Capacitors and Capacitance: Capacitor physics and circuit operation

Main Ideas

Why a Single Metal Plate Cannot Readily Accumulate Extra Electrons

In a metal, positive particles stay in place while electrons can move. Adding electrons to an isolated plate becomes increasingly difficult because the electrons repel one another.

How a Capacitor Stores Charge

Place a second metal plate nearby. Electrons pushed onto the first plate repel electrons away from the second, leaving it positively charged. The plates acquire equal and opposite charges.

An external force, such as a battery, can push more charge onto the plates. Removing that force leaves the capacitor charged.

Charging and Discharging in a Circuit

• Connecting a capacitor to a battery transfers electrons between its plates until the capacitor’s voltage matches the battery’s voltage.
• Connecting the charged capacitor across a light bulb gives electrons a path from the negatively charged plate to the positively charged plate. Their movement through the bulb makes it light until the capacitor discharges.
• Batteries connected in series can charge the capacitor to a higher voltage. A higher charging voltage means more charge on the plates and makes the bulb stay lit longer during discharge.

What Capacitance Means

Capacitance describes how much charge a capacitor can hold for a given applied voltage. Increasing capacitance lets the plates hold more charge at the same voltage. A higher-capacitance capacitor takes longer to charge from a given battery and longer to discharge through the bulb.

Three Ways to Increase Capacitance

1. Increase the plate area. Larger plates can hold more charge without requiring a greater applied force.
2. Insert an insulating material between the plates. The material’s molecules reorient in response to the plate charges, helping the plates accumulate more charge at the same voltage.
3. Move the plates closer together. The stronger interaction between the plates helps more charge accumulate at the same voltage.

Design Principle

To make capacitance as large as possible, use large plate areas, place an insulating material between the plates, and keep the plates close together. A larger capacitance stores more energy at the same voltage.

Diagram Convention

The video shows voltage “upside down” so its blue arrows can indicate electron flow.

Speakers or Sources Featured

• Narrator: An explanatory voice describes capacitor physics and circuit operation. No other speakers are identified.