The current is the flow of electrical charge around a circuit per second; the greater the flow of electrical charge per second, the greater the current. We measure current in ampere (A); amperes is sometimes referred to as amps.

There are two things that are needed in order for electrical charge to flow around a circuit.

Resistance opposes the flow of electrical charge (current) in a circuit.

The circuit below contains a cell and a filament bulb.

There are two things that are needed in order for electrical charge to flow around a circuit.

- The first is that the circuit must be closed. No current can flow if the circuit is broken, which can be when a switch is turned off or a component is connected incorrectly.
- The second is that there must be a source of potential difference. The source of potential difference is usually a battery or cell. Potential difference is the driving force that pushes the electrical charge around a circuit. Potential difference is measured in volts (V) and is sometimes referred to as voltage.

Resistance opposes the flow of electrical charge (current) in a circuit.

The circuit below contains a cell and a filament bulb.

An electrical current will flow around the circuit because the circuit is a closed loop with no gaps and there is a source of potential difference (for the circuit above, the source of potential difference is the cell).

In the single closed loop above, the current will be the same everywhere around the circuit. We measure the current by using an ammeter. I have added two ammeters to the circuit and the readings on both of the ammeters will be the same.

In the single closed loop above, the current will be the same everywhere around the circuit. We measure the current by using an ammeter. I have added two ammeters to the circuit and the readings on both of the ammeters will be the same.

**Charge Flow**

Charge flow is the total amount of charge that has flowed through a point on the circuit during a given period of time. We calculate charge flow by multiplying the current by time.

In the above formula, Q is charge flow measured in coulombs (C), I is current measured in amps (A) and t is time measured in seconds (s). Let’s now have a few examples of using this formula.

**Example 1**

A charger passes a current of 4 A through the battery of a drone. The charger is on for 50 seconds. Calculate the charge flow.

We work out charge flow by multiplying the current by the time.

The question tells us that the current is 4 amps (A) and the time is 50 seconds. Both of these values are in the correct units, so we just sub them straight into the formula.

The charge flow is 200 coulombs (C).

**Example 2**

A current of 1.5 A passes through a filament bulb for 35 minutes. Work out the charge flow.

Like the question before, we work out the charge flow by multiplying the current by the time.

For the above calculation, current needs to be measured in amps (A) and time needs to be measured in seconds. We are told in the question that the current is 1.5 A, which is in the correct units. The question also tells us that the time 35 minutes, which is not in the correct units because time needs to be in seconds rather than minutes. There are 60 seconds in 1 minute, so we convert minutes to seconds by multiplying by 60.

The charge flow is 3,150 coulombs (C).

**Example 3**

A phone charger passes a current of 0.9 A through the battery of a phone. The charge flow is 4,860 C. How long was the phone charger on for? Give your answer in hours.

The question is asking us to work out time. When we cover time up in the formula triangle, we can see that we can work out time by dividing the charge flow (Q) by the current (I).

We are told in the question that the charge flow is 4,860 C and the current is 0.9 A. Both of these are in the correct units, so we can sub them into the formula.

The time that the charger is on for is 5,400 seconds. The question asks us to find the time in hours. There are 60 seconds in 1 minute and 60 minutes in 1 hour. Therefore, we convert the time from seconds to hours by dividing by 60 twice (÷ 60 ÷ 60).

The charger was on for 1.5 hours.