Electrostatics is the branch of physics that deals with the study of electric charges at rest. It focuses on the forces, fields, and potentials associated with static electric charges.
Electric Charge
Electric charge is a fundamental property of matter that governs the interaction between particles through electromagnetic forces. It is responsible for electric and magnetic phenomena.
- Charge is measured in Coulombs (C).
- The smallest unit of charge is the charge of an electron or proton, denoted as:e=1.6×10−19Ce = 1.6 \times 10^{-19} Ce=1.6×10−19C
- Proton charge = +e
- Electron charge = -e
- Neutron has no charge (neutral).
Types of Electric Charge
There are two types of electric charges:
- Positive Charge (+q):
- Possessed by protons.
- Example: Glass rod rubbed with silk loses electrons and becomes positively charged.
- Negative Charge (-q):
- Possessed by electrons.
- Example: Plastic or rubber rod rubbed with fur gains electrons and becomes negatively charged.
Properties of Electric Charge
- Charge is Quantized
- Charge always exists in integral multiples of the elementary charge (eee).
- Formula: Q=n⋅e,n∈ZQ = n \cdot e, \quad n \in \mathbb{Z}Q=n⋅e,n∈Z
- Example: A charge of 3.2×10−19C3.2 \times 10^{-19} C3.2×10−19C means n=2n = 2n=2, meaning two excess electrons or protons.
- Charge is Conserved
- The total charge in an isolated system remains constant.
- Example: When a glass rod is rubbed with silk, electrons transfer from the rod to the silk, ensuring total charge remains the same.
- Like Charges Repel, Unlike Charges Attract
- Positive charges repel positive charges.
- Negative charges repel negative charges.
- Positive and negative charges attract each other.
- This explains why protons and electrons are attracted to each other.
- Charge Can Be Transferred
- Conductors (like metals) allow charge to move freely.
- Insulators (like rubber, plastic, glass) do not allow charge to move easily.
- Charge is Invariant
- The charge of a particle remains the same regardless of its speed or location in space.
Methods of Charging a Body
A neutral object can be charged using different methods:
1. Charging by Friction
- When two different materials are rubbed together, electrons transfer from one object to another.
- Example:
- Glass rod rubbed with silk → Glass becomes positively charged, silk becomes negatively charged.
- Plastic rod rubbed with fur → Plastic becomes negatively charged, fur becomes positively charged.
2. Charging by Conduction (Contact)
- When a charged object touches a neutral conductor, charge is transferred.
- Example: If a negatively charged rod touches a metal sphere, the sphere will also become negatively charged.
3. Charging by Induction
- A charged object is brought near (but does not touch) a neutral object, causing charge separation.
- Example: A negatively charged rod brought near a metal sphere pushes electrons away, making one side positive and the other negative.
Coulomb’s Law (Force Between Charges)
The force between two point charges is given by:F=k∣q1q2∣r2F = k \frac{|q_1 q_2|}{r^2}F=kr2∣q1q2∣
Where:
- FFF = Electrostatic force
- kkk = Coulomb’s constant (9×109 Nm2/C2)(9 \times 10^9 \, Nm^2/C^2)(9×109Nm2/C2)
- q1,q2q_1, q_2q1,q2 = Magnitude of charges
- rrr = Distance between charges
- If FFF is positive, it is a repulsive force (same charges).
- If FFF is negative, it is an attractive force (opposite charges).
Example Calculation:
Find the force between two charges of 2C2C2C and 3C3C3C placed 1 meter apart.F=(9×109)×(2×3)12F = (9 \times 10^9) \times \frac{(2 \times 3)}{1^2}F=(9×109)×12(2×3) F=54×109NF = 54 \times 10^9 NF=54×109N
Electric Field Due to a Charge
An electric field is the region around a charge where another charge experiences force.E=Fq=kQr2E = \frac{F}{q} = k \frac{Q}{r^2}E=qF=kr2Q
Where:
- EEE = Electric field (N/C)
- QQQ = Source charge
- rrr = Distance from the charge
- Direction of Field:
- For positive charges: Field points outward.
- For negative charges: Field points inward.
Electric Potential (V)
Electric potential is the work done in bringing a unit positive charge from infinity to a point.V=kQrV = k \frac{Q}{r}V=krQ
- Unit: Volt (V)
- Potential Difference (ΔV\Delta VΔV) is the work done to move a charge between two points.
What is an Electric Field?
The electric field is a region around a charged object where another charge experiences a force. It is created by static or moving electric charges.
Definition of Electric Field
The electric field at a point is defined as the force per unit charge exerted on a small positive test charge at that point.E=FqE = \frac{F}{q}E=qF
Where:
- EEE = Electric field strength (N/C or V/m)
- FFF = Force on the charge (N)
- qqq = Test charge (C)
SI Unit: Newton per Coulomb (N/C) or Volts per meter (V/m).
What is a Magnetic Field?
A magnetic field is a region around a moving charge or magnet where another moving charge or magnetic material experiences a force.
Definition of Magnetic Field
The magnetic field at a point is defined as the force per unit current per unit length experienced by a conductor placed perpendicular to the field.B=FILB = \frac{F}{I L}B=ILF
Where:
- BBB = Magnetic field strength (Tesla, T).
- FFF = Force (N).
- III = Current (A).
- LLL = Length of conductor (m).
SI Unit: Tesla (T) or N/A⋅mN/A \cdot mN/A⋅m.
Relationship Between Electric and Magnetic Fields
- A changing electric field produces a magnetic field (Maxwell’s Law).
- A changing magnetic field produces an electric field (Faraday’s Law of Induction).
- Electromagnetic waves (like light, radio waves) consist of perpendicular electric and magnetic fields oscillating together.
Difference Between Charge and Current
| Property | Electric Charge (QQQ) | Electric Current (III) |
|---|---|---|
| Definition | The property of matter responsible for electrical interactions. | The rate of flow of electric charge. |
| Formula | Q=n⋅eQ = n \cdot eQ=n⋅e | I=QtI = \frac{Q}{t}I=tQ |
| SI Unit | Coulomb (C) | Ampere (A) |
| Symbol | QQQ | III |
| Nature | Static or moving | Always in motion |
| Example | Charge on an electron is −1.6×10−19C-1.6 \times 10^{-19} C−1.6×10−19C | Current in a circuit is 2A when 2C flows per second |
Relationship Between Charge and Current
Electric current is the rate of flow of electric charge:I=QtI = \frac{Q}{t}I=tQ
Where:
- III = Current (A)
- QQQ = Charge (C)
- ttt = Time (s)
For example, if 10 Coulombs of charge flow through a wire in 2 seconds, the current is:I=10C2s=5AI = \frac{10C}{2s} = 5AI=2s10C=5A
Key Takeaways
- Charge (QQQ) is the fundamental property of particles like electrons and protons.
- Current (III) is the movement of charge per second.
- Charge can be present without current, but current cannot exist without charge flow
Real-Life Example: Charge vs. Current
Example 1: Static Electricity (Charge Without Current)
Imagine rubbing a balloon on your hair.
- Electrons from your hair transfer to the balloon, making it negatively charged.
- Your hair loses electrons and becomes positively charged.
- If you bring the charged balloon near a wall, it sticks because opposite charges attract.
- Here, charge is present, but there is no flow of charge (no current).
Example 2: Electric Circuit (Charge Flowing as Current)
Now, consider a torch (flashlight):
- When you turn on the switch, the battery pushes electrons through the circuit.
- The electric charge moves through the wire, creating electric current.
- The moving charge powers the bulb, converting electrical energy into light.
- Here, charge is moving, so there is current.
Summary
| Situation | Charge Present? | Current Present? |
|---|---|---|
| Rubbing a balloon on hair | ✅ Yes | ❌ No |
| Lightning in a thunderstorm | ✅ Yes | ✅ Yes (briefly) |
| A battery in an unused circuit | ✅ Yes | ❌ No |
| A battery in a working flashlight | ✅ Yes | ✅ Yes |
Leave a Reply