P2aL14 Charge

Key Words

Attract - pull towards.

Electric current - flow of charge.

Electrical conductor - material that allows electrons to flow.

Electrons - tiny particles that carry negative charge which orbit the nucleus.

Electrostatic induction - movement of electrons due to the presence of a charged object

Gold leaf electroscope - simple instrument to detect and measure the presence of electrical charge.

Negative electric charge - occurs in a particle having more electrons than protons.

Positive electric charge - occurs in a particle having fewer electrons than protons.

Static electrical charge - charge on a charged insulated object.

Uncharged - object has the same number of protons as neutrons; it's neutral.

Grade E

We know from previous work that static electrical charges exert a pushing or pulling force on each other:

• Like charges repel;
• Unlike charges attract.

If electrical charges are in a conductor, they will come close together, or get further apart.  A flow of charge occurs; this is a current.  When static electricity flows, the voltages are high, but the currents are tiny.  If you charge yourself up on a nylon carpet, you can get a small shock.  The voltage is about 30 000 V, but the current is about 10^-12 A.  It's the current that matters.  Should you be stupid enough to touch a 25 000 V overhead cable, you will be killed.

We can detect charges using a very simple instrument called the Gold Leaf Electroscope.

When we bring up a negatively charged rod near the cap, the electrons are repelled down the rod as shown in this animation.  The red circles are the protons; the blue circles are the electrons.  The like negative charges at the bottom of the rod and the gold leaf repel, lifting the gold leaf.

It's only electrons that move.  Protons NEVER move.

If we bring a positively charged rod to the cap, the electrons are attracted up the rod.  There is positive charge left behind.  The like positive charges repel, lifting the gold leaf.

Again, the protons are not free to move.   So they stay where they are.

Grade C

The gold leaf  in the electroscope rises as we bring a charged rod towards the cap.  It only tells us that the rod is charged, but not what the charge is.  Now if we scrape some positive charge onto the electroscope (by rubbing the cap with a charged acetate rod), we see the following:

• If the second rod is positively charged, the leaf rises further, as more electrons are attracted to the cap.
• If the second rod is negatively charged, the leaf falls, because the electrons are repelled to the rod and the leaf, reducing the overall positive charge on the rod and the leaf.

If we scrape negative charge from a polythene rod, the leaf rises as before.  We now do the same as before, but:

• If the second rod is positively charged, the leaf falls, because the electrons are attracted to the cap from the rod and the leaf.  This reduces the overall negative charge on the rod and the leaf.
• If the second rod is negatively charged, the leaf rises, because the electrons are repelled to the rod and the leaf.  This increases the overall negative charge on the rod and the leaf.

Electrostatic induction is behind a number of things that happen due to static electricity.  Dust is attracted to a TV screen that is negatively charged.  Electrons in the dust particle are repelled and go round the back of the particle, making the side of the particle close to the TV screen positively charged.  (Remember that for something on the scale of an electron, the dust particle is the size of a planet.)

A negatively charged balloon sticks to the wall, because electrons are repelled further into the wall, leaving the top surface positively charged.

The ultimate electrostatic event is a thunderstorm.  Hot air moves upwards in the cloud.  Ice particles are carried upwards carrying a positive charge.  Downwards moving ice particles get a negative charge.  Nobody knows why this happens.  The separation of charge results in a huge potential difference (voltage), and a spark (lightning) jumps.

Notice how the electrons are repelled into the ground.

Grade A

In the thunderstorm, the repulsion of like charges results in the characteristic anvil cloud.  The animation above shows an Earth strike (forked lightning) but it is just as likely for there to be a cloud discharge (sheet lighting) within the cloud, or between clouds.

The voltage is typically about 10⁹ V.  The current is about 20 000 A, but the discharge is less than 1/1000th of a second.  About 20 C of charge flows.  The amount of energy released in a thunderstorm is massive.  The detonation of a nuclear weapon has less energy than a typical summer thunderstorm.