Electricity & simple circuits (intro)
The idea
Flip a switch and energy arrives instantly — here is the machinery behind that trick. A circuit is a complete loop of conducting material; electric current is the flow of charge around that loop, measured in amperes (A). The battery provides the push, called voltage and measured in volts (V), and everything in the loop resists the flow a little, a property called resistance, measured in ohms (Ω). The three are tied together by one rule worth memorizing: current = voltage ÷ resistance. More push means more current; more resistance means less.
A good mental picture is a bike chain: the battery is your legs, the current is the chain looping around, and the bulb or motor is the wheel where the energy gets delivered. The misconception to retire is that current gets 'used up' by a bulb. The same current flows into and out of the bulb — what gets transferred is energy, carried by the moving charge and handed over as light and heat. Break the loop anywhere, and the whole flow stops at once.
Worked example
A flashlight bulb with a resistance of 18 Ω is connected to a 9 V battery in a simple loop. How much current flows in the circuit?
- Identify the roles: the 9 V battery supplies the push, and the bulb's 18 Ω is the only thing resisting the flow.
- Apply the rule: current = voltage ÷ resistance = 9 V ÷ 18 Ω = 0.5 A.
- Interpret the loop: that same 0.5 A flows through every point of the circuit — out of the battery, through the bulb, and back. The bulb takes energy from the charge, not the charge itself.
- Sanity-check with a swap: a 9 Ω bulb on the same battery would draw 9 ÷ 9 = 1 A. Halving the resistance doubles the current, exactly as the rule predicts.
Answer. A current of 0.5 A flows everywhere in the loop.
Check your understanding
- Why does the entire string of lights go dark when a single bulb in a simple loop burns out?
- How would you explain the difference between current and voltage using water in pipes or riders on a ski lift?
- What happens to the current if you keep the same battery but add a second bulb into the loop, and why?
- If current is not used up by a bulb, what exactly does the battery run out of over time?
Build the foundations first
Electricity & simple circuits (intro) builds on these concepts. If any feel shaky, start there.