Electroscopes: how to find an electric charge

Safe­ty pre­cau­tions

Warn­ing! Only un­der adult su­per­vi­sion.

Equip­ment

• rub­ber lid;
• 2 pa­per clips;
• 2 pieces of foil;
• glass;
• bal­loon;
• sty­ro­foam;
• pin.

Step-by-step in­struc­tions

Make two holes in a rub­ber lid equal dis­tances from the cen­ter. Put an un­fold­ed pa­per clip through each hole. Hang iden­ti­cal pieces of foil on the pa­per clips and put the set­up on a glass. Rub a bal­loon and some sty­ro­foam to­geth­er. First, hold each item to the lid sep­a­rate­ly – the pieces of foil should re­pel one an­oth­er. Hold the items to the lid to­geth­er – the foil pieces should at­tract each oth­er.

Process de­scrip­tion

The fric­tion of two ob­jects against each oth­er caus­es the trans­fer of elec­trons from one ob­ject to the oth­er. When this hap­pens, the ob­ject that gives elec­trons gains a pos­i­tive charge, and the ob­ject that ac­cepts elec­trons gains a neg­a­tive charge. When the bal­loon and the sty­ro­foam are rubbed against each oth­er, their sur­faces gain op­po­site charges: the bal­loon be­comes pos­i­tive, and the sty­ro­foam be­comes neg­a­tive. You can prove the pres­ence of a charge on the sur­face of each ob­ject us­ing a sim­ple de­vice – a DIY elec­tro­scope. If you bring a charged ob­ject to the elec­trodes (the pa­per clips), this caus­es a re­dis­tri­bu­tion of charges in the con­duct­ing clip-foil sys­tem. As a re­sult, the pa­per clips and foil re­ceive in­duced charges. When one charged ob­ject is held close to the pa­per clips, the pieces of foil ac­quire the same charge and re­pel each oth­er. When op­po­site­ly-charged ob­jects are held close to the pa­per clips, the pieces of foil will at­tract each oth­er. When they touch, the pieces of foil ex­change charges and their charges even­tu­al­ly equal­ize. At this point, they sim­ply stop at­tract­ing one an­oth­er.

Cool ex­per­i­ments are wait­ing for you in the MEL Chem­istry sub­scrip­tion!