We usually think of an electric circuit as a flow of positive electric charge from the positive terminal of the battery or power supply to the negative terminal. In fact the current is carried by negatively-charged electrons which flow from negative to positive.
At first sight it may seem odd that the current in a series circuit is the same after passing through a bulb (or other component) as before it. In other words current is not used up by a bulb. What is used up (or, strictly, converted into another form) is the energy carried by the current. In a battery or other power supply, electrons pick up energy (the greater the voltage of the supply, the more energy) and carry this round the circuit to bulbs (or other components). In each bulb electrical energy is converted into light energy and each electron will emerge from the bulb with less energy that with which it entered.
One analogy that could be used with students is the electrons are like postmen and the energy they carry is like letters. The postmen fill their bags with letters at the post office (which represents the battery). As they do their rounds, they gradually get rid of all their letters (energy) at various houses (bulbs) but the same number of postmen return to the post office.
Where a circuit has no branches (a series circuit), the current is the same no matter where it is measured.
Where a circuit has two or more branches (a parallel circuit) the sum of the currents in each branch adds up to the current in the main part of the circuit.
The actual value of the current in any part of the circuit depends on the voltage of the battery (or power supply) and the resistance of the bulb (or other component) and is given by the relationship current = voltage / resistance (I = V/R using the conventional symbols). In the virtual laboratory the bulbs have a notional resistance of 5 ohms (5 Ω) and the batteries a voltage of 1.5 V each.
In fact the resistance of a metal increases with temperature so the filament of a bulb will have a greater resistance when it is hot than when it is cold. No attempt has been made to include this effect in the virtual laboratory simulation.
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| Sc4 | Physical processes | |
| Electricity | ||
| 1 | Pupils should be taught: | |
| a | about everyday appliances that use electricity | |
| b | about simple series circuits involving batteries, wires, bulbs and other components [for example buzzers, motors] | |
| c | how a switch can be used to break a circuit | |
| Sc4 | Physical processes | |
| Electricity | ||
| 1 | Pupils should be taught: | |
| Simple circuits | ||
| a | to construct circuits incorporating a battery or power supply and a range of switches to make electrical devices work | |
| b | how changing the number or type of components [for example batteries, bulbs, wires] in a circuit can make bulbs brighter or dimmer | |
| c | how to represent series circuits by drawings and conventional symbols and how to construct series circuits on the basis of drawings and diagrams using conventional symbols | |
| Sc4 | Physical processes | |
| Electricity and magnetism | ||
| 1 | Pupils should be taught: | |
| Circuits | ||
| a | how to design and construct series and parallel circuits, and how to measure current and voltage | |
| b | that the current in a series circuit depends on the number of cells and the number and nature of other components and that the current is not ‘used up’ by components | |
| c | that energy is transferred from batteries and other sources to other components in electrical circuits | |
| Sc4 | Physical processes | |
| Electricity | ||
| 1 | Pupils should be taught: | |
| Circuits | ||
| a | that resistors are heated when charge flows through them | |
| b | the qualitative effect of changing resistance on the current in a circuit | |
| c | the quantitative relationship between resistance, voltage and current | |
| d | how current varies with voltage in a range of devices [for example, resistors, filament bulbs, diodes, light dependent resistors (LDRs) and thermistors] | |
| e | that voltage is the energy transferred per unit charge | |
| f | the quantitative relationship between power, voltage and current | |
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