Electricity is the invisible force that powers our modern lives, but it remains a mystery to many. To truly grasp how your smartphone charges or how a light bulb glows, you need to understand the fundamental concepts of electric charges and electric current.
What is an Electric Charge?
In physics, electric charge is an intrinsic property of matter, much like mass. However, unlike mass, the charge can be positive, negative, or neutral. We conventionally represent a positive charge with \(+\) and a negative charge with \(-\). This duality of charges is responsible for all electrical phenomena.
Coulomb’s Law: The Force of Attraction and Repulsion
The behavior of electric charges is dictated by Coulomb’s Law, a fundamental principle in physics. This law states that:
- Like charges repel each other (positive repels positive, negative repels negative).
- Opposite charges attract each other (positive attracts negative).
The force between two charges, \(( F )\), is directly proportional to the product of their magnitudes \(( q_1 )\) and \(( q_2 )\) and inversely proportional to the square of the distance \(( r )\) between them.
This can be expressed mathematically as:
$$ F = k \frac{q_1 q_2}{r^2} $$
where \( k \) is Coulomb’s constant.
Static Electricity
Static electricity occurs when electric charges accumulate on an object’s surface and remain immobile. This phenomenon is responsible for the small shock you sometimes feel when touching a metal doorknob after walking on the carpet. The friction between your shoes and the carpet transfers electrons (negatively charged particles), leaving you with a net charge that discharges upon contact with the doorknob.
What is an Electric Current?
Electric current is the flow of electric charges. Imagine a river – the water molecules are the charges, and the river’s flow represents the electric current.
We measure electric current in amperes (A), often shortened to “amps”. One ampere is equal to one coulomb (C) of charge passing a point in a circuit per second, which can be represented as:
$$ I = \frac{Q}{t} $$
where:
- \(I\) is the current in amperes \(A\)
- \( Q \) is the charge in coulombs \(C\)
- \( t \) is the time in seconds \(s\)
Direct Current (DC) vs. Alternating Current (AC)
There are two main types of electric current:
- Direct Current (DC): In DC, the flow of charge is always in one direction, from the positive to the negative terminal of the power source. Batteries and solar cells are common sources of DC.
- Alternating Current (AC): In AC, the flow of charge periodically reverses direction. This is the type of current that powers our homes and most electrical appliances.
Ohm’s Law
Ohm’s Law, a fundamental principle in electrical engineering, defines the relationship between voltage ( V ), current ( I ), and resistance ( R ). It states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them. In mathematical terms:
$$ I = \frac{V}{R} $$
where:
- \( I \) is the current in amperes \(A\)
- \( V \) is the voltage in volts \(V\)
- \( R\) is the resistance in ohms \(Ω\)
Applications of Electric Charge and Current
The principles of electric charge and current are the bedrock of countless technologies we depend on daily.
| Application | How it Works |
|---|---|
| Power generation | Generators convert mechanical energy (from sources like steam or water) into electrical energy using magnets and coils. |
| Electronics | Transistors, diodes, and other components control the flow of current in electronic devices like computers and smartphones. |
| Electric motors | Motors use electromagnetic forces to convert electrical energy into mechanical energy, powering everything from fans to electric vehicles. |
| Lighting | Light bulbs, LEDs, and fluorescent lights convert electrical energy into light, illuminating our homes and cities. |
| Heating and cooling | Electric heaters and air conditioners use electrical energy to change temperatures, providing comfort in our homes and workplaces. |
| Communication | Signals are transmitted as electrical currents in wires or electromagnetic waves, enabling communication through telephones, radios, and the internet. |
Example Questions
Q1: What is the current flowing through a circuit with a voltage of 12V and a resistance of 4Ω?
Answer: Using Ohm’s Law (\( I = \frac{V}{R} )\), we can calculate the current as:
$$ I = \frac{12V}{4Ω} = 3A $$
Q2: How much charge passes through a point in a circuit in 10 seconds if the current is 2A?
Answer: Using the equation for current \(( I = \frac{Q}{t} )\), we can rearrange to solve for charge \( Q \):
$$Q = I \cdot t = 2A \cdot 10s = 20C $$
Q3: What is the force between two charges of \(3 \times 10^{-6} \, C\) and \(4 \times 10^{-6} \, C\) separated by a distance of 0.5m? (Use \( k = 8.99 \times 10^9 \, Nm^2/C^2 )\)
Answer: Using Coulomb’s Law \(( F = k \frac{q_1 q_2}{r^2} )\), we can calculate the force as:
$$ F = 8.99 \times 10^9 \frac{(3 \times 10^{-6})(4 \times 10^{-6})}{(0.5)^2} = 0.4316 \, N $$
Q4: If a light bulb has a resistance of 240Ω and is connected to a 120V power source, what is the current flowing through the bulb?
Answer: Using Ohm’s Law \(( I = \frac{V}{R} )\), we can calculate the current as:
$$ I = \frac{120V}{240Ω} = 0.5A $$
Q5: How long will it take for a charge of 30C to pass through a point in a circuit if the current is 5A?
Answer: Using the equation for current \(( I = \frac{Q}{t} )\), we can rearrange to solve for time \( t \):
$$ t = \frac{Q}{I} = \frac{30C}{5A} = 6s $$
Q6: A circuit has a current of 3A and a resistance of 10Ω. What is the voltage across the circuit?
Answer: Using Ohm’s Law \(( V = I \cdot R )\), we can calculate the voltage as:
$$ V = 3A \cdot 10Ω = 30V $$
Q7: What is the power consumed by an appliance that draws 2A of current from a 220V source?
Answer: Power \( P \) is given by the product of current \( I \) and voltage \( V \):
$$ P = I \cdot V = 2A \cdot 220V = 440W $$
Q8: If the distance between two charges is tripled, by what factor does the force between them change?
Answer: According to Coulomb’s Law \(( F \propto \frac{1}{r^2} )\), if the distance \( r \) is tripled, the force \( F \) will be reduced by a factor of \( 3^2 \):
$$ \text{New Force} = \frac{1}{3^2} = \frac{1}{9} $$