuss: the field pattern inside and outside; the concept of poles; how the solenoid field can be controlled; and one similarity and one difference between the two.