Electrical charge and current — AQA GCSE Combined Science
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Electrical charge and current explained
Electrical charge flows only when there is a complete closed conducting path and a source of potential difference, such as a cell or power supply.
Read the full explanation
The source does work on the charge, giving energy to the charges so they move around the circuit; without it, charge does not flow even if the wires form a closed loop. A useful model is a battery pushing charge around a track, like a pump pushing water around a closed pipe system. If the switch is open, the path is not closed and current stops. If the source is removed, there is no potential difference to drive the charge, so again no current flows. Current is the rate of flow of charge, and it is measured in amperes with an ammeter connected in series.
Electric current is a flow of electrical charge. The size of the electric current is the rate of flow of electrical charge. Charge flow, current and time are linked by the equation:
Electric current is the movement of electrical charge through a conductor, like electrons flowing through a copper wire. The size of the current indicates how quickly charge passes a point: a larger current means more charge flows each second. Current is measured in amperes (A) using an ammeter in series. Charge flow is measured in coulombs (C) and time in seconds (s). The relationship is charge flow = current × time, or Q = I × t. For example, a steady direct current of 2 A flowing for 5 s transfers 10 C of charge. Rearranging gives current = charge flow ÷ time. This equation lets you calculate any one quantity when the other two are known, and it applies to steady direct currents.
charge flow = current × time
The equation charge flow = current × time links three quantities in an electric circuit. Charge flow is measured in coulombs (C), current in amperes (A) and time in seconds (s). If a current of 3 A flows for 4 s, the charge flow is 3 × 4 = 12 C. The equation can be rearranged: current = charge flow ÷ time, and time = charge flow ÷ current. This is useful when a question gives charge and time but asks for current, or gives charge and current but asks for time. Always convert time to seconds before substituting; for example, 5 minutes = 300 s. The equation applies when the current is steady. In calculations, write the equation, substitute the known values with units, then calculate and give the unit.
Q = I t
This equation links charge flow to current and time. Current is the rate of flow of electrical charge: one ampere means one coulomb of charge passes a point each second. Rearranged, Q = I × t, I = Q ÷ t and t = Q ÷ I. For example, a current of 2 A flowing for 30 s transfers Q = 2 × 30 = 60 C. If 120 C passes in 40 s, I = 120 ÷ 40 = 3 A. Always convert time to seconds: 5 minutes is 300 s. The equation applies to steady currents in series circuits and to any component, including filament lamps and resistors, where charge carriers pass through. It also underpins Q = ne, linking total charge to the number of electrons, each of charge 1.6 × 10⁻¹⁹ C.
charge flow, Q, in coulombs, C
Charge flow, Q, is the total quantity of electrical charge that passes a point in a circuit. It is measured in coulombs, symbol C. One coulomb passes when a current of one ampere flows for one second, so Q = I t. Charge is carried by electrons in metal wires; each electron carries 1.6 × 10⁻¹⁹ C, so 1 C corresponds to about 6.25 × 10¹⁸ electrons. For example, a current of 0.2 A for 50 s transfers 10 C. Charge is not used up around a series circuit: the same charge passes each point, though energy is transferred to components. In parallel branches, charge splits between branches and recombines. Always give Q in coulombs and avoid confusing charge with current or energy.
current, I, in amperes, A (amp is acceptable for ampere)
Electric current measures the rate of flow of electric charge around a circuit. The symbol for current is I, and its unit is the ampere, symbol A; in written work 'amp' is also accepted for ampere. One ampere means one coulomb of charge passing a point each second, so a current of 2 A transfers 2 C every second. Current is measured with an ammeter connected in series with the component being investigated, because the same current must pass through the meter and the component. In a series circuit the current is the same at every point, while in parallel branches the current splits and recombines. When calculating, substitute current in amperes into Q = I t, and remember that a current can be direct or alternating but the unit remains the ampere.
time, t, in seconds, s
Time, symbol t, is measured in seconds, symbol s, when calculating electrical charge. The equation Q = I t links charge in coulombs to current in amperes and time in seconds, so time must be in seconds before substitution. If a time is given in minutes, multiply by 60; if given in hours, multiply by 3600; if given in milliseconds, divide by 1000. For example, a current of 0.5 A flowing for 3 minutes transfers charge Q = 0.5 × 180 = 90 C. Time is measured in the laboratory with a stopwatch or timer, and in circuit questions it is often the duration for which a current flows. Rearranging Q = I t gives t = Q ÷ I, which allows the time to be found when charge and current are known.
A current has the same value at any point in a single closed loop.
In a single closed loop, charge cannot accumulate or escape, so the rate of flow of charge — the current — is identical at every point. Imagine a simple series circuit with a cell, a lamp and a resistor. The cell drives charge around the loop; the same number of coulombs passes through the cell, the lamp and the resistor each second. If the current were larger in one part, charge would build up there, which does not happen in a steady circuit. This conservation of charge explains why ammeters placed anywhere in the loop give the same reading. It also means that adding components in series increases resistance and reduces the current everywhere, but the current remains equal at all points. Students should be able to predict ammeter readings and explain why they are equal using charge conservation.
Your focus
- Explain why both a closed circuit and a source of potential difference are needed for charge to flow.
- Describe current as the rate of flow of charge and use Q = It to calculate charge, current or time.
- Predict the effect on current of opening a switch or removing the source of potential difference from a circuit.
Show all 24 objectives
- Describe electric current as a flow of electrical charge and explain that current is the rate of flow of charge.
- Use the equation charge flow = current × time to calculate charge flow, current or time.
- Apply correct units for charge flow, current and time in calculations and answers.
- Recall and write the equation charge flow = current × time.
- Use the equation to calculate charge flow, current or time from given values.
- Convert time to seconds and give answers with correct units.
- State and apply the equation Q = I t to calculate charge flow, current or time.
- Convert time values into seconds and use consistent SI units in calculations.
- Explain the meaning of one coulomb in terms of one ampere flowing for one second.
- Define charge flow and state its unit, the coulomb.
- Calculate charge flow using Q = I t with correct units.
- Explain that charge is conserved in circuits and relate charge to the number of electrons.
- Define electric current as the rate of flow of charge and give its unit, the ampere (A).
- Select and use the symbol I and the unit A correctly in written and calculated answers.
- Connect an ammeter in series and interpret its reading as the current through a component.
- State that time is measured in seconds (s) and identify t as its symbol.
- Convert times given in minutes, hours or milliseconds into seconds before substitution.
- Rearrange and use Q = I t to determine a time from measured charge and current.
- Describe current as the rate of flow of charge in a circuit.
- Explain why the current is the same at all points in a single closed loop using conservation of charge.
- Predict and compare ammeter readings at different positions in a series circuit.
Electrical charge and current exam tips
Marking Points
- States that a closed circuit is needed so charge can complete a continuous path.
- States that a source of potential difference, such as a cell or battery, is needed to make charge flow.
- Explains that the source does work on the charge, transferring energy to it and causing it to move.
- Uses the equation Q = It, where Q is charge in coulombs, I is current in amperes and t is time in seconds.
- Describes current as the rate of flow of electrical charge.
- Applies the idea that removing the source or opening the switch stops the current.
- State that electric current is a flow of electrical charge, commonly electrons in a metal conductor.
- Explain that current is the rate of flow of charge, so current equals charge flow divided by time.
- Recall and apply the equation charge flow = current × time, with charge in coulombs, current in amperes and time in seconds.
- Rearrange the equation to find current or time when the other two quantities are given.
- Use correct units and unit symbols: coulombs (C), amperes (A) and seconds (s).
- Substitute numerical values correctly into the equation and evaluate the answer with an appropriate unit.
- Recall the equation charge flow = current × time.
- Identify the correct units: charge flow in coulombs (C), current in amperes (A) and time in seconds (s).
- Substitute known values into the equation and calculate the unknown quantity.
- Rearrange the equation to calculate current as charge flow ÷ time or time as charge flow ÷ current.
- Convert time to seconds before using the equation, for example minutes × 60.
- Give the final answer with the correct unit and an appropriate number of significant figures.
- State the equation as Q = I t, identifying Q as charge flow in coulombs, I as current in amperes and t as time in seconds.
- Substitute values correctly, converting minutes to seconds by multiplying by 60 before calculating.
- Rearrange the equation to find current or time when charge and the other quantity are given.
- Calculate charge flow for a stated current and time, for example 0.5 A for 120 s gives 60 C.
- Explain that one coulomb is the charge transferred when one ampere flows for one second.
- Use the equation with series circuits, recognising that current is the same at every point in a series loop.
- Identify Q as charge flow and state that its unit is the coulomb, symbol C.
- Define one coulomb as the charge passing a point when one ampere flows for one second.
- Use Q = I t to calculate charge flow, ensuring current is in amperes and time in seconds.
- Recognise that charge is conserved around a circuit and is not used up by components.
- Link charge to the number of electrons using the charge on one electron, 1.6 × 10⁻¹⁹ C.
- Distinguish charge in coulombs from current in amperes and energy in joules.
- States that current is the rate of flow of electric charge, measured in amperes (A).
- Uses the correct symbol I for current and the correct unit symbol A, accepting 'amp' for ampere in written answers.
- Explains that an ammeter is connected in series so that the current to be measured passes through it.
- Applies the relationship charge = current × time, using current in amperes, to calculate charge or current.
- Recognises that one ampere equals one coulomb per second, linking the unit to the definition of current.
- Describes current in series and parallel circuits, including that current is the same throughout a series circuit and divides between parallel branches.
- States that time is measured in seconds, with the unit symbol s, in the equation Q = I t.
- Uses the symbol t for time and substitutes time in seconds into charge calculations.
- Converts minutes to seconds by multiplying by 60 and hours to seconds by multiplying by 3600.
- Converts milliseconds to seconds by dividing by 1000 before substitution.
- Rearranges Q = I t to t = Q ÷ I and calculates time when charge and current are known.
- Measures or records the duration of a current with a stopwatch or timer, reading to an appropriate precision.
- State that current is the rate of flow of charge, measured in amperes (A).
- Explain that in a single closed loop there is only one path, so charge cannot leave or accumulate.
- Apply conservation of charge to conclude that the current is the same at every point in the loop.
- Predict that ammeters connected at different positions in the same series loop give identical readings.
- Use the equation I = Q ÷ t to support the idea that equal charge transfer per second means equal current.
Examiner Tips
- 💡In explanations, name both requirements: a closed circuit and a source of potential difference.
- 💡When calculating charge, write the equation, substitute values with units, and give the answer in coulombs.
- 💡Use the water-pump analogy only as a model, then link it back to the physics of charge, energy and potential difference.
- 💡Write the equation, then substitute values with units before calculating to reduce errors.
- 💡Check that time is in seconds and convert minutes or hours if necessary.
- 💡Write down the equation before substituting numbers to make rearranging clearer.
- 💡Convert time to seconds as the first step if it is given in minutes or hours.
- 💡Check the size and unit of your answer to see whether it is sensible.
- 💡Write the equation, then substitute numbers with units before calculating so method marks are visible.
- 💡Check the unit of time in the question and convert to seconds immediately if it is given in minutes.
- 💡Give the unit C with your final charge answer and check the size is sensible for the current and time stated.
- 💡Underline the quantity asked for, then choose the matching unit before calculating.
- 💡Show the equation and substitution so the examiner can credit correct physics even if arithmetic slips.
- 💡Use standard form for very small charges such as the charge on an electron, 1.6 × 10⁻¹⁹ C.
- 💡Check whether the question asks for the quantity, its symbol or its unit, and answer with exactly that.
- 💡In calculations, convert milliamperes to amperes by dividing by 1000 before substituting into Q = I t.
- 💡When describing a circuit, state where the ammeter is placed and why the reading is the current through that component.
- 💡Write down the time in seconds before starting the calculation, showing the conversion if the question gives minutes or hours.
- 💡Include the unit s with a calculated time, and check that the value is sensible for the current and charge involved.
- 💡When timing an experiment, start and stop the stopwatch at the same point in the cycle to reduce timing error.
- 💡When asked to compare ammeter readings in a series loop, state that they are equal and justify with conservation of charge.
- 💡Use the phrase 'rate of flow of charge' when defining current to secure the definition mark.
- 💡If a circuit diagram shows one loop, check whether the question asks about current, pd or resistance before writing your answer.
Common Mistakes
- Thinking that a closed circuit alone is enough for charge to flow; correction: a closed circuit must also contain a source of potential difference.
- Confusing current with potential difference; correction: potential difference is the energy transferred per unit charge and drives the current, while current is the rate of flow of charge.
- Using Q = It with time in minutes; correction: convert time to seconds before substituting into the equation.
- Writing the equation as charge flow = current ÷ time. Correction: charge flow is current multiplied by time, because more current or more time transfers more charge.
- Using minutes instead of seconds without converting. Correction: convert time to seconds before substituting, for example 2 minutes = 120 s.
- Confusing current with charge. Correction: current is the rate of charge flow measured in amperes, while charge flow is measured in coulombs.
- Multiplying current by time but forgetting to convert minutes to seconds. Correction: convert all times to seconds first.
- Rearranging incorrectly, for example writing current = charge flow × time. Correction: current = charge flow ÷ time.
- Giving the answer without a unit or with the wrong unit. Correction: include C for charge, A for current or s for time.
- Using minutes directly instead of converting to seconds; correct by multiplying minutes by 60 before substitution.
- Confusing current with charge; correct by remembering current is charge per second, measured in amperes, while charge is measured in coulombs.
- Rearranging incorrectly, for example writing I = Q t; correct by dividing both sides by t to give I = Q ÷ t.
- Writing the unit as C s⁻¹ or A; correct by using C for charge, since C s⁻¹ is the unit of current.
- Treating charge as used up by a lamp; correct by stating charge is conserved and energy is transferred instead.
- Mixing up Q and I in the equation; correct by checking that Q is the quantity in coulombs and I is the rate in amperes.
- Writing the unit as 'amps' when the symbol A is required in a calculation answer; correction: give the unit as A, or write 'ampere' or 'amp' in words.
- Connecting an ammeter in parallel with a component; correction: connect the ammeter in series so the current flows through it.
- Confusing current with charge or with potential difference; correction: current is the rate of charge flow in amperes, charge is measured in coulombs and potential difference in volts.
- Substituting minutes directly into Q = I t; correction: convert minutes to seconds by multiplying by 60 first.
- Treating milliseconds as seconds; correction: divide milliseconds by 1000 to obtain seconds.
- Rearranging Q = I t incorrectly, for example writing t = Q × I; correction: divide charge by current, t = Q ÷ I.
- Thinking current is used up as it passes through components: correct this by stating that charge is conserved and energy, not current, is transferred to components.
- Believing the current before a component is larger than after it: correct this by explaining that the same charge passes through every point each second.
- Confusing current with potential difference or energy: correct this by defining current as charge flow per second and pd as energy transfer per unit charge.