In 1820, a Danish scientist named Hans Christian Oersted was giving a lecture. He placed a compass near a wire carrying an electric current — and the compass needle jumped. In that moment, he discovered something profound: electricity and magnetism are connected. An electric current creates a magnetic field.
This discovery changed the world. It led to the invention of electric motors, generators, transformers, relays, and countless other devices that we rely on every day. Every time you use a fan, a washing machine, a car, or a phone, you are using the magnetic effect of electric current. In this chapter, we are going to explore this beautiful connection. Let us begin.
Oersted's discovery was accidental, but it was not luck. He was curious, he was prepared, and he noticed something unexpected. That is how science works. The universe reveals its secrets to those who are watching carefully. Keep your eyes open — you might notice something no one else has seen.By the end of this chapter, you should be able to:
In 1820, Hans Christian Oersted discovered that a compass needle deflects when placed near a wire carrying an electric current. This showed that an electric current produces a magnetic field.
Oersted's experiment:
The deflection of the compass needle shows that a magnetic field is present around the current-carrying wire. The direction of deflection depends on the direction of the current.
This discovery was a turning point in physics. Before Oersted, electricity and magnetism were thought to be completely separate. After Oersted, they were seen as two aspects of a single force: electromagnetism. This unification is one of the great achievements of physics.When a current flows through a straight wire, it produces a magnetic field around the wire. The field lines form concentric circles around the wire.
Properties of the field:
Think of the wire as a long, thin magnet. The field lines circle around it, like ripples around a stone dropped in water. The closer you are to the wire, the stronger the field.
| Distance from Wire | Field Strength |
|---|---|
| Very close | Strong |
| Moderate distance | Weaker |
| Far away | Very weak |
How do we know which way the magnetic field points? We use the right-hand grip rule.
Right-hand grip rule for a straight wire:
This rule works for any current-carrying wire. It allows you to predict the direction of the magnetic field without any complicated calculations.
Right-hand grip rule for a solenoid:
Circular loop: When a wire is formed into a circular loop and current flows through it, the magnetic field lines pass through the centre of the loop. The field is strongest at the centre. The loop behaves like a short bar magnet.
Solenoid: A solenoid is a long coil of wire. When current flows through it, the magnetic field inside the solenoid is strong and uniform. The field outside is weak and similar to that of a bar magnet.
| Configuration | Field Pattern | Field Strength |
|---|---|---|
| Straight wire | Concentric circles | Strongest near wire |
| Circular loop | Field through centre | Strongest at centre |
| Solenoid | Uniform inside, like a bar magnet outside | Strongest inside |
Factors affecting the strength of a solenoid's magnetic field:
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is the principle behind the electric motor.
The force depends on:
Force on a current-carrying conductor: F = B × I × L
F = force (N), B = magnetic field strength (T), I = current (A), L = length of conductor in the field (m)
(This formula applies when the conductor is perpendicular to the magnetic field.)
Fleming's left-hand rule: To find the direction of the force, use your left hand:
This rule is essential for understanding electric motors and other electromagnetic devices.
Fleming's left-hand rule is a gift to students. It turns a complex three-dimensional relationship into a simple hand gesture. Use it every time you need to find the direction of the force. It will never let you down.An electric motor converts electrical energy into mechanical energy. It uses the force on a current-carrying conductor in a magnetic field to produce rotation.
Parts of a simple electric motor:
How it works:
This simple design is the basis of every electric motor, from the tiny motor in your phone to the massive motors in electric trains.
The electric motor is one of the most important inventions in human history. It transformed industry, transport, and daily life. And it all comes down to a simple idea: a current-carrying wire in a magnetic field experiences a force. Physics changes the world.An electromagnet is a coil of wire wrapped around a soft iron core. When current flows, the core becomes magnetised. When the current stops, the core loses its magnetism.
Advantages of electromagnets over permanent magnets:
Uses of electromagnets:
| Device | How It Uses Electromagnets |
|---|---|
| Electric bell | An electromagnet pulls a hammer to strike a bell, then the circuit breaks and the hammer returns. |
| Relay | An electromagnet switches a larger current circuit on or off. |
| Loudspeaker | An electromagnet moves a coil attached to a cone, producing sound. |
| Electric motor | Electromagnets provide the magnetic field that drives rotation. |
| Magnetic crane | An electromagnet lifts and drops scrap metal. |
| Circuit breaker | An electromagnet trips a switch when the current is too high. |
A relay is a switch operated by an electromagnet. It allows a small current to control a much larger current.
How a relay works:
Relays are used in cars, washing machines, and industrial control systems. They allow low-power circuits to control high-power devices safely.
The relay is a brilliant invention. It allows a tiny signal to control a massive machine. This is how automation began. Understanding relays is understanding the foundation of modern control systems.The electric bell is a classic application of electromagnets. It consists of:
How it works:
This is a beautiful example of a self-interrupting circuit. The bell rings because the circuit is constantly making and breaking.
The electric bell is one of my favourite demonstrations. It shows how a simple electromagnet can produce a continuous, repeating action. It is physics in motion — and it makes a satisfying sound too.Force on a current-carrying conductor: F = B × I × L
F = force (N), B = magnetic field strength (T), I = current (A), L = length of conductor (m)
Right-hand grip rule: Thumb points in direction of current; fingers curl in direction of magnetic field.
Fleming's left-hand rule: First finger = field; Second finger = current; Thumb = force (motion).
Factors affecting solenoid strength: Number of turns, current, core material.
Question: A current flows upwards through a straight wire. Use the right-hand grip rule to determine the direction of the magnetic field on the east side of the wire.
Answer: Grip the wire with your right hand, thumb pointing upwards (direction of current). Your fingers curl anticlockwise when viewed from above. On the east side, the field points south.
Units check: not applicable. But notice how the right-hand grip rule gives a clear, unambiguous answer. It is a powerful tool for visualising magnetic fields.Question: A straight wire of length 0.2 m carries a current of 3 A in a magnetic field of strength 0.5 T. The wire is perpendicular to the field. Calculate the force on the wire.
Answer:
Formula: F = B × I × L
Substitution: F = 0.5 T × 3 A × 0.2 m
Answer: F = 0.3 N
Units check: T × A × m = N. The answer is in newtons, correct for force. Does 0.3 N make sense? Yes, it is a small but measurable force.Question: A wire carries a current to the east in a magnetic field pointing north. Use Fleming's left-hand rule to determine the direction of the force on the wire.
Answer: Point your first finger north (field). Point your second finger east (current). Your thumb points upwards — the force is upward.
Units check: not applicable. But notice how Fleming's left-hand rule gives a clear, three-dimensional answer. It is an essential tool for understanding electric motors.Question: A solenoid has current flowing clockwise when viewed from one end. Which end is the north pole?
Answer: Using the right-hand grip rule for a solenoid: grip the solenoid with your right hand so your fingers curl in the direction of the current (clockwise). Your thumb points toward the end that is the north pole. So the end you are viewing is the south pole, and the opposite end is the north pole.
Units check: not applicable. But this is a common exam question. Remember: the right-hand grip rule for a solenoid is different from the rule for a straight wire. Practice both until you can apply them instantly.Question: State three ways to increase the strength of an electromagnet.
Answer: 1. Increase the current. 2. Increase the number of turns in the coil. 3. Use a soft iron core instead of air.
Units check: current is measured in amperes (A). The strength of an electromagnet is directly related to the current and the number of turns. This is why powerful electromagnets use many turns and large currents.Graphs can help us understand the relationship between current and magnetic field strength.
Magnetic field strength vs current: If you plot the magnetic field strength on the vertical axis and the current on the horizontal axis, you get a straight line through the origin. The field is directly proportional to the current.
Force on conductor vs current: If you plot the force on a current-carrying conductor on the vertical axis and the current on the horizontal axis, you also get a straight line through the origin. The force is directly proportional to the current.
Force vs magnetic field strength: If you plot force on the vertical axis and magnetic field strength on the horizontal axis, you get another straight line. The gradient is I × L.
These graphs help us visualise the relationships in electromagnetism. They turn abstract equations into pictures.
Think of a graph as a map of a relationship. The shape tells you how one quantity changes with another.
Apparatus: A solenoid (coil of wire), a power supply, a rheostat, an ammeter, a compass, iron filings, and a card.
Method:
Observation: The field inside the solenoid is strong and uniform. Outside, it resembles the field of a bar magnet. The field strength increases with current.
Explanation: The magnetic field of a solenoid is the combined field of many current-carrying loops. Inside, the fields add up to give a strong, uniform field. Outside, the field is similar to that of a bar magnet.
Precautions:
Likely source of error: The Earth's magnetic field may affect the compass readings. Keep the solenoid away from other magnetic sources.
This experiment is a beautiful way to visualise the magnetic field of a solenoid. The iron filings reveal the invisible field lines, and the compass shows the direction. It is a perfect combination of observation and theory.Oersted discovered that a compass needle deflects when placed near a wire carrying an electric current, showing that a current produces a magnetic field.
Grip the wire with your right hand, thumb pointing in the direction of the current. Your fingers curl in the direction of the magnetic field lines.
A solenoid is a long coil of wire that produces a strong, uniform magnetic field inside when current flows through it.
First finger = magnetic field direction; second finger = current direction; thumb = force (motion) direction.
F = B × I × L = 0.4 × 2 × 0.3 = 0.24 N.
Current flows through a coil in a magnetic field. The force on the coil creates a turning effect. A split-ring commutator reverses the current every half-turn to keep the coil rotating in the same direction.
Electric bells, relays, loudspeakers, magnetic cranes, circuit breakers.
Increase the current; increase the number of turns; use a soft iron core.
(a) Describe Oersted's experiment and explain its significance. [4]
(b) State the right-hand grip rule for a straight wire. [3]
(c) A current flows downwards through a vertical wire. Use the right-hand grip rule to determine the direction of the magnetic field on the north side of the wire. [3]
(a) A compass needle is placed near a wire carrying a current. The needle deflects, showing that a current produces a magnetic field. This showed that electricity and magnetism are connected. (4 marks.)
(b) Grip the wire with your right hand, thumb pointing in the direction of the current. Your fingers curl in the direction of the magnetic field. (3 marks.)
(c) Thumb points downwards. Fingers curl clockwise when viewed from above. On the north side, the field points east. (3 marks.)
(a) Describe the construction and working of a simple electric motor. [5]
(b) Explain the function of the split-ring commutator. [3]
(c) State two ways to increase the speed of the motor. [2]
(a) A simple motor has a coil placed between the poles of a magnet. Current flows through the coil, and the force on each side creates a turning effect. The coil rotates. (5 marks.)
(b) The commutator reverses the current every half-turn, so the force continues to push the coil in the same direction. Without it, the coil would stop after half a turn. (3 marks.)
(c) Increase the current; increase the magnetic field strength. (2 marks.)
(a) Describe how an electric bell works. [5]
(b) Explain why the circuit in an electric bell is self-interrupting. [3]
(c) State two other uses of electromagnets. [2]
(a) When the switch is pressed, current flows through the electromagnet, which attracts the armature. The hammer strikes the gong. The movement breaks the circuit, the electromagnet loses magnetism, and the armature springs back, closing the circuit again. This repeats rapidly. (5 marks.)
(b) The movement of the armature breaks the circuit at the contact screw, so the current stops. This causes the electromagnet to release the armature, which closes the circuit again. (3 marks.)
(c) Relays, loudspeakers, magnetic cranes, circuit breakers. (2 marks.)
Explain why a current-carrying wire experiences a force in a magnetic field, but a stationary charge does not.
A current is a flow of charges. When these charges move through a magnetic field, they experience a force (the Lorentz force). A stationary charge has no velocity, so it does not experience a magnetic force. The force depends on the velocity of the charge relative to the magnetic field.
A student says, "An electromagnet is just a permanent magnet that you can switch on and off." Is this correct? Explain.
Not quite. An electromagnet uses a current to produce a magnetic field. It can be switched on and off, and its strength and polarity can be controlled. A permanent magnet has a fixed field. While both produce magnetic fields, their behaviour and applications are different.
Why is soft iron used for the core of an electromagnet instead of steel?
Soft iron magnetises quickly when current flows and demagnetises quickly when current stops. Steel retains magnetism, which is not desirable for an electromagnet that needs to be switched on and off rapidly. Soft iron gives a stronger field for the same current and responds faster to changes in current.
Electricity and magnetism are two sides of the same coin — and when current flows, it weaves an invisible field that moves the world.
