Electrostatics & Electric Circuits

≈ 30 min

Two kinds of charge

Matter contains two kinds of electric charge: positive (carried by protons) and negative (carried by electrons). The rule for how they interact is short:

Like charges repel (push apart).
Unlike charges attract (pull together).

When you rub a plastic ruler on your sleeve it can pick up tiny bits of paper. Rubbing transfers electrons from one surface to the other. No charge is created or destroyed - it is only moved. This is conservation of charge: the total charge before and after stays the same.

Making charge flow: current, emf and potential difference

In a circuit a cell (battery) pushes charge around a conducting loop.

Current ( $I$ ) is the rate of flow of charge, measured in amperes ( $\text{A}$ ):

I = \frac{Q}{\Delta t}

where $Q$ is charge in coulombs ( $\text{C}$ ).

Emf (electromotive force) is the total energy the cell gives each coulomb of charge to move it around the whole circuit, measured in volts ( $\text{V}$ ).
Potential difference ( $V$ ) across a component is the energy transferred per coulomb as charge passes through that component:

V = \frac{W}{Q}

Resistance is the opposition a component offers to the flow of charge. A larger resistance means it is harder for charge to flow for a given push.

Two light spheres hang side by side. Both carry a negative charge. What will they do?

Attract each other

Repel each other

Neither attract nor repel

First attract, then repel

A charge of $6\,\text{C}$ flows through a torch bulb in $3\,\text{s}$ . Calculate the current in the bulb, in amperes.

As charge flows through a resistor, $24\,\text{J}$ of energy is transferred for every $4\,\text{C}$ that passes through it. Calculate the potential difference across the resistor, in volts.