Define current, potential difference, and resistance, and apply Ohm's law in calculations

What this dot point is asking

SEAB wants you to define electric current, potential difference (voltage), and resistance, to use $I = Q/t$ and Ohm's law $V = IR$, and to know the difference between ohmic and non-ohmic conductors. The big idea is that a voltage pushes a current of charge around a circuit, and resistance opposes that flow.

The answer

Electric current

Electric current is the rate of flow of electric charge:

where $Q$ is the charge in coulombs and $t$ the time in seconds, giving current in amperes ($\text{A}$). In a metal wire the moving charges are electrons. Conventional current is taken to flow from the positive to the negative terminal of the supply.

Potential difference (voltage)

Potential difference, or voltage, is the energy transferred per unit charge as charge moves between two points:

measured in volts ($\text{V}$), where one volt is one joule per coulomb. A battery's voltage is the energy it gives each coulomb of charge.

Resistance and Ohm's law

Resistance opposes the flow of current. It is the ratio of voltage to current:

measured in ohms ($\Omega$). A larger resistance means a smaller current for the same voltage. This relationship is Ohm's law.

Ohmic and non-ohmic conductors

• An ohmic conductor (such as a metal wire at constant temperature) has a constant resistance, so current is directly proportional to voltage, and a current-voltage graph is a straight line through the origin.
• A non-ohmic conductor (such as a filament lamp) has a resistance that changes; its current-voltage graph is a curve. A filament lamp's resistance rises as it heats up, so the graph bends over.

Examples in context

Example 1. A dimmer switch. A dimmer changes the brightness of a lamp by changing the resistance in the circuit. Increasing the resistance reduces the current (Ohm's law), so the lamp receives less power and glows more dimly, a direct everyday use of $V = IR$.

Example 2. Thick versus thin cables. High-power appliances need thick connecting cables because a thicker wire has a lower resistance. Lower resistance means less energy is wasted as heat in the cable for a given current, which is why kettle leads are thicker than phone-charger leads.

Try this

Q1. Define electric current and state its unit. [2 marks]

• Cue. Current is the rate of flow of charge, $I = Q/t$; unit is the ampere ($\text{A}$).

Q2. A $9.0\ \text{V}$ supply drives a current of $0.50\ \text{A}$ through a resistor. Calculate its resistance. [2 marks]

• Cue. $R = \dfrac{V}{I} = \dfrac{9.0}{0.50} = 18\ \Omega$.

Q3. Explain why the current-voltage graph of a filament lamp is a curve rather than a straight line. [2 marks]

• Cue. As the current rises the filament heats up, increasing its resistance, so the current grows less than in proportion and the graph bends over.