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Physics · 4 Electricity and magnetism

4.3.1 Circuit diagrams and circuit components

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Before you start

  • Current, p.d. and resistance, and how each is measured (4.2.2–4.2.4).
  • That an ammeter goes in series and a voltmeter in parallel.
  • The I–V behaviour of a diode from 4.2.4.
Syllabus objectives covered (2)
  • Draw and interpret circuit diagrams containing cells, batteries, power supplies, generators, potential dividers, switches, resistors (fixed and variable), heaters, thermistors (NTC only), light-dependent resistors (LDRs), lamps, motors, bells, ammeters, voltmeters, magnetising coils, transformers, fuses and relays and know how these components behave in the circuit
  • Draw and interpret circuit diagrams containing diodes and light-emitting diodes (LEDs) and know how these components behave in the circuit

Part 1 of 5

Why circuit diagrams are drawn the way they are

  1. A circuit diagram is not a picture of the apparatus. It is a map of what is connected to what, using agreed symbols so that anyone can read it.
  2. Draw wires as straight lines with right-angled corners, and keep components spaced along them. A neat rectangle is easier to read and easier to mark than a tangle.
  3. A junction where wires genuinely connect is marked with a solid dot. Wires that merely cross without connecting have no dot.
  4. That dot is worth a mark on its own in a parallel-circuit question, because without it the diagram says something different from what you meant.
  5. Every component symbol must be drawn accurately. A lamp is a circle with a cross; a fixed resistor is a plain rectangle. An approximate squiggle will not be credited.

Part 2 of 5

Sources and switches

  1. A cell is a long thin line and a short thick line. The long line is the positive terminal — a detail that decides which way a diode points.
  2. A battery is two or more cells in series, drawn as repeated cell symbols. Strictly, one cell is not a battery.
  3. A power supply or generator provides the e.m.f. in place of a cell. A generator is drawn as a circle containing the letter G.
  4. A switch is a break in the line with a hinged arm. Open means the circuit is broken and no current flows anywhere in that loop; closed means the circuit is complete.
  5. A fuse is a rectangle with a line through it. It contains a thin wire that melts and breaks the circuit if the current exceeds its rating — a deliberate weak point.
  6. A relay uses a small current in one circuit to operate a switch in another. The small current magnetises a coil, which pulls a contact closed and switches on the second, larger circuit.
  7. The point of a relay is isolation: a low-voltage, low-current circuit (a sensor, a switch you touch) can safely control a high-power one (a motor, a heater) without the two being electrically joined.

Part 3 of 5

Resistors and the two that sense

  1. A fixed resistor is a plain rectangle. Its resistance does not change.
  2. A variable resistor is a rectangle with an arrow through it. Turning or sliding it changes its resistance, which changes the current in the circuit.
  3. A potential divider is two resistors in series, with an output taken from the connection between them. It supplies a chosen fraction of the supply p.d. You will meet this properly in 4.3.3.
  4. Two components change their resistance in response to the world, and both are exam favourites.
  5. A thermistor (NTC only in this syllabus) has a resistance that decreases as the temperature increases. NTC stands for negative temperature coefficient, which is what that sentence means.
  6. An LDR, or light-dependent resistor, has a resistance that decreases as the light intensity increases. Bright light, low resistance; darkness, high resistance.
  7. Both go the 'wrong' way round compared with intuition — more of the thing means less resistance. Learn them as a pair, since they behave identically with different inputs.
  8. That behaviour is what makes them useful as sensors. Put one in a potential divider and the output p.d. changes as the temperature or light changes, which can then switch something on.

Part 4 of 5

Components that do a job

  1. A lamp is a circle with a cross inside. It transfers electrical energy to light and thermal energy, and its resistance rises as it heats (4.2.4).
  2. A heater is drawn as a rectangle, sometimes with hatching. It transfers electrical energy to thermal energy.
  3. A motor is a circle containing the letter M. It transfers electrical energy to kinetic energy.
  4. A bell is drawn as a dome shape. It transfers electrical energy to sound, and works using an electromagnet that repeatedly makes and breaks the circuit.
  5. A magnetising coil is a series of loops. Current through it produces a magnetic field, which is the basis of electromagnets, relays and transformers.
  6. A transformer is two coils drawn either side of a core, shown as parallel lines. It changes the size of an alternating p.d. — and works only with a.c.
  7. The meters you already know: an ammeter is a circle with A, connected in series; a voltmeter is a circle with V, connected in parallel.

Part 5 of 5

Diodes and LEDs

  1. A diode is drawn as a triangle pointing at a bar. The triangle points in the direction the conventional current is allowed to flow.
  2. That is the whole behaviour: very low resistance in the forward direction, very high resistance in the reverse direction, so current passes one way only.
  3. To decide whether a diode conducts in a given circuit, trace the conventional current from the positive terminal of the cell. If it reaches the triangle first and flows towards the bar, the diode conducts.
    +conductslit+blocksnot lit
    Same components, one reversed. Trace from the positive terminal and follow the arrow.
  4. If the diode is the wrong way round, effectively no current flows and nothing else in that series loop works either.
  5. This makes a diode useful for protecting a circuit against a battery inserted backwards, and for converting a.c. to d.c. by blocking one half of the cycle.
  6. An LED, or light-emitting diode, is a diode that emits light when a current flows through it in the forward direction. Its symbol is a diode symbol with two small arrows pointing away.
  7. An LED behaves exactly like a diode electrically — connect it backwards and it neither conducts nor lights. It is usually protected by a resistor in series, because a small increase in p.d. gives a large increase in current and would destroy it.

Worked examples

Example 1. Describe how the resistance of a thermistor and of an LDR change with their surroundings, and explain why this makes them useful in sensing circuits.

Show the working
  1. Take the thermistor first. An NTC thermistor's resistance decreases as the temperature increases.
  2. Now the LDR. Its resistance decreases as the light intensity increases — so bright light gives low resistance.
  3. Both therefore convert a physical condition into an electrical property that a circuit can respond to.
  4. Placed in a potential divider, a change in resistance changes the share of the supply p.d. that appears across the output.
  5. That changing p.d. can then switch something on — a heater when it gets cold, a lamp when it gets dark — which is exactly what a sensing circuit is for.

Answer: A thermistor's resistance falls as temperature rises; an LDR's resistance falls as light intensity rises. Each converts a physical condition into a change in resistance, so in a potential divider the output p.d. changes with temperature or light and can control another circuit.

Example 2. A cell, a lamp and a diode are connected in series and the lamp does not light. The components are all working. Explain the observation, and state how to make the lamp light.

Show the working
  1. Since the components work and the circuit is complete, the problem must be the direction of the diode.
  2. A diode has a very high resistance in the reverse direction, so effectively no current flows.
  3. The lamp is in series with it, so the same current — near zero — passes through the lamp, and it cannot light.
  4. To check, trace the conventional current from the positive terminal of the cell and see whether it meets the bar of the diode symbol before the triangle.
  5. The fix is to reverse the diode, so that conventional current enters at the triangle and flows towards the bar.

Answer: The diode is connected in reverse, so its very high resistance means almost no current flows in the series circuit and the lamp cannot light. Reversing the diode, so conventional current flows in the direction the triangle points, will light the lamp.

Example 3. Explain how a relay allows a circuit containing an LDR and a low-voltage cell to switch on a mains-powered lamp safely.

Show the working
  1. Describe the sensing circuit: the LDR is in a low-voltage circuit, and its resistance changes as the light level changes.
  2. When the light falls, the changing p.d. causes a current to flow through the relay's coil in that low-voltage circuit.
  3. The current magnetises the coil, which becomes an electromagnet and attracts an iron armature.
  4. The armature pushes a pair of contacts together, completing the separate mains circuit and switching on the lamp.
  5. The two circuits are connected only magnetically, never electrically, so the dangerous mains voltage never reaches the sensing circuit or the person using it.

Answer: A small current in the low-voltage LDR circuit magnetises the relay's coil, which attracts an armature and closes the contacts of the separate mains circuit. The two circuits are linked only by the magnetic field, so the mains voltage is kept isolated from the sensing circuit.

Where people go wrong

Mistake: Saying a thermistor's resistance increases as it gets hotter.

Instead: For an NTC thermistor — the only type in this syllabus — resistance decreases as temperature increases.

It feels backwards, but NTC means negative temperature coefficient. Pair it with the LDR, which behaves the same way with light.

Mistake: Drawing crossing wires without checking whether they connect.

Instead: Put a solid dot only where wires genuinely join.

The dot is the only thing distinguishing a junction from a crossing, and it changes what the circuit does.

Mistake: Thinking a diode the wrong way round just dims the lamp.

Instead: Effectively no current flows at all, so nothing in that series loop works.

The reverse resistance is enormous, not merely large.

Mistake: Connecting an LED without a series resistor.

Instead: An LED normally needs a protective resistor in series.

Its current rises very steeply with p.d. beyond about 0.7 V, so a small excess destroys it.

Mistake: Describing a relay as an amplifier or a transformer.

Instead: A relay is a switch operated by an electromagnet, linking two separate circuits.

It does not increase current or change p.d. — it isolates one circuit from another while letting the first control the second.

Check yourself

  1. How do you show on a circuit diagram that two crossing wires are connected?
    AnswerDraw a solid dot at the junction. Without the dot, the wires simply cross without connecting.
  2. State how the resistance of an NTC thermistor changes as it is cooled.
    AnswerIts resistance increases as the temperature falls.
  3. State how the resistance of an LDR changes when it is covered up.
    AnswerThe light intensity falls, so its resistance increases.
  4. Explain how a fuse protects a circuit.
    AnswerIt contains a thin wire that melts if the current exceeds its rating, breaking the circuit and stopping the current.
  5. What is the purpose of a relay?
    AnswerIt lets a small current in one circuit operate an electromagnet that closes a switch in a separate circuit, so a low-power circuit can safely control a high-power one.
  6. In a diode symbol, what does the direction of the triangle indicate?
    AnswerThe direction in which conventional current is allowed to flow.
  7. Why is a resistor usually connected in series with an LED?
    AnswerTo limit the current. Beyond about 0.7 V the LED's current rises very steeply with p.d., and without a resistor it would be damaged.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A circuit is required in which a lamp switches on automatically when it gets dark.

(a) Identify the component that should be used to detect the change in light level. 1 mark No calculator

Mark scheme
  • B1 a light-dependent resistor / LDR

(b) Describe how the resistance of this component changes as darkness falls. 1 mark No calculator

Mark scheme
  • B1 its resistance increases as the light intensity decreases

(c) The lamp runs from the mains supply. Explain why a relay is included in the circuit. 3 marks No calculator

Mark scheme
  • B1 a small current in the sensing circuit magnetises the relay coil
  • B1 this closes the contacts of the separate mains circuit, switching the lamp on
  • B1 the two circuits are not electrically connected, so the mains voltage is kept away from the low-voltage circuit

Question 2. A student connects a cell, a switch, an ammeter, a fixed resistor and an LED in series, with a voltmeter across the resistor.

(a) Draw the circuit diagram for this arrangement. 3 marks No calculator

Mark scheme
  • B1 correct symbols for cell, switch, resistor and LED
  • B1 all five components in a single series loop
  • B1 voltmeter drawn in parallel across the resistor only

(b) State what happens if the cell is connected the other way round, and give a reason. 2 marks No calculator

Mark scheme
  • B1 the LED does not light and effectively no current flows
  • B1 the LED is now reverse biased and has a very high resistance, ORA

(c) Explain the purpose of the fixed resistor. 1 mark No calculator

Mark scheme
  • B1 to limit the current through the LED and prevent it being damaged

Question 3. A heating circuit uses a thermistor so that a heater switches off when a room becomes warm enough.

(a) Describe how the resistance of the thermistor changes as the room warms up. 1 mark No calculator

Mark scheme
  • B1 its resistance decreases as the temperature increases

(b) Suggest why a fuse is also included in this circuit. 2 marks No calculator

Mark scheme
  • B1 if the current becomes too large the fuse wire melts
  • B1 breaking the circuit and preventing overheating or fire, owtte

Words to know

Thermistor (NTC)
A resistor whose resistance decreases as its temperature increases.
Light-dependent resistor (LDR)
A resistor whose resistance decreases as the light intensity on it increases.
Relay
An electromagnetically operated switch that lets a small current in one circuit control a separate, higher-power circuit.
Fuse
A thin wire that melts and breaks the circuit if the current exceeds its rating.
Diode
A component that allows conventional current to flow in one direction only, in the direction its triangle points.
LED
A light-emitting diode: a diode that emits light when forward current flows through it.
All Physics lessons Next: 4.3.2 Series and parallel circuits