Current Electricity
Easy Overview
Electricity is just charges moving through wires. But once you have more than one battery and more than one bulb, things get interesting. This chapter is about how current flows through circuits — how it splits at junctions, how it adds up, and how we measure things like resistance and potential difference.
Kirchhoff's Laws
Kirchhoff had two simple rules. First: at any junction, the total current coming in equals the total current going out. Current can't pile up at a junction — it's like water in pipes, what flows in must flow out. Second: if you go around any closed loop in a circuit, the total voltage gains (from batteries) equal the total voltage drops (across resistors). Think of a roller coaster — the lift hill gives you potential energy (voltage gain), and the drops and turns use it up (voltage drops). By the time you're back at the start, it all balances out.
Wheatstone Bridge and Meter Bridge
The Wheatstone bridge is a clever circuit to measure an unknown resistance. You set up four resistors in a diamond shape and adjust one until no current flows through the middle. When it's balanced, the ratio of the two resistors on one side equals the ratio on the other. Simple cross-multiplication gives you the unknown value. The meter bridge is just a practical version of the same idea using a long wire. It's like a seesaw — when it's perfectly balanced, the products of opposite sides are equal.
Electrical Power and Energy
Power in a circuit is just voltage times current — P = VI. A 100W bulb running on 220V draws about 0.45A of current. The energy consumed is power multiplied by time. That's what your electricity bill measures — kilowatt-hours (kWh). One kWh is using 1000 watts for one hour. The heat produced in a resistor is I²Rt — that's Joule's law. That's how electric heaters and toasters work — current passing through a high-resistance wire generates heat.
Temperature Dependence of Resistance
Resistance usually increases with temperature for metals. As metals heat up, their atoms vibrate more, making it harder for electrons to flow through. For a typical copper wire, resistance goes up by about 0.4% per degree Celsius. That's why your phone charger cable gets warm when fast charging — the current causes heating, which increases resistance, which causes more heating. For semiconductors like silicon, resistance actually decreases with temperature — more heat frees up more charge carriers.
Key Points
- •Kirchhoff's current law (KCL): ΣI_in = ΣI_out at a junction.
- •Kirchhoff's voltage law (KVL): ΣVoltage gains = ΣVoltage drops in a closed loop.
- •Wheatstone bridge balanced when P/Q = R/S. Used to measure unknown resistance.
- •Meter bridge works on the same principle as Wheatstone bridge using a wire of length 1 m.
- •Electrical power P = VI = I²R = V²/R.
- •Resistance of metals increases with temperature. For semiconductors, it decreases.
Practice Questions
- State and explain Kirchhoff's laws. Use them to find the current in each branch of a given circuit.
- Describe the principle and working of a Wheatstone bridge. How is a meter bridge used to find unknown resistance?
- An electric bulb rated 60 W, 220 V is connected to a 220 V supply. Find the current drawn and resistance of the bulb.
- Explain how the resistance of a conductor changes with temperature. Give the relation.