Physics - Electrical charge carriers, Potential difference (voltage), Ohm’s law

National 5

Electrical Current (Definition)

The rate of flow of electric charge. (How much charge passes a point per second)

How is electrical current measured?

Using an ammeter, connected in series in the circuit.

Unit of Electrical Current

Ampere (A)

Formula linking Charge, Current, and Time

Q = It

Variables in Q = It

Q = Charge (in Coulombs, C), I = Current (in Amperes, A), t = time (in seconds, s)

Force on a charged particle in an electric field

A charged particle (like an electron or proton) experiences a force when it is in an electric field.

Path of a charged particle fired between two parallel charged plates

It follows a parabolic path, curving towards the oppositely charged plate.

Path of a charged particle released from rest in a uniform electric field

It accelerates in a straight line towards the oppositely charged plate.

Potential Difference (Voltage) (Definition)

The energy given to each coulomb of charge

Unit of Potential Difference (Voltage)

Volt (V), which is one Joule per Coulomb ($J/C$)

Gradient of a Voltage vs. Current (V-I) graph

The gradient ($m = \Delta V / \Delta I$) is equal to the Resistance ($R$).

Formula linking Potential Difference, Current, and Resistance (Ohm's Law)

V = IR

Variables in V = IR

$V$ = Potential Difference (in Volts, V), I = Current (in Amperes, A), R = Resistance (in Ohms, Omega

Experiment to verify Ohm's Law (Method)

Set up a circuit with a power supply, ammeter (in series), voltmeter (in parallel across the component), and a variable resistor. Record V and I for different settings of the variable resistor.

Experiment to verify Ohm's Law (Analysis)

Plot a graph of V (y-axis) against I (x-axis). If the graph is a straight line through the origin and the component is "ohmic". The resistance R is the gradient of this line.