Charge & Current (OCR Gateway GCSE Physics: Combined Science)

• Electric current is defined as the rate of flow charge
  • In other words, the size of an electric current is the amount of charge passing through a component per second

• The wires in an electric circuit are made of metal, because metal is a good conductor of electric current
• In the wires, the current is a flow of electrons

In metal wires, the current is a flow of electrons. This image shows the electrons flowing through a lattice of metal ions

Measuring Current

• The amount of current flowing through a component is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

An ammeter can be used to measure the current around a circuit and is always connected in series

• The current, charge and time are related by the equation:

• Where:
  • I = current, measured in amperes (or amps, A)
  • Q = charge, measured in coulombs (C)
  • t = time, measured in seconds (s)

• The equation for current can be rearranged using the formula triangle below:

Charge, current, time formula triangle

ANSWER:   B

Step 1: Write out the equation relating current, charge and time

Step 2: Rule out any obviously incorrect options

• • Option A does not mention time, so can be ruled out

Step 3: Try the rest of the options by applying the equation to determine the correct answer

• • Consider option B:

I = 4 ÷ 500 = 8 × 10^–3 = 8 mA

• • Consider option C:

I = 8 ÷ 100 = 80 × 10^–3 = 80 mA

• • Consider option D:

I = 1 ÷ 8 = 125 × 10^–3 = 125 mA

• • Therefore, the correct answer is B

• A current will flow in a circuit if:
  • The circuit includes a source of potential difference
  • The circuit is closed - i.e. there are no gaps in the circuit

• Sources of potential difference include:
  • A cell
  • Batteries (multiple cells)
  • Electrical generator

• In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point
• • This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part

• This means that all components in a closed-loop have the same current

The current is the same at each point in a closed-loop

Current at a Junction

• At a junction in a circuit (where two or more wires meet) the current is conserved
  • This means the amount of current flowing into the junction is equal to the amount of current flowing out of it

• This is because charge is conserved

The current coming into a junction must equal the current coming out of a junction

• In electrical wires, the current is a flow of electrons
• Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
• Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
  • This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood

By definition, conventional current always goes from positive to negative (even through electrons go the other way)

Step 1: Recall that at a junction, the current is conserved

• • This means that the total amount of current flowing into a junction is equal to the total amount flowing out

Step 2: Consider the first junction in the circuit where current splits

• • The diagram below shows the first junction in the circuit

Step 3: Calculate the missing amount of current

• • Since 10 A flows into the junction (the total current from the battery), 10 A must flow out of the junction
  • The question says that 6 A flows through ammeter A₁ so the remaining current flowing through ammeter A₂ must be:

10 A − 6 A = 4 A

• • Therefore, 4 A flows through ammeter A₂