P4.5.3P4. Electricity and Magnetism
Magnetic effect of a current — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 14 of 19 1 quiz
Lesson summary
P4.5.3 Magnetic effect of a current — COMBINED SCIENCE notes: Describe the magnetic fields around wires and solenoids and the uses of electromagnets.Magnetic field — a magnetic field forms wherever a current flows, in straight wires, solenoids and circular coilsField pattern — co…
- Right-hand grip rule
- Solenoid field
- Larger current
- More turns on the coil
- Soft iron core
- Soft iron advantage
Section 1 of 10Field around a straight wire
Magnetic field
- Magnetic field a magnetic field forms wherever a current flows, in straight wires, solenoids and circular coils
Field pattern
- Field pattern concentric circles centred on the wire
- A circular pattern means the field around the wire has no poles.
- The lines are closer together near the wire, so the field is strongest there.
Direction
- Direction the field lines run clockwise or anticlockwise depending on the current direction
- Reversing the current reverses the field direction.
Right-hand grip rule
- Right-hand grip rule thumb along the conventional current, curled fingers show the field direction
Strength
- Strength a larger current gives a stronger field and the field gets weaker with distance from the wire
Section 2 of 10Investigating the field around a wire
1
Pass a thick wire vertically through a hole in the middle of a card, and clamp it so it is perpendicular to the card.
2
Connect the wire in series with a cell, an ammeter and a variable resistor.
3
Place a plotting compass on the card, mark dots at both ends of the needle, then move the compass to the new dot and repeat.
4
Join the dots with a smooth curve to draw one field line, then repeat for other field lines.
5
Reverse the current; the compasses point the opposite way, showing the field reverses.
6
As an alternative, sprinkle iron filings on the card and tap it until they settle into the field pattern.
Section 3 of 10Field around a solenoid and a circular coil
Solenoid
Solenoid a coil of wire that gives a stronger field by putting more turns in a small region
Solenoid field
- Solenoid field similar to a bar magnet, with field lines emerging from the north pole and returning to the south pole
- Inside the solenoid the field lines are parallel and evenly spaced, so the field is strong and uniform.
Poles
- Poles found with the right-hand grip rule, with the thumb pointing to the north pole
- The end where the current flows anticlockwise is the north pole.
- The end where the current flows clockwise is the south pole.
Circular coil
- Circular coil equivalent to one turn of a solenoid, with field lines emerging from one face and entering the other
- Many circular coils combined make a solenoid.
Section 4 of 10Strengthening an electromagnet
1Larger current
- Larger current gives a stronger field
2More turns on the coil
- More turns on the coil gives a stronger field
3Soft iron core
- Soft iron core becomes an induced magnet when current flows, so the overall magnet is much stronger
4Soft iron advantage
- Soft iron advantage it loses its magnetism when the current is switched off, so the electromagnet can be turned on and off
5Reversing the current
- Reversing the current reverses the poles of the electromagnet
Section 5 of 10Relay circuits
- Relay = a switch that is opened and closed by an electromagnet
- It has one circuit containing an electromagnet and a second circuit with a switch near the electromagnet.
1
A current passes through the coil in circuit 1.
2
The coil produces a magnetic field that attracts the switch in circuit 2.
3
The switch pivots and closes its contacts, so a current flows in circuit 2.
4
When the current in circuit 1 stops, the magnetic force stops and the contacts open.
Section 6 of 10Electric bell
1
Pressing the button K sends a current through the electromagnet E.
2
The electromagnet attracts the iron armature A, so the hammer strikes the bell B.
3
The movement of the armature breaks the circuit at T.
4
The current stops, the field collapses and the armature returns to its original position.
5
The circuit is made again and the process repeats, giving a continuous ringing.
Section 7 of 10Scrapyard crane and loudspeaker
Scrapyard crane
- Scrapyard crane an electromagnet attracts magnetic materials when switched on and drops them when switched off
Loudspeaker
- Loudspeaker converts an electrical signal into sound using the motor effect
- An alternating current in the coil makes a changing magnetic field around the coil.
- This interacts with the field of the permanent magnet and exerts a force on the coil that keeps changing direction.
- The coil and the speaker cone oscillate, making the air oscillate to create sound waves.
Section 8 of 10More from the specification
1
Force on a current-carrying conductor
- A current-carrying conductor positioned in an external magnetic field experiences a mechanical force perpendicular to both the field and current,
- as determined by Fleming's left-hand rule.
2
Electromagnetic induction and moving conductors
- Moving a conductor through a magnetic field induces an electromotive force (e.m.f.) detected on a sensitive voltmeter,
- increasing in magnitude with wire speed and magnetic strength.
3
Transformer voltage and turn ratio equation
- An alternating voltage applied to the primary coil of an iron-core transformer induces an alternating voltage in the secondary coil, governed by the transformer equation Vp / Vs = Np / Ns.
4
Electrical power transmission in transformers
- Assuming 100% efficiency, electrical power input equals power output (IpVp = IsVs),
- meaning increasing voltage in step-up transformers proportionally reduces transmission current and heat losses.
5
Force on a current-carrying conductor
- A current-carrying conductor positioned in an external magnetic field experiences a mechanical force perpendicular to both the field and current,
- as determined by Fleming's left-hand rule.
6
Electromagnetic induction and moving conductors
- Moving a conductor through a magnetic field induces an electromotive force (e.m.f.) detected on a sensitive voltmeter,
- increasing in magnitude with wire speed and magnetic strength.
7
Transformer voltage and turn ratio equation
- An alternating voltage applied to the primary coil of an iron-core transformer induces an alternating voltage in the secondary coil, governed by the transformer equation Vp / Vs = Np / Ns.
8
Electrical power transmission in transformers
- Assuming 100% efficiency, electrical power input equals power output (IpVp = IsVs),
- meaning increasing voltage in step-up transformers proportionally reduces transmission current and heat losses.
9
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
10
Transformer
- A device with a primary coil and secondary coil on an iron core that changes an alternating voltage.
11
Voltage
- A measure of the electrical energy supplied or changed in a circuit.
12
Conductor
- Material that lets current flow.
13
Voltmeter
- Meter that measures voltage, connected in parallel.
14
Generator
- A device that converts movement into an electrical output by electromagnetic induction.
15
Potential difference
- The work done by a unit charge passing through a component.
16
Step-up transformer
- A transformer with more turns on the secondary coil than the primary, raising the voltage.
17
Step-down transformer
- A transformer with fewer turns on the secondary coil, lowering the voltage.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Electromagnet | A solenoid wound on a soft iron core that is a magnet only while current flows. |
| Solenoid | A coil of wire that produces a bar-magnet-like field when carrying a current. |
| Right-hand grip rule | Thumb along the conventional current gives the curl of the field lines. |
| Soft iron | A magnetic material that is easily magnetised and demagnetised. |
| Relay | A switch operated by an electromagnet that lets a small current control a larger one. |
| Loudspeaker | A device that uses a coil in a permanent magnet's field to turn an alternating electrical signal into sound. |
Section 10 of 10Exam technique
1
Thinking the field lines around a wire are straight — they are concentric circles around the wire
2
Thinking steel is used for the core of an electromagnet — soft iron is used because it loses its magnetism when the current is off
3
Reading the field direction from the electron flow — the grip rule uses conventional current
4
Thinking the field is the same strength everywhere around a wire — it is strongest close to the wire and weakens with distance
Marked practiceQuizzes for this lesson
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