IB MYP 4-5 Physics- Electric charge and electrons- Study Notes - New Syllabus
IB MYP 4-5 Physics-Electric charge and electrons- Study Notes
Key Concepts
- Electric charge and electrons
Electric charge and electrons
Electric Charge
Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric field.
- There are two types of charges: positive (+) and negative (–).
- Like charges repel each other, while unlike charges attract each other.
- The SI unit of charge is the coulomb (C).
- A small amount of charge is carried by subatomic particles such as electrons and protons:
- Proton: positive charge \( +1.6 \times 10^{-19} \,\text{C} \).
- Electron: negative charge \( -1.6 \times 10^{-19} \,\text{C} \).
- Neutron: no charge (neutral).
Electrons
Electrons are negatively charged particles that orbit the nucleus of an atom.
- They are very light compared to protons and neutrons.
- Electrons are responsible for electric current, because current is simply the movement of charges (mostly electrons) through a conductor.
- When electrons move, they create an electric field and magnetic effects.
- In solids (like metals), electrons are free to move, making them good conductors of electricity.
Quantization of Charge
Charge is always a whole-number multiple of the elementary charge \( e = 1.6 \times 10^{-19} \,\text{C} \).
- So, total charge \( Q \) is given by:
\( Q = n \times e \)
where \( n \) is an integer (number of electrons or protons).
Example:
Two balloons are rubbed with a wool cloth. Both balloons acquire a negative charge. Explain why they repel each other.
▶️ Answer/Explanation
When the balloons are rubbed, electrons are transferred from the wool cloth to the balloons. Both balloons gain excess electrons and become negatively charged.
Since like charges repel each other, the two balloons push away from each other due to electrostatic repulsion.
Example:
A plastic rod has a charge of \( Q = -3.2 \times 10^{-19} \,\text{C} \). How many extra electrons are present on the rod?
▶️ Answer/Explanation
We use the formula:
\( Q = n \times e \)
where \( Q = -3.2 \times 10^{-19} \,\text{C} \) and \( e = 1.6 \times 10^{-19} \,\text{C} \).
\( n = \dfrac{Q}{e} = \dfrac{3.2 \times 10^{-19}}{1.6 \times 10^{-19}} = 2 \).
So, the rod has \(\boxed{2 \text{ extra electrons}}\).
Example:
A neutral metal sphere is brought near a negatively charged rod without touching it. Describe what happens to the charges inside the sphere.
▶️ Answer/Explanation
When the negatively charged rod is brought near the neutral sphere, the free electrons in the sphere are repelled and move to the far side.
This leaves the near side of the sphere positively charged (due to lack of electrons) and the far side negatively charged.
This process is called electrostatic induction. The sphere remains overall neutral, but charges are separated inside it.