Lesson 8: Static Electricity

In the figure : In the figure similar charges repel (push each other away) and opposite charges attract (pull each other). The green arrows show attraction and the yellow arrows show repulsion.

Rule 1: Two types of charge — All objects, even our bodies, hold a lot of charge, but it is hidden because positive and negative charges are equal in number. Benjamin Franklin named the charges positive and negative. If they are not balanced, the object is charged.

• Easy : A new balloon is neutral.
• Medium : A rubbed balloon has unbalanced charge, so it is charged.
• Exam-type : A body with 5 positive and 5 negative units of charge is neutral; with 5 positive and 3 negative units it is charged.

In the figure : In the figure a charged comb attracts small pieces of uncharged paper: charged objects attract uncharged objects.

Rule 2: Repulsion and attraction — Like charges repel; unlike charges attract. Charged objects also attract uncharged objects. Repulsion is the sure test of a charged object, because attraction can also happen with an uncharged object.

• Easy : Two glass rods rubbed with silk push each other away.
• Medium : A plastic rod rubbed with wool and a rubbed glass rod pull towards each other.
• Exam-type : Object P repels object Q, so both are charged with the same type of charge. If P only attracted Q, Q might be uncharged.

Spot the difference

In the figure : In the figure object A has lost two negative particles and has become positively charged, while object B has gained them and has become negatively charged.

Rule 1: How an object gets charged — Each atom has a stationary positive charge and moving negative charges in balance. On rubbing, negative particles go from one object to the other. The one that gains them is negative; the one that loses them is positive.

• Easy : A ruler gains negative particles: it becomes negative.
• Medium : Hair loses negative particles: it becomes positive.
• Exam-type : If A loses 3 negative particles and B gains the same 3, A is positive and B is negative with equal amounts of charge.

In the figure : In the figure a timeline shows that Thomas Browne named the property "electricity" in 1646 AD. Franklin's kite experiment came later, in 1752.

Rule 2: History and conditions — About 2500 years ago Thales found that a rubbed amber rod attracts feathers. Amber is called elektron in Greek. Static charge stays only at the place of rubbing and for a short time. Moist air absorbs the charge, so experiments are done in dry weather, especially in winter.

• Easy : Amber rubbed with fur attracts feathers.
• Medium : The year 1752 − 1646 = 106 years between Browne's naming and Franklin's kite.
• Exam-type : On a humid monsoon day charge escapes into the moist air, so the experiment fails.

Spot the difference

In the figure : In the figure a negatively charged rod near an uncharged ball pushes the negative particles to the far side. The side nearest to the rod becomes positive, so the ball is attracted. The charge lasts only while the rod is near.

Rule 1: Induction — A charged object near a neutral one pushes away the like charge in the nearest part, so that part gets the opposite charge. Induced charge lasts only while the charged object is near. A negatively charged comb near a trickle of water attracts it; remove the comb and the water is neutral again.

• Easy : A charged balloon sticks to a wall by induction.
• Medium : A negative comb near a water trickle makes the near side positive, so the trickle bends towards it.
• Exam-type : Take the comb away and the induced charge disappears, since it lasts only while the charged object is near.

In the figure : In the figure after touching the negatively charged rod, the ball has the same charge as the rod, so it is pushed away.

Rule 2: Contact — A charged comb touched to an uncharged comb charges it. A neutral aluminium ball is first attracted to a negative balloon by induction; after touching, both have the same charge and repel each other.

• Easy : Contact gives the same type of charge.
• Medium : First attract (induction), then touch, then repel (same charge).
• Exam-type : A ball that first jumps to a rod and then flies away has been charged by contact.

Spot the difference

In the figure : In the figure the parts of the gold leaf electroscope are: A the metal disc, B the copper rod, C the two thin gold leaves and D the bottle.

Rule 1: Construction — A copper rod with a metal disc at the top and two thin gold leaves at the bottom is placed inside a bottle.

• Easy : The disc is at the top and the leaves at the bottom.
• Medium : Number of gold leaves = 2.
• Exam-type : If A is the disc and B the rod, then C (leaves) hang from the end of the rod B.

In the figure : In the figure a charged rod near the disc gives both leaves the same charge, so they repel each other and open. If the object is uncharged, the leaves stay closed.

Rule 2: Working — An uncharged object near the disc leaves the leaves closed. A charged object gives both leaves the same charge, so they repel and open. Touching the disc with a hand makes the leaves collapse, because the charge flows to the earth through the body.

• Easy : Leaves open → object charged.
• Medium : Leaves stay closed → object uncharged (neutral).
• Exam-type : Touch the disc with a hand: the charge goes to the earth and the leaves collapse.

Rule 3: Franklin's experiment — In 1752 Benjamin Franklin flew a kite (silk, deodar wood and metal wire) with his son William during lightning. Charge from the clouds travelled down the string and a spark was seen at the earth. He showed that lightning is a form of electric charge.

• Easy : Franklin's experiment was in 1752.
• Medium : Lightning is a form of electric charge.
• Exam-type : From the year of Thales' amber (about 2500 years ago) to Franklin took many centuries, but the idea of charge stayed the same.

In the figure : In the figure of Franklin's experiment (1752) the charge of the cloud A travelled down the string and wire C of the kite B, and a spark D was seen near the earth.

Spot the difference

In the figure : In the figure of a thundercloud the upper part is positive (A) and the lower part is negative (B). The tall building (C) gets a positive charge by induction, and when the charge of the cloud becomes large it flows to the building as lightning.

Rule 1: How lightning happens — In clouds the upper part becomes positive and the lower part negative. When the negative charge at the base becomes much larger than that on the ground, it flows to the ground in stages in much less than a second, giving heat, light and sound. Tall buildings and trees get an opposite charge by induction, so the flow goes to them.

• Easy : Lightning gives heat, light and sound.
• Medium : The cloud's base is negative; the tall building gets positive charge by induction.
• Exam-type : The discharge is over in much less than a second, in stages.

In the figure : In the figure of a lightning conductor A is the forked end, B the long copper strip, C the cast iron plate and D the pit filled with coal and salt. It carries the charge of lightning safely into the earth.

Rule 2: Benefits and protection — Lightning forms nitrogen oxide from the air, which comes down with rain and improves soil fertility. It also changes oxygen into ozone, which protects us from ultraviolet rays. A lightning conductor is a long copper strip with a forked end at the top of a building and the other end joined to a cast iron plate in a pit filled with coal and salt, with a provision for water.

• Easy : Ozone protects us from ultraviolet rays.
• Medium : Nitrogen oxide comes down with rain and improves soil fertility.
• Exam-type : Without a lightning conductor, the charge would flow through the building; with it, the charge goes safely into the ground.

Spot the difference