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4.4.5Electricity and Magnetism

Investigating the Field Around a Wire — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 48 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.4.5 Investigating the Field Around a Wire — IGCSE PHYSICS notes: Describe experiments to investigate the magnetic field around a current-carrying wire and solenoid and interpret the results.Wire and coil — a thick wire and a solenoid (a wire wrapped into a coil), such as a meta…

  • Straight wire
  • Field against current
  • Independent variable
  • Control variables
  • Earth's field
  • Instrumentation

Section 1 of 11Apparatus

Wire and coil
  • Wire and coil a thick wire and a solenoid, such as a metal slinky.
    • (a wire wrapped into a coil)
Circuit
  • Circuit a cell, ammeter, variable resistor and connecting wires.
Support
  • Support cardboard with holes large enough for the wire and a clamp stand.
Detectors
  • Detectors iron filings or a plotting compass.

Section 2 of 11Experiment 1: plotting the field around a wire with compasses

1
Pass the thick wire vertically through a hole in the middle of the card and clamp it so it is perpendicular to the card.
2
Connect the wire to a series circuit with the variable resistor, ammeter and cell.
3
Place a plotting compass on the card and mark dots at each end of the needle once it settles.
4
Move the compass so it points away from the new dot and repeat to make a chain of dots.
5
Remove the compass and join the dots with a smooth curve to make one field line.
6
Repeat to draw several field lines.
7
Reverse the current: the compasses point the opposite way, so the field reverses.

Section 3 of 11Experiment 1: plotting with iron filings

Experiment 1: plotting with iron filings
Sprinkle iron filings evenly on the card.
Switch on a current of a few amperes for a short time.
Tap the card gently until the filings settle in the field pattern.
Result
  • Result the filings form concentric circles about the wire, closer together near the wire.
Heating
  • Heating use short bursts of current because a large current heats the wire.

Section 4 of 11Experiment 2: field around a circular coil or solenoid

Experiment 2: field around a circular coil or solenoid
Thread the wire through holes on opposite sides of the card to form a circular loop, or thread a solenoid through pre-made holes.
Connect it in a series circuit with the variable resistor, ammeter and cell.
Plot the field with compasses as in Experiment 1, or sprinkle iron filings and tap the card.
Solenoid
  • Solenoid it is essentially many circular loops, so the pattern around a loop extends to a solenoid.
Result
  • Result the field around a solenoid resembles that of a bar magnet.

Section 5 of 11Results and conclusions

Straight wire
  • Straight wire the field lines are circles centred on the wire in planes at right angles to it.
Direction
  • Direction it follows the right-hand grip rule and reverses when the current reverses.
Current
  • Current the field is stronger when the current is larger.
Distance
  • Distance the field is stronger closer to the wire and weaker farther away.
Field against current
  • Field against current a magnetic field sensor gives a straight line through the origin, so the field is proportional to the current.

Section 6 of 11Variables for the investigation

Independent variable
  • Independent variable the current or the distance from the wire.
    • (changed with the variable resistor)
Dependent variable
  • Dependent variable the field strength, read from a sensor or estimated from the turn of the compass needle.
Control variables
  • Control variables the distance when changing current, the current when changing distance, the sensor orientation and the absence of other magnets or metal objects.

Section 7 of 11Improving accuracy

Earth's field
  • Earth's field note the compass reading with no current and keep other magnets away.
Repeats
  • Repeats repeat readings and average them.
Heating
  • Heating switch the current off between readings so the wire cools.
Instrumentation
  • Instrumentation a Hall probe or data logger gives numerical readings, while a compass gives direction well but strength only roughly.

Section 8 of 11More from the specification

1
Transformer
  • Device that changes the size of an alternating voltage.
2
Secondary
  • Output coil.
3
Primary
  • Input coil.
4
Conductor
  • Material that allows current to flow.
5
Voltmeter
  • Instrument that measures p.d., connected in parallel.
6
Generator
  • Device that turns kinetic energy into electrical energy.
7
Brushes
  • Carbon contacts pressing on the slip rings.
8
Alternating current
  • Current that continually reverses direction.
9
IpVp
IsVs
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
10
Magnetic Field Around a Current-Carrying Conductor
  • Electric current flowing through a straight wire generates concentric circular magnetic field lines;
    • coiling the wire into a solenoid intensifies the field, mimicking a permanent bar magnet.
11
Principles of Electromagnetic Induction
  • Relative motion between a conductor and a magnetic field induces an electromotive force (EMF) across the coil;
    • in alternating current circuits, mutual induction across primary and secondary transformer coils transfers electrical power.
12
Step-Up and Step-Down Transformers
  • Step-up transformers elevate voltage for efficient grid transmission,
    • reducing resistive heating losses (P = I2 R); step-down transformers reduce voltage to safe consumer levels.
      What the symbols mean: P = power (W) · I = current (A) · R = resistance (Ω)
      • (e.g. 230 V or 115 V)
    • (e.g. from 500 V to 20 000 V)
13
Transformer Power Calculations
  • For an ideal 100% efficient transformer, input electrical power equals output power: Vp Ip = Vs Is,
  • meaning stepping voltage up causes a proportional drop in current.

Section 9 of 11More from the specification

More from the specification
Field Around a Current-Carrying Conductor
  • Electric current flowing through a straight wire induces circular, concentric magnetic field lines around the conductor, oriented via the right-hand grip rule.
Motor Effect and Deflecting Force
  • A current-carrying conductor located in an external magnetic field experiences a mechanical force perpendicular to both field and current, given by F = B I L.
Electromagnetic Induction
  • Moving a conductor relative to a magnetic field induces an electromotive force (voltage) across its terminals, proportional to the rate of cutting magnetic flux.
Transformer Voltage Equation
  • Alternating current in the primary coil induces a changing magnetic flux through an iron core, generating a secondary voltage according to Vp / Vs = Np / Ns.
Potential difference
  • Work done by a unit charge passing through a component.

Section 10 of 11Key terms

TermMeaning
Plotting compassSmall compass used to show field direction at a point.
Iron filingsSmall iron pieces that show the shape of a field.
SolenoidLong coil of wire that produces a magnetic field when carrying current.
Field lineLine showing the direction and strength of a magnetic field.
Hall probeSensor that measures magnetic field strength.

Section 11 of 11Exam technique

1
Leaving the current on continuously — it heats the wire and changes the resistance.
2
Forgetting that the Earth's magnetic field also affects the compass.
3
Stating that the field is the same strength at all distances from the wire.
4
Concluding that the field is circular because filings form straight lines.

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