Episode 102: Current as a flow of charge
Lesson for 16-19
- Activity time 85 minutes
- Level Advanced
Here, you are trying to illustrate the idea of electric charge and its relationship to a flow of current.
- Demonstration: Spooning charge (15 minutes)
- Worked example: Calculating numbers of electrons (10 minutes)
- Demonstration: The shuttling ball (15 minutes)
- Worked example: Calculating charge per second (10 minutes)
- Discussion: Defining current, the coulomb (10 minutes)
- Student questions: On charge and current (30 minutes)
Demonstration: Spooning charge
Start by reminding your students of the nature of electric charge. They should be familiar with the concept of charge from pre-16 science course. Remind them that this is a fundamental property of some types of particles (e.g. protons and electrons) and that there is a law of force: like charges repel, unlike charges attract. These are the forces that push charges around electric circuits.
Spooning Charge demonstration to reinforce the idea that charge can be taken from one place to another and does not just disappear.
Worked examples: Calculating numbers of electrons
Remind them that charge is measured in coulombs and tell them that the size of the charge on an electron is a tiny fraction of a coulomb (1.6 × 10-19 C). A quick calculation will confirm that even tiny charge transfers on our scale involve enormous numbers of electrons.
Calculate the number of electrons transferred when you
spoon charge and show that this depends on the voltage of the supply. (The result is, of course, linked to capacitance; this could be mentioned for a strong group.)
Demonstration: The shuttling ball
Shuttling Ball demonstration to link charge and current. The greater the rate of transfer of charge the greater the current.
Worked examples: Calculating charge per second
Calculate the charge transferred per second in the shuttling ball experiment. To do this, use a coulomb-meter to measure the charge on the ball and then measure the number of transfers in a given time. Alternatively you could work back from the current.
Discussion: Defining current, the coulomb
Define current as rate of change of charge. At this level it might be best to do this graphically. Current is the gradient of a graph of charge transferred against time, leading to I = Δ Q Δ t or (in the limit) I = d Q d t. Exactly how you represent this will depend on the requirements of your specification and the mathematical experience of your students. The idea of the gradient can be introduced by asking how the charge transferred by the shuttling ball increases with time – it will go up in a series of steps but, given a large number of transfers, these will approximate to a constant slope. The average current is equal to its gradient. The essential outcome is that they realise that a current of one amp is equivalent to a flow of one coulomb per second. The equation I = Qt (familiar from pre-16 science lessons) is useful but stress that this refers to an average current I and they must take care when I is changing.
Define the coulomb as the charge passed by a current of 1 A in 1 s, i.e. 1 C = 1 A s. (Note that the ampere is an SI base unit, and its definition is beyond requirements at this stage.)