Current

1. Current, symbol $I$, is the rate of flow of charge around a circuit.
2. Current is measured in amperes, unit $A$, often shortened to amps, using an ammeter connected in series.
3. Small currents are often given in milliamperes, where $1mA=0.001A$.
4. A current can only flow when there is a closed circuit and a source of potential difference, such as a cell, to push the charge around it.

Potential difference

1. Potential difference, symbol $V$, is the energy transferred per unit charge as the charge moves between two points in a circuit.
2. Potential difference is measured in volts, unit $V$, using a voltmeter connected in parallel across the component.
3. The definition can be written as an equation, $V=\frac{E}{Q}$, where $E$ is the energy transferred in joules, $J$, and $Q$ is the charge in coulombs, $C$.
   1. This means that $1V=1J/C$, so a potential difference of $1V$ transfers $1J$ of energy for every coulomb of charge.
4. The potential difference across a cell tells you how much energy the cell gives to each coulomb of charge that passes through it.
   1. A $9V$ battery gives each coulomb of charge $9J$ of energy.
5. The potential difference across a component tells you how much energy each coulomb transfers in that component, for example to the thermal store of a resistor or away from a lamp by light.
6. The potential difference of the supply is what drives charge around a circuit, so a larger supply potential difference pushes more charge through a component every second.

Resistance

1. Resistance, symbol $R$, is a measure of how strongly a component opposes the flow of charge through it.
2. Resistance is measured in ohms, unit $\Omega$, and large resistances are often given in kilohms, where $1k\Omega =1000\Omega$.
3. In a metal, the current is a flow of free electrons, which keep colliding with the ions that make up the metal.
   1. Each collision slows the electrons down and transfers energy to the ions, so the more often the electrons collide, the greater the resistance.
4. Connecting wires are made of copper and have a very small resistance, while resistors are made to have a particular, much larger resistance.

How the three quantities are linked

1. The current through a component depends on both the potential difference across it and its resistance.
2. For a component with a fixed resistance, a larger potential difference produces a larger current, because each coulomb is pushed through with more energy.
3. For a fixed potential difference, a larger resistance produces a smaller current, because the component opposes the flow of charge more strongly.
4. The resistance of a component is found by dividing the potential difference across it by the current through it, $R=\frac{V}{I}$.
   1. To find a resistance, the potential difference and the current must be measured at the same time on the same component.
5. Current: symbol $I$, measured in amperes, $A$, with an ammeter in series.
6. Potential difference: symbol $V$, measured in volts, $V$, with a voltmeter in parallel.
7. Resistance: symbol $R$, measured in ohms, $\Omega$, and calculated from readings of $V$ and $I$.