Electric Current Revision Notes for HSC SSCE Physics

Electric Current

What is electric current?

Electric current is the flow of electrically charged particles through a material. Every time you use an electrical device like a light, computer, or phone, electric current is flowing through circuits, transferring and transforming energy.

In a circuit, current usually involves the flow of electrons. However, in other systems like the human body, current can be carried by larger charged particles such as calcium ions ( $\text{Ca}^{2+}$ ) and potassium ions ( $\text{K}^{+}$ ).

Defining current mathematically

Current, represented by the symbol $I$ , is defined as the quantity of charge passing through a point per unit time:

$I = \frac{q}{t}$

where:

$I$ is the current
$q$ is the quantity of charge (measured in coulombs, C)
$t$ is the time (measured in seconds, s)

The unit of current is amperes (A), named after French mathematician and physicist André-Marie Ampère. One ampere equals one coulomb per second:

$1 \text{ A} = 1 \text{ C s}^{-1}$

infoNote

While current is defined as a scalar quantity, it has a sign because charges can move in different directions and can be either positive or negative. This is why understanding the direction convention is essential.

Understanding the direction of current

It's often difficult to determine whether moving charges are positive or negative. The same observable effect can result from different scenarios. Consider the following three possibilities:

In all three cases shown above, the right-hand side becomes more positive by the same amount. This can happen through:

Positive charges moving to the right
Negative charges moving to the left
A combination of both

chatImportant

Direction Convention

To avoid confusion, physicists have established a convention: we define the direction of current as the direction that positive charges would move if positive charges were creating the current.

This means:

In your body, the current direction matches the direction that calcium and sodium ions move
In an electric circuit, the current direction is opposite to the direction that electrons move

Remember: regardless of the sign of the actual charge carriers, the result is the same, so we always describe current in terms of positive charge flow.

Worked example: Calculating current from ionic movement

lightbulbExample

Worked Example: Current from Ionic Movement

Problem: A cell membrane has 500 $\text{Ca}^{2+}$ ions move across it in 1.0 s. What is the current flow through this membrane?

Solution:

Given information:

Time: $t = 1.0 \text{ s}$
Number of ions: 500 $\text{Ca}^{2+}$

Each $\text{Ca}^{2+}$ ion carries a charge of $+2e$ , where $e$ is the electron charge ( $1.6 \times 10^{-19} \text{ C}$ ).

Step 1: Calculate total charge $q = 500 \times 2e = 500 \times 2 \times 1.6 \times 10^{-19} \text{ C}$ $q = 1.6 \times 10^{-16} \text{ C}$

Step 2: Apply the current formula $I = \frac{q}{t} = \frac{1.6 \times 10^{-16} \text{ C}}{1.0 \text{ s}}$

Step 3: Calculate the final answer $I = 1.6 \times 10^{-16} \text{ C s}^{-1} = 1.6 \times 10^{-16} \text{ A}$

Conditions required for current flow

For electric current to flow, three conditions must be satisfied:

A path for the current to flow along
Charge carriers (charged particles) that are free to move
An applied force to make the charge carriers start moving

Without all three conditions, no current can flow.

Conductors, insulators, and semiconductors

Materials can be classified based on their ability to allow current flow:

Conductors are materials that allow current to flow through them easily. Metals are excellent conductors because they contain many free electrons that can move throughout the material.

Insulators are materials that do not allow current to flow through them. Insulators lack free electrons, so charge carriers cannot move even when a force is applied.

Semiconductors are materials with a small number of free electrons. These materials allow current to flow, but not as easily as conductors.

infoNote

Whether a material acts as a conductor, insulator, or semiconductor depends on its atomic structure and how its atoms bond together. The key factor is the availability of free charge carriers.

Atomic structure and free electrons

To understand why metals conduct electricity, we need to examine their atomic structure.

A single copper atom has many electrons arranged in shells around its nucleus. When the atom is isolated, all electrons are bound to the nucleus by electrostatic forces. The electrons in the outermost shell are called valence electrons.

However, when many copper atoms come together to form metallic copper, something remarkable happens. A small number of outer-shell electrons (one in copper, but one, two, or three in other metals) become unbound from their original nuclei. These electrons become free electrons or conduction electrons that can move throughout the metal.

Metallic bonding and the sea of electrons

The free electrons in a metal play a crucial role in holding the material together. They act as a negative "sea" or "glue" surrounding the positive metal ions (which consist of the nuclei and their remaining bound electrons).

This arrangement is called metallic bonding – a type of bonding between many atoms enabled by these free electrons. The sea of free electrons holds the positive ions together while simultaneously providing mobile charge carriers that can flow when a force is applied.

This explains why metals satisfy two of the three conditions for current flow:

They have free charge carriers (the free electrons)
They provide a path for these carriers to move through

Measuring current with an ammeter

Current in a circuit is measured using an ammeter. A multimeter can function as an ammeter when set to the appropriate current setting.

Ammeters have two different settings:

DC (direct current): Used when current flows in one direction continuously, such as in battery-powered circuits
AC (alternating current): Used when current oscillates in direction, such as in mains power supply

Because an ammeter measures the current flowing through a specific point, it must be inserted directly into the circuit at that location. We say the ammeter is connected in series with the circuit.

chatImportant

Critical Safety Rule

Never connect an ammeter in parallel (across) components, as this can damage the ammeter or the circuit. Ammeters must always be connected in series.

Investigation 13.1: Current flow in metals

Aim: To investigate the flow of current through metal wire

Note: A resistor is used in this experiment to ensure the current in the circuit does not become too high.

Materials:

1.5 V battery in holder
10 Ω resistor
Current meter (ammeter or multimeter on current setting)
Fine copper wire in various lengths (minimum 20 m total; for example: 20 m, 40 m, 60 m, 80 m, 100 m)
Micrometer
Crocodile clips for making connections

Risk assessment:

What are the risks?	How can you manage these risks?
Electricity can cause shocks	Do not touch both terminals of the battery at once
A length of wire across the battery can cause a short circuit, damaging the battery	Always keep the resistor in the circuit

Method:

Read the instructions for your multimeter or current meter, or ask your teacher for guidance
Connect the resistor to the positive terminal of the battery
Connect one probe of your current meter to the resistor
Connect the other probe of the current meter to one end of the wire
Connect the other end of the wire to the negative terminal of the battery

Record the current flowing through the wire as shown on the current meter (remember to include units)
Remove the wire from the circuit
Repeat steps 4-7 for each different length of wire
Use the micrometer to measure the diameter of the metal conductor within the wire

infoNote

Tip: If the current doesn't change as you change wire lengths, try using a smaller resistor (e.g., 5 Ω or 1 Ω).

Results:

Record your data in a spreadsheet with columns headed "length" and "current". Include units in the heading cells or in the cell below.

Analysis of results:

Use the spreadsheet to create a scatter plot with current on the vertical axis and length on the horizontal axis. Observe the shape of your graph
Add another column headed $\frac{1}{\text{length}}$ and calculate this value for each wire length
Plot a scatter graph of current as a function of $\frac{1}{\text{length}}$ . If this graph is linear, add a line of best fit and display the equation
Write an equation describing the relationship between current and wire length

Discussion:

Were you able to answer your research question?
What are the limitations of this experiment?
Can you suggest better ways of making the measurements or further investigations?

Conclusion:

Write a conclusion based on the aim of this investigation, making reference to your data and its analysis. Answer your research question.

bookmarkSummary

Key Points to Remember:

Electric current is the flow of charged particles (charge carriers) through a material
Current is calculated using the formula $I = \frac{q}{t}$ , where $q$ is charge in coulombs and $t$ is time in seconds
Current is measured in amperes (A), where $1 \text{ A} = 1 \text{ C s}^{-1}$
By convention, current direction is defined as the direction positive charges would flow, even if the actual charge carriers are negative electrons
Three conditions must be met for current to flow: a path, free charge carriers, and an applied force
Conductors (like metals) allow current flow easily because they have many free electrons; insulators prevent current flow because they lack free electrons; semiconductors have few free electrons and conduct poorly
In metals, outer valence electrons become free and form a "sea of electrons" that holds the material together through metallic bonding while also serving as charge carriers
Ammeters measure current and must always be connected in series in a circuit

Electric Current (HSC SSCE Physics): Revision Notes