# Electrical Current

## Key Stage 2

### Meaning

Electrical Current is the amount of electricity flowing through a wire.

The bigger the electrical current the brighter a bulb and the louder a buzzer.
If an electrical current goes through an animal it is called an electrical shock.

## Key Stage 3

### Meaning

An electrical current is a flow of charge.

Current is measured using an Ammeter.
The units of current are amperes, which are also called amps (A).
A current in a wire is a flow of electrons which are negatively charged particles.
Conventional Current flows from positive to negative. This is because electricity was discovered before scientists knew about electrons.
In a salt solution current is the flow of both positive and negative ions.

## Key Stage 4

### Meaning

Electrical current is the rate of flow of charge.

Current is measured using an Ammeter.
The SI Units of current are amperes, which are also called amps (A).
A current in a wire is a flow of electrons which are negatively charged particles.
Conventional Current flows from positive to negative. This is because electricity was discovered before scientists knew about electrons.
In a salt solution current is the flow of both positive and negative ions.
In a series circuit the current is the same everywhere in the circuit.
In a parallel circuit the current splits at a junction.

### Equation

#### Equation Linking Current, Charge and Time

NB: You should remember this equation with charge as the subject of the formula.

Current = (Charge)/(time)

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

Where

$$I$$ = The electrical current

$$Q$$ = The amount of charge flowing past a point.

$$t$$ = The time taken for the charge to flow.

This can give the definition "Current (I) is the (=) amount of charge flowing past a point (Q) per (÷) unit time (t)."

#### Equation Linking Current, Potential Difference and Resistance

NB: You should remember this equation.

Current = (Potential Difference)/(Resistance)

$$I=\frac{V}{R}$$

Where

$$I$$ = The electrical current

$$V$$ = The potential difference across a component.

$$R$$ = The resistance of an component.

### Example Calculations

#### Finding Current from Charge and Time

 A charge of 15 Coulombs passes through a point in a circuit ever 0.52 seconds. Calculate the current flowing past this point correct to two significant figures. A capacitor stores a charge of 10C. It discharges in 12ms. Calculate the current flowing out of the capacitor correct to two significant figures. 1. State the known quantities in correct units. Q = 15C t = 0.52s 1. State the known quantities in correct units. Q = 10C t = 12ms = 12x10-3s 2. Substitute the numbers into the equation and solve. $$I=\frac{Q}{t}$$ $$I=\frac{15}{0.52}$$ $$I=28.846153A$$ $$I\approx29A$$ 2. Substitute the numbers into the equation and solve. $$I=\frac{Q}{t}$$ $$I=\frac{10}{12 \times 10^{-3}}$$ $$I=833.3A$$ $$I\approx830A$$

#### Finding Charge from Current and Time

 A battery supplies 4.7Amps to a bulb over a period of 14 seconds. Calculate the charge leaving the battery in this time correct to two significant figures. A hairdryer uses a current of 7.2A for 5 minutes to dry a person’s hair. Calculate the charge flowing through the hairdryer in this time correct to two significant figures. 1. State the known quantities in correct units. I = 4.7A t = 14s 1. State the known quantities in correct units. I = 7.2A t = 5min = 300s 2. Substitute the numbers and evaluate. $$I=\frac{Q}{t}$$ $$4.7=\frac{Q}{14}$$ 2. Substitute the numbers and evaluate. $$I=\frac{Q}{t}$$ $$7.2=\frac{Q}{300}$$ 3. Rearrange the equation and solve. $$Q=4.7 \times 14$$ $$Q = 65.8C$$ $$Q \approx 66C$$ 3. Rearrange the equation and solve. $$Q=7.2 \times 300$$ $$Q = 2160C$$ $$Q \approx 2200C$$

#### Finding Time from Current and Charge

 A battery charger uses a current of 150mA to deliver a charge of 245 Coloumbs to a battery. Calculate the time taken to charge this battery correct to two significant figures. A cloud in a thunderstorm loses 15C in one lightening strike. At a current of 31,000kA. Calculate how long this lightning strike lasts correct to two significant figures. 1. State the known quantities in correct units. I = 150mA = 150x10-3A Q = 245C 1. State the known quantities in correct units. I = 31,000kA = 3.1x107A Q = 15C 2. Substitute the numbers and evaluate. $$I=\frac{Q}{t}$$ $$150 \times 10^{-3} = \frac{245}{t}$$ 2. Substitute the numbers and evaluate. $$I=\frac{Q}{t}$$ $$3.1 \times 10^7 = \frac{15}{t}$$ 3. Rearrange the equation and solve. $$t=\frac{245}{150 \times 10^{-3}}$$ $$t=1633.3s$$ $$t\approx1633.3s$$ 3. Rearrange the equation and solve. $$t=\frac{15}{3.1 \times 10^7}$$ $$t = 4.8387 \times 10^{-7}s$$ $$t\approx4.8 \times 10^{-7}s$$

#### Finding Current from Potential Difference and Resistance

 A potential difference of 9.9V is placed across an 19 Ohm resistor. Calculate the current flowing through the resistor correct to two significant figures. A toaster has a resistance of 27 Ohms is plugged into the mains which has a potential difference of 230V. Calculate the current flowing through the toaster correct to two significant figures. 1. State the known quantities in correct units. V = 9.9V R = 19Ω 1. State the known quantities in correct units. V = 230V R = 27Ω 2. Substitute the numbers into the equation and solve. $$I=\frac{V}{R}$$ $$I=\frac{9.9}{19}$$ $$I=0.52105A$$ $$I\approx0.52A$$ 2. Substitute the numbers into the equation and solve. $$I=\frac{V}{R}$$ $$I=\frac{230}{27}$$ $$I=8.519A$$ $$I\approx8.5A$$

#### Finding Resistance from Potential Difference and Current

 A student measures a potential difference of 5.4V and a current of 0.13mA across a component. Calculate the resistance of the component. Calculating the resistance of a buzzer connected in series to a 9V battery with an ammeter reading of 23mA. 1. State the known quantities in correct units. V = 5.4V I = 0.13mA = 0.13x10-3 1. State the known quantities in correct units. V = 9V I = 23mA = 23x10-3 2. Substitute the numbers and evaluate. $$I=\frac{V}{R}$$ $$0.13 \times 10^{-3}=\frac{5.4}{R}$$ 2. Substitute the numbers and evaluate. $$23 \times 10^{-3}=\frac{9}{R}$$ 3. Rearrange the equation and solve. $$R=\frac{5.4}{0.13 \times 10^{-3}}$$ $$R=41538.46\Omega$$ $$R\approx 42000\Omega$$ 3. Rearrange the equation and solve. $$R=\frac{9}{23 \times 10^{-3}}$$ $$R=391.3043\Omega$$ $$R\approx 390\Omega$$

#### Finding Potential Difference from Current and Resistance

 A current of 55mA flows through a component with a resistance of 93 Ohms. Calculate the potential difference across this component correct to two significant figures. A 22kΩ resistor has a current flowing through it of 6mA. Calculate the potential difference across the resistor correct to two significant figures. 1. State the known quantities in correct units. I = 55mA = 55x10-3A R = 93Ω 1. State the known quantities in correct units. I = 6mA = 6x10-3A R = 22kΩ = 22x103Ω 2. Substitute the numbers and evaluate. $$I=\frac{V}{R}$$ $$55 \times 10^{-3}=\frac{V}{93}$$ 2. Substitute the numbers and evaluate. $$I=\frac{V}{R}$$ $$6 \times 10^{-3}=\frac{V}{22 \times 10^3}$$ 3. Rearrange the equation and solve. $$V= 55 \times 10^{-3} \times 93$$ $$V= 5.115V$$ $$V\approx5.1V$$ 3. Rearrange the equation and solve. $$V= 6 \times 10^{-3} \times 22 \times 10^3$$ $$V= 132V$$ $$V\approx130V$$