Current, Potential Difference & Resistance
Electric Charge and Current
Electric current is the rate of flow of charge around a circuit. In metals, the current is carried by electrons, which are negatively charged. When a potential difference is applied across a circuit, electrons flow from the negative terminal of the battery through the wires and components to the positive terminal.
Current is measured in amperes (A) using an ammeter, which is always connected in series with the component being measured.
Charge, current, and time are linked by the equation: \(Q = I \times t\), where \(Q\) is charge in coulombs (C), \(I\) is current in amperes (A), and \(t\) is time in seconds (s).
Potential Difference
Potential difference (often called voltage) is the energy transferred per unit charge that passes between two points in a circuit. One volt means one joule of energy is transferred for every coulomb of charge that passes. It is measured using a voltmeter, which is always connected in parallel across the component being measured.
The equation linking energy transferred, charge, and potential difference is:
\(E = Q \times V\)
Where \(E\) is energy transferred in joules (J), \(Q\) is charge in coulombs (C), and \(V\) is potential difference in volts (V).
Resistance
Resistance is a measure of how difficult it is for charge to flow through a component. A high resistance means less current flows for a given potential difference. Resistance is measured in ohms (\(\Omega\)).
The relationship between potential difference, current, and resistance is given by:
\(V = I \times R\)
Where \(V\) is potential difference in volts (V), \(I\) is current in amperes (A), and \(R\) is resistance in ohms (\(\Omega\)). This can be rearranged to \(I = \frac{V}{R}\) or \(R = \frac{V}{I}\).
A resistor has a resistance of 20 \(\Omega\). The current through it is 0.5 A. Calculate the potential difference across it.
\(V = I \times R = 0.5 \times 20 = 10\) V
The three core circuit equations are: \(V = I \times R\) (Ohm’s law), \(Q = I \times t\) (charge), and \(E = Q \times V\) (energy transferred).
I–V Characteristics
The relationship between current and potential difference can be shown on an I–V graph. Different components produce different characteristic graphs:
Fixed resistor: The I–V graph is a straight line through the origin. Current is directly proportional to potential difference, meaning the resistance stays constant. This component obeys Ohm’s law.
Filament lamp: The graph is a curve that becomes less steep at higher potential differences. As the current increases, the filament heats up, which increases its resistance. The lamp does not obey Ohm’s law.
Diode: Current flows in one direction only (forward bias). In reverse bias, essentially no current flows. The I–V graph shows current increasing rapidly once the threshold voltage (about 0.7 V for silicon) is reached in the forward direction, and zero current in the reverse direction.
Thermistors and LDRs
Some components have a resistance that changes with conditions:
Thermistor (NTC type) — resistance decreases as temperature increases. Used in temperature sensors and thermostats. LDR (light-dependent resistor) — resistance decreases as light intensity increases. Used in automatic lighting systems and light meters.
Thermistor (NTC): resistance decreases as temperature increases. LDR: resistance decreases as light intensity increases. Both are used as sensors in control circuits.
Exam Tip: Always connect an ammeter in series and a voltmeter in parallel. If asked to draw a circuit to investigate resistance, include a battery, ammeter in series, voltmeter in parallel across the component, and a variable resistor to change the current.
Current, potential difference, and resistance are the three fundamental quantities in circuit physics. Master the equations \(V = I \times R\), \(Q = I \times t\), and \(E = Q \times V\), and understand how different components (fixed resistors, filament lamps, diodes, thermistors, and LDRs) behave.