Demonstrating Electric Charges (CIE IGCSE Physics)

• When certain insulating solids are rubbed against each other, they can become electrically charged
  • This is called charging by friction

• The charges remain on the insulators and cannot immediately flow away
  • One gains a net positive charge and the other gains a net negative charge

• An example of this is a plastic or polythene rod being charged by rubbing it with a cloth
  • Both the rod and cloth are insulating materials

Charging solids by friction

A polyethene rod may be given a charge by rubbing it with a cloth

• When an uncharged cloth is rubbed against an uncharged polythene rod

  • Electrons are transferred away from the cloth to the rod

    • The cloth has lost electrons so it becomes positively charged

    • The polythene rod has gained electrons so it becomes negatively charged

  • These oppositely charged objects are also attracted to each other

• When an uncharged cloth is rubbed against an uncharged acetate plastic rod, however

  • Electrons are transferred away from the acetate rod to the cloth

    • The cloth has gained electrons so it becomes negatively charged

    • The rod has lost electrons so it becomes positively charged

Experiment 1: investigating electrostatic charging by friction

• The aim of this experiment is to investigate how insulating materials can be charged by friction

Variables

• Independent variable = Rods of different material
• Dependent variable = Charge on the rod
• Control variables:
  • Time spent rubbing the rod
  • Using the same type of cloth
  • Using the same length of rod

Equipment

Equipment list

Equipment	Purpose
Polythene rod	to charge and hang from a cradle to test against each material
Rods of different materials (acrylic, acetate, glass, wood)	to observe the effects of these on the polythene rod
Cloths (one per material)	to rub the materials to charge them
Cradle	to suspend rods from, allowing them to move freely when subjected to a force
Nylon string	to hang the rods
Wooden stand	to suspend the string and cradle from

 

Method

Apparatus for investigating charging by friction

1. Take a polythene rod, hold it at its centre and rub both ends with a cloth
2. Suspend the rod, without touching the ends, from a stand using a cradle and nylon string
3. Take a second polythene rod and rub one end with a different cloth
4. Bring the second polythene rod close to the suspended rod
5. Record any observations of the suspended rod's motion, i.e. whether it is attracted or repelled by the second rod
6. Repeat using an acetate rod and rods of different materials

Example results table

charged material 1	charged material 2	attracted or repelled
polythene	polythene
polythene	acetate

Analysis of results

• When two insulating materials are rubbed together, negative charge (electrons) will transfer from one insulator to the other
• A polythene rod gains a negative charge when rubbed with a cloth
  • This is because electrons are transferred to the polythene from the cloth

• An acetate rod gains a positive charge when rubbed with a cloth
  • This is because electrons are removed from the acetate by the cloth

Transfer of electrons between charged insulators

Electrons are transferred to the polythene rod giving it a negative charge, and they move from the acetate rod giving it a positive charge

• If the material is repelled by the polythene rod, then the materials have the same charge
  • For example, the polythene rod would be repelled by a second polythene rod, as they have the same charge

• If the material is attracted to the polythene rod, then they have opposite charges
  • For example, the polythene rod would be attracted to an acetate rod, as they have opposite charges

Evaluating the experiment

• Reduce the effects of environmental factors (e.g. close windows to reduce drafts) to ensure the motion of the polythene rod is due to electric forces only
• Make sure not to touch the ends of the rods once they have been charged (if the ends are touched, the rods will discharge and the forces will no longer be present)
• Produce greater deflections by rubbing the rods for a longer period to transfer more charge (ensuring that the time spent rubbing each rod is the same)

Experiment 2: detecting charge using a gold-leaf electroscope

• The aim of this experiment is to detect charge using a gold leaf electroscope

Variables

• Independent variable = Rods of different material
• Dependent variable = Charge on the rod

Equipment

Equipment list

Equipment	Purpose
Gold-leaf electroscope	to detect charge
Polythene and acetate rods (or strips)	to observe the effects of these on the gold-leaf electroscope when charged
Cloths (one per material)	to charge the rods (or strips) by rubbing them

 

Method

The gold-leaf electroscope is a device consisting of a metal plate, a metal stem and a thin gold leaf. The stem and leaf are housed in an airtight container to prevent draughts.

1. Before beginning the experiment, ensure the plate of the electroscope is uncharged by touching it with your finger. The leaf should hang straight down next to the stem
2. Charge a polythene rod by rubbing it with a cloth
3. Bring the charged rod towards the plate of the electroscope and record any observations
4. Bring the charged rod away from the plate of the electroscope and record any observations
5. Touch the charged rod to the plate of the electroscope and record any observations
6. Repeat using an acetate rod

Example results table

charged material	action	gold leaf rises or falls	movement of electrons
polythene	moved towards plate
polythene	moved away from plate
polythene	touched plate
acetate	moved towards plate
acetate	moved away from plate
acetate	touched plate

 

Analysis of results

• When a charged object is brought near the plate of the electroscope, the leaf rises
  • The negatively charged polythene rod repels electrons away from the surface of the plate down the stem and leaf, giving them a negative charge, hence, they repel
  • The positively charged acetate rod attracts electrons to the surface of the metal plate from the stem and leaf, giving them a positive charge, hence, they repel

• When the charged object is moved away from the plate, the leaf falls
  • Electrons in the electroscope are no longer repelled or attracted by the rod so they redistribute themselves
  • The stem and leaf become electrically neutral

• When a charged object touches the plate of the electroscope, the leaf stays risen
  • The charge from the rod is transferred to the metal plate and travels down the stem and leaf of the electroscope
  • The stem and leaf therefore carry the same charge and repel each other
  • The electroscope has been charged

• When a finger touches the plate, the leaf falls
  • The charge from the electroscope is transferred to the person and travels to the earth
  • The stem and leaf become electrically neutral
  • The electroscope has been discharged

Evaluating the experiment

• Make sure not to touch the ends of the rods once they have been charged (if the ends are touched, the rods will discharge)
• When the electroscope is charged by contact with a rod, it should stay risen. If it doesn't, repeat the process but ensure to press harder and draw the rod along the edge of the plate