IB MYP 4-5 Physics- Static electricity and Charging Methods- Study Notes - New Syllabus
IB MYP 4-5 Physics-Static electricity and Charging Methods- Study Notes
Key Concepts
- Static electricity and Charging Methods
Static Electricity
Static Electricity
Static electricity is the buildup of electric charge on the surface of an object. It occurs when electrons are transferred from one material to another, usually by rubbing or contact.
- Charges can be positive (deficiency of electrons) or negative (excess of electrons).
- Like charges repel, unlike charges attract.
- Static electricity remains on the surface until it is discharged (e.g., through a spark or conduction).
Methods of Charging
1. Charging by Friction
- When two insulators are rubbed together, electrons transfer from one object to the other.
- The object that loses electrons becomes positively charged, while the one that gains electrons becomes negatively charged.
- Example: Rubbing a balloon with wool or hair causes the balloon to gain electrons and stick to walls.
2. Charging by Conduction
- Occurs when a charged object comes in direct contact with a neutral conductor.
- Electrons move between the objects until both reach the same charge.
- The neutral object becomes charged with the same type of charge as the charged object.
3. Charging by Induction
- A charged object is brought near a neutral conductor but does not touch it.
- Electrons inside the neutral conductor are either attracted or repelled, causing charge separation.
- If the conductor is then grounded, it can become permanently charged with the opposite type of charge to the inducing object.
- This is the principle used in devices like the electroscope.
4. Charging by Grounding
- Grounding allows charges to flow between a charged object and the Earth.
- Since the Earth is an infinite charge reservoir, it can accept or supply electrons easily.
- For example, touching a negatively charged object to the ground allows excess electrons to escape, neutralizing the object.
(a) When a charged rod is brought near the metal sphere without touching it, some of the positive and negative charges in the sphere are separated. (b) Some of the electrons leave the sphere through the grounding wire, with the result (c) that the sphere acquires a positive net charge.
Example:
When a plastic comb is rubbed on dry hair, the comb can attract small pieces of paper. Explain why this happens.
▶️ Answer/Explanation
Rubbing the comb on hair transfers electrons from the hair to the comb. The comb becomes negatively charged, while the hair becomes positively charged.
When the charged comb is brought near small pieces of paper, it induces opposite charges in them (electrostatic induction). The paper is attracted to the comb.
Example:
A polythene rod is rubbed with a wool cloth and gains \( 6.4 \times 10^{-19} \,\text{C} \) of charge. How many electrons did it gain?
▶️ Answer/Explanation
We use \( Q = n \times e \).
\( n = \dfrac{Q}{e} = \dfrac{6.4 \times 10^{-19}}{1.6 \times 10^{-19}} = 4 \).
Thus, the rod gained \(\boxed{4 \text{ electrons}}\).
Example:
Why is it dangerous to refuel a car at a petrol station without grounding (earthing) the fuel nozzle?
▶️ Answer/Explanation
When fuel flows through the nozzle, friction between the liquid and the nozzle can build up static charge.
If this static charge is not discharged safely, it may cause a spark.
Since petrol vapour is highly flammable, the spark can ignite it, leading to fire or explosion.