CIE IGCSE Physics (0625) The magnetic effect of a current Study Notes - New Syllabus
CIE IGCSE Physics (0625) The magnetic effect of a current Study Notes
LEARNING OBJECTIVE
- Understanding the concepts of The magnetic effect of a current
Key Concepts:
- Magnetic Field Around a Straight Current-Carrying Wire
- Experiment: Identifying Magnetic Field Patterns Due to Current in Wires and Solenoids
- Magnetic Effect of Current in Relays and Loudspeakers
Magnetic Field Around a Straight Current-Carrying Wire
Magnetic Field Around a Straight Current-Carrying Wire
The magnetic field forms concentric circles around the wire.
- Use the Right-hand Thumb Rule:
- Thumb points in the direction of current.
- Fingers curl in the direction of magnetic field lines.
- The magnetic field is strongest near the wire and weakens with distance.
Field Strength ∝ 1 / distance from wire
Magnetic Field Around a Solenoid
- The magnetic field inside the solenoid is uniform and strong, similar to that of a bar magnet.
- Field lines are parallel and evenly spaced inside; they loop outside from the N to S pole.
- Field outside is weaker and non-uniform.
Effect of Changing the Magnitude of Current
- Increasing the current increases the strength of the magnetic field around both a wire and a solenoid.
- Field lines become denser (closer together), showing increased field strength.
Field Strength ∝ Current
Effect of Reversing the Direction of Current
- Reversing the current reverses the direction of the magnetic field.
- For a straight wire: the circular field reverses direction (use Right-hand rule again).
- For a solenoid: the poles of the solenoid swap – North becomes South and vice versa.
Example:
A student connects a solenoid to a DC power supply and places a compass near one end. When she reverses the terminals of the supply, she observes the compass needle swings the opposite way. Explain why.
▶️ Answer/Explanation
Reversing the terminals reverses the direction of current in the solenoid.
This causes the magnetic field to reverse direction, swapping the North and South poles of the solenoid.
The compass detects this reversal and swings to align with the new field direction.
Experiment: Identifying Magnetic Field Patterns Due to Current in Wires and Solenoids
Experiment: Identifying Magnetic Field Patterns Due to Current in Wires and Solenoids
For a Straight Current-Carrying Wire
Apparatus: Power supply (DC), straight vertical copper wire, rheostat, switch, iron filings or magnetic compasses, cardboard sheet.
Procedure:
- Place the wire vertically through a hole in the center of the cardboard.
- Sprinkle iron filings evenly on the cardboard surface.
- Close the circuit to allow current through the wire.
- Gently tap the cardboard to let filings align with the magnetic field.
Observation: The filings form concentric circles around the wire.
Direction of Field: Use the right-hand thumb rule – if thumb points in current direction, curled fingers show magnetic field direction.
For a Solenoid (Coiled Wire)
Apparatus: Power supply (DC), insulated wire wound into a solenoid (coil), magnetic compasses or iron filings, rheostat, switch.
Procedure:
- Connect the solenoid to the power supply.
- Place small magnetic compasses along and around the solenoid.
- Close the switch to allow current through the solenoid.
Observation: The compasses align to show field lines running from one end to the other inside the solenoid, and looping outside – like a bar magnet.
Field Direction: Use the right-hand grip rule – fingers curl in direction of current in coils, and thumb points to the solenoid’s North Pole.
Conceptual Note:
Iron filings show the pattern of magnetic field lines.
Compasses show the direction of the magnetic field.
Magnetic Effect of Current in Relays and Loudspeakers
Magnetic Effect of Current in Relays and Loudspeakers
1. Relays
A relay is an electrically operated switch that uses the magnetic effect of current to control one circuit with another.
Structure:
- Contains a small coil (electromagnet), iron armature, spring, and switch contacts.
- Usually separates a low-voltage control circuit from a high-voltage output circuit.
Working Principle:
- When current flows through the coil, it becomes magnetised (electromagnet).
- This attracts the iron armature, which moves and closes (or opens) another switch in a different circuit.
- When the current is turned off, the spring returns the switch to its original position.
Use of Magnetic Effect: The coil becomes an electromagnet when current flows, using the magnetic field to move mechanical parts.
Applications:
- Car ignition systems.
- Automatic door locks.
- Remote control switching of household appliances using low-power circuits.
2. Loudspeakers
A loudspeaker converts electrical signals (AC) into sound waves using the magnetic effect of current.
Structure:
- A coil of wire (voice coil) attached to a cone (diaphragm).
- A permanent magnet surrounds the coil.
Working Principle:
- An AC signal passes through the coil, generating a changing magnetic field.
- This interacts with the field of the permanent magnet.
- The coil experiences a force (from the motor effect), causing it to move back and forth.
- This motion vibrates the cone, producing sound waves.
Use of Magnetic Effect: The force on the coil (due to interaction between current and magnetic field) produces motion and sound.
Applications:
- Speakers in radios, phones, televisions.
- Public address systems (PA).
- Headphones and earphones.
Example :
A water level sensor detects when a tank is empty. The sensor operates on a 6V battery. When the tank is empty, the sensor circuit turns on a relay, which in turn starts a 230V water pump motor to refill the tank.
▶️ Answer/Explanation
The sensor circuit contains a coil (electromagnet) of the relay. When the water level is low, current flows through the coil, magnetising it.
The magnetic field attracts the iron armature, which closes the contacts in the high-voltage pump circuit.
The high-voltage pump turns on and fills the tank. When the tank is full, the sensor switches off, de-energising the coil and opening the high-voltage circuit.
Concept: A small current controls a large current using the magnetic effect of the relay.
Example :
A music player sends an audio signal (alternating current) into a loudspeaker connected to it. Explain how the loudspeaker produces sound.
▶️ Answer/Explanation
The alternating current flows through the voice coil placed in a magnetic field provided by a permanent magnet.
Due to the magnetic effect of current, the coil experiences a force that varies with the direction of the current (motor effect).
The coil vibrates back and forth rapidly, and the cone attached to it moves with it.
These vibrations produce pressure waves in air — i.e., sound waves that match the audio signal.
Concept: Alternating current causes a varying magnetic force, converting electrical signals into mechanical vibrations (sound).