4.2.2Electricity and Magnetism

Demonstrating Electric Charges — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

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📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision

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Lesson summary

4.2.2 Demonstrating Electric Charges — IGCSE PHYSICS notes: Describe experiments that show charging by friction and by induction and use an electroscope to detect and compare charge.Detectors — small pieces of tissue paper, a suspended polythene strip, a gold-leaf electroscope an…

Law of charges
Polythene with polythene
Polythene with acetate
Sign of an unknown charge
Improvements
Construction

Section 1 of 10Detecting charge

Detectors

Detectors small pieces of tissue paper, a suspended polythene strip, a gold-leaf electroscope and a charge sensor with a data logger.

Law of charges

Law of charges each detector relies on like charges repelling and unlike charges attracting.

Section 2 of 10Charging by friction: suspended rods

Hold a polythene rod at its centre and rub both ends with a cloth.

Hang the rod from a nylon string and cradle on a wooden stand without touching its ends.

Rub one end of a second polythene rod with a cloth and bring it close to the hanging rod.

Record whether the hanging rod is attracted or repelled.

Repeat with an acetate rod and rods of other materials such as acrylic, glass and wood.

Variables

Variables the independent variable is the rod material and the dependent variable is the charge on the rod.

Controls

Controls rubbing time, type of cloth and length of rod.

Section 3 of 10Results and conclusions

1Polythene with polythene

Polythene with polythene they repel because they have the same charge.

2Polythene with acetate

Polythene with acetate they attract because their charges are opposite.

3Sign of an unknown charge

Sign of an unknown charge repulsion means it has the same sign as the known charge.
Only repulsion is a sure test because a neutral object is also attracted.

4Improvements

Improvements use dry materials, close windows to avoid draughts, do not touch charged ends and rub for the same longer time.

Section 4 of 10The gold-leaf electroscope

The gold-leaf electroscope

Construction

Construction a metal cap joined to a metal rod with a thin gold leaf beside a stem, all inside a case to avoid draughts.

Operation

Operation when the cap is charged, the stem and leaf have the same charge and repel, so the leaf rises.

Size of charge

Size of charge a bigger charge gives a larger leaf angle.

Touching with a charged rod

Touching with a charged rod transfers charge and leaves the electroscope with the same sign as the rod.

Touching with a finger

Touching with a finger lets the charge flow to earth, the leaf falls and the electroscope is discharged.

Section 5 of 10Electroscope with a charged rod

Action	Leaf	Explanation
Charged rod brought near the cap	Rises	Electrons are repelled or attracted, so the stem and leaf gain like charges
Rod taken away	Falls	Electrons redistribute and the electroscope is neutral again
Rod touches the cap	Stays raised	Charge is transferred, so the electroscope is charged
Finger touches the cap	Falls	Charge flows through the person to earth

Section 6 of 10Charging by induction

Charging by induction

Bring a negative rod near the cap without touching, so electrons are repelled down to the leaf and the leaf rises.

Touch the cap with a finger so electrons flow to earth and the leaf drops.

Remove the finger first, then remove the rod.

Result

Result the electroscope is left with a net charge opposite to the rod, because the electrons that left cannot return.

Contact gives the same sign as the rod, but induction gives the opposite sign.

Section 7 of 10Testing an unknown charge

Method

Method charge an electroscope with a known sign and bring up the unknown object without touching.

Leaf rises further

Leaf rises further the object has the same sign.

Leaf falls

Leaf falls the object has the opposite sign.

Charge sensor

Charge sensor a data logger shows the two rubbed objects carry equal and opposite charges, supporting conservation of charge.

Section 8 of 10More from the specification

Section 9 of 10Key terms

Term	Meaning
Electroscope	Instrument with a metal cap, stem and gold leaf used to detect charge.
Induction	Charging an object without touching it, giving it the opposite sign to the charged object.
Discharge	Removing charge, for example by touching with a finger.
Charging by friction	Rubbing insulators so that electrons transfer.
Earth	Ground connection through which charge can flow away.

Section 10 of 10Exam technique

Removing the rod before the earth connection during induction — the charge redistributes and the electroscope is left uncharged.

Forgetting that induction gives the opposite sign to the rod, whereas contact gives the same sign.

Using damp materials or humid air — moisture removes charge and gives unreliable results.

Saying the leaf rises because it is attracted to the case — it rises because the leaf and stem have like charges and repel.

Last updated: 18 October 2026