What you'll learn
- How protons, neutrons and electrons relate to electric charge.
- How to draw and interpret GCSE circuit diagrams using standard symbols.
- The difference between series and parallel circuits.
- How to calculate charge, current, potential difference and energy transferred.
1. Starting point: atoms and charge
All matter is made from atoms. For this topic, you only need a simple model of the atom: a tiny central nucleus with electrons arranged around it.
Subatomic particles
- A proton is found in the nucleus, has relative charge +1, and relative mass 1.
- A neutron is found in the nucleus, has relative charge 0, and relative mass 1.
- An electron is found in shells around the nucleus, has relative charge -1, and has a much smaller relative mass, about one 1836th of a proton.
Electricity is about charge moving. In metal wires, the moving charged particles are electrons.
Electric charge
Electric charge is a property of particles that can be positive or negative. Charge is measured in coulombs (C).
2. What a circuit needs
An electric circuit is a complete path that charge can move around.
A cell is a source of potential difference. A battery is made from two or more cells. In circuit symbols, the long line of a cell is the positive terminal and the short line is the negative terminal.
A circuit will only have a current if:
- there is a closed loop, with no gaps
- there is a source of potential difference, such as a cell or battery
Closed circuit plus potential difference
When a closed circuit includes a source of potential difference, charge can flow around the circuit, so there is a current.
An open switch makes a gap, so current cannot flow. A closed switch completes the circuit.
3. Circuit diagrams and symbols
Circuit diagrams use standard symbols instead of realistic pictures. This makes circuits quicker to draw and easier to compare.
You should recognise and use these symbols:
- cell and battery
- open switch and closed switch
- ammeter and voltmeter
- resistor and variable resistor
- lamp and motor
- diode, LED, thermistor and LDR
A diode allows current more easily in one direction than the other. An LED is a light-emitting diode. A thermistor changes resistance with temperature, and an LDR changes resistance with light intensity.
Remembering LED and LDR arrows
For an LED, arrows point away because light is emitted. For an LDR, arrows point towards because light falls onto the component.
4. Series and parallel circuits
Components can be connected in different ways.
A series circuit has one loop. The components are connected one after another, so there is only one path for current.
A parallel circuit has branches. Charge reaches a junction, splits between different paths, and then rejoins later.
Series circuits
In a series circuit:
- there is one path for current
- the same current passes through every component
- if one component breaks or is removed, the whole circuit is broken
Parallel circuits
In a parallel circuit:
- there is more than one path for current
- current splits at junctions
- if one branch breaks, current can still flow in other complete branches
Thinking current is used up
Current is not “used up” by components. Energy is transferred by components, but charge still has to keep flowing around the circuit.
5. Potential difference: energy per charge
Potential difference is often called voltage. It tells you how much energy is transferred for each coulomb of charge.
Potential difference
Potential difference is the energy transferred per unit charge passed. It is measured in volts (V). One volt means one joule per coulomb: 1 V=1 J/C1\,\text{V} = 1\,\text{J/C}1V=1J/C.
A voltmeter measures potential difference. It is connected in parallel with the component, because it compares the energy per charge at two points: before and after the component.
The equation for energy transferred is:
E=Q×VE = Q \times VE=Q×Vwhere:
- EEE is energy transferred in joules (J)
- QQQ is charge moved in coulombs (C)
- VVV is potential difference in volts (V)
This is a recall and use equation for Edexcel Combined Science, so you need to learn it.
Calculating energy transferred
A charge of 15 C passes through a lamp with a potential difference of 6.0 V across it. Calculate the energy transferred.
- Choose the equation that links energy, charge and potential difference: E=Q×VE = Q \times VE=Q×V.
- Substitute the values with units: E=15 C×6.0 VE = 15\,\text{C} \times 6.0\,\text{V}E=15C×6.0V.
- Calculate and use 1 V=1 J/C1\,\text{V} = 1\,\text{J/C}1V=1J/C, so C×V\text{C} \times \text{V}C×V gives joules: E=90 JE = 90\,\text{J}E=90J.
6. Current: charge per second
An electric current is the rate of flow of charge. “Rate” means how much happens each second.
Current
Current is the charge flowing per second. It is measured in amperes, usually shortened to amps (A).
In metal wires, current is caused by a flow of electrons. The electrons are already in the metal; a cell makes them drift around the circuit.
An ammeter measures current. It is connected in series with a component, because it must measure the charge flowing through that component.
Connecting meters to a lamp
You want to measure the current through a lamp and the potential difference across the lamp.
- Put the ammeter in series with the lamp, so the same moving charge passes through both the lamp and the ammeter.
- Put the voltmeter in parallel across the lamp, with one connection on each side of the lamp.
- If the lamp is in a parallel branch, place the ammeter in that branch; placing it before the junction would measure the total current instead.
The equation linking charge, current and time is:
Q=I×tQ = I \times tQ=I×twhere:
- QQQ is charge in coulombs (C)
- III is current in amps (A)
- ttt is time in seconds (s)
This is also a recall and use equation.
Calculating charge moved
A current of 0.40 A flows for 3 minutes. Calculate the charge moved.
- Convert the time into seconds: 3 min=180 s3\,\text{min} = 180\,\text{s}3min=180s.
- Choose the equation linking charge, current and time: Q=I×tQ = I \times tQ=I×t.
- Substitute and calculate: Q=0.40 A×180 s=72 CQ = 0.40\,\text{A} \times 180\,\text{s} = 72\,\text{C}Q=0.40A×180s=72C.
Electron flow versus conventional current
In metals, the moving charges are electrons, which drift from the negative terminal towards the positive terminal. However, circuit current direction is often shown as conventional current, from positive to negative.
7. Current at junctions
A junction is a point where a circuit splits into branches or where branches rejoin.
Current is conserved at a junction. This means the total current going into a junction equals the total current coming out.
Conservation of current
At a junction: total current in = total current out. This is because charge cannot disappear or build up in the wire.
Finding current in a branch
A current of 2.5 A reaches a junction. It splits into two branches. One branch has a current of 1.1 A. Find the current in the other branch.
- Apply conservation of current: total current in equals total current out.
- Write the relationship for the two branches: 2.5 A=1.1 A+I2.5\,\text{A} = 1.1\,\text{A} + I2.5A=1.1A+I.
- Rearrange and calculate: I=2.5 A−1.1 A=1.4 AI = 2.5\,\text{A} - 1.1\,\text{A} = 1.4\,\text{A}I=2.5A−1.1A=1.4A.
In the exam
- Draw circuit diagrams using standard symbols: long line positive, short line negative, ammeter in series, voltmeter in parallel.
- For calculations, write the recall equation first, substitute values with units, and convert time into seconds if needed.
- At junctions, use current in = current out; do not say current is used up by components.
Check yourself
- Where are protons, neutrons and electrons found, and what charge does each have?
- How would you connect an ammeter and a voltmeter to measure a lamp in a circuit?
- A current of 0.25 A flows for 40 s. What charge passes through the component?
