Static Electricity
Electrostatics explains how stationary electric charges are created, typically on insulators, by the transfer of electrons. It covers the fundamental forces between these charges and the practical applications and hazards, such as static shocks and the importance of earthing for safety.
Part of the ESAT Physics syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Charging insulators by friction involves rubbing two materials together, causing electrons to move from one surface to the other.
- The material that gains electrons becomes negatively charged, and the one that loses electrons becomes positively charged. Only electrons move; protons are fixed in the nucleus.
- The fundamental law of electrostatics is that like charges repel each other (e.g., positive and positive), while unlike charges attract each other (e.g., positive and negative).
- A charged object can attract a neutral object. It does this by inducing a separation of charge within the neutral object, causing the side nearer to the charged object to have an opposite charge.
- Earthing (or grounding) an object provides a path for excess charge to flow to or from the Earth, which acts as a vast reservoir of charge, thus neutralizing the object.
- Static charge can be hazardous, as a large build-up can lead to a spark, which can ignite flammable materials. Earthing is used to prevent this.
› Why does this happen?
Why does rubbing materials together create charge?
Atoms in different materials hold onto their outer electrons with different strengths. When you rub two insulators together, the friction provides energy. This energy is enough to pull some of the weakly-held electrons away from the surface of one material and transfer them to the other. The material that loses electrons is left with more positive protons than negative electrons, so it becomes positively charged. The material that gains the extra electrons becomes negatively charged.
How can a charged object attract a neutral one?
When a charged object is brought near a neutral insulator, like paper, it causes charges inside the paper's atoms to shift. For example, a negatively charged rod will push the electrons in the paper's atoms slightly away from it. This makes the surface of the paper closer to the rod slightly positive, and the surface further away slightly negative. The attraction between the rod and the closer, positive side is stronger than the repulsion with the further away, negative side. This results in an overall force of attraction.
Definitions
- Static Charge
- An imbalance of electric charge on the surface of an object, created by a surplus (negative charge) or deficit (positive charge) of electrons.
- Insulator
- A material in which electrons are not free to move. This allows static charge to build up on its surface rather than flowing away.
- Charging by Induction
- The process where a charged object brought near a neutral conductor causes a separation of charge within the conductor, without direct contact.
- Earthing
- Connecting an object to the Earth via a conductor to allow charge to flow, safely neutralizing any static charge build-up.
Worked example
A student holds a negatively charged plastic rod near, but not touching, an isolated, uncharged metal sphere. While the rod is held in place, the student briefly touches the far side of the sphere with their finger. The finger is then removed, and finally the rod is taken away. What is the final charge on the sphere, and why?
- 1
Step 1:
The negatively charged rod is brought near the sphere.
As the sphere is a metal conductor, its free electrons are repelled by the rod's negative charge.
These electrons move to the far side of the sphere.
- 2
Step 2:
This movement of electrons (induction) leaves the side of the sphere nearer the rod with a net positive charge and the far side with a net negative charge.
The sphere is still neutral overall.
- 3
Step 3:
When the student touches the sphere, they provide an earthing path.
The repelled electrons on the far side of the sphere flow away from the sphere, through the student, and into the Earth.
- 4
Step 4:
The finger is removed, breaking the earthing path.
The sphere now has a deficit of electrons and therefore a net positive charge.
This charge is held on the near side by the attraction of the negative rod.
- 5
Step 5:
When the rod is finally removed, the net positive charge on the sphere spreads out evenly over its surface.
Answer: The sphere is left with a net positive charge.
Common mistakes
- ×Assuming positive charge is due to the movement of protons. Protons are locked in the nucleus; positive charge is always caused by a loss of electrons.
- ×Believing that only charged objects can be attracted. A charged object can induce charge separation in a neutral object and attract it.
- ×Confusing the properties of insulators and conductors. Static charge builds up on insulators because charge cannot flow, whereas conductors allow charge (electrons) to move freely and can be neutralized by earthing.
No-calculator tips
- ✓Always track the movement of electrons. Ask 'where did they go?' or 'where did they come from?' to determine the resulting charges.
- ✓Sketch the situation. Drawing the objects and using '+' and '-' symbols to show charge distribution helps visualize the forces of attraction and repulsion.
- ✓Remember that earthing is a path for charge to escape or be drawn from. It always results in the object becoming neutral, unless an external charge is holding a charge separation (as in the worked example).