Current and Charge

Overview

This lesson builds the electrical meaning of current from the movement of charge. You should be able to describe what is flowing, use charge-current-time calculations, and explain why a large current can still involve a very small drift speed in a metal wire.

What You Need to Know

• Electric current is the rate of flow of charge. In metals the mobile charge carriers are electrons; in electrolytes they are ions.
• Charge is quantised. This means charge comes in whole-number multiples of the elementary charge, so an object cannot have any arbitrary amount of charge.
• Use Q = It when charge, current, and time are linked. Charge is measured in coulombs, current in amperes, and time in seconds.
• Conventional current is shown from positive to negative, even though electrons in a metal drift from negative to positive.
• For a current-carrying conductor, use I = Anvq, where A is cross-sectional area, n is number density, v is drift speed, and q is the charge on one carrier.
• A small drift speed can still give a useful current because there are many charge carriers in each cubic metre of a metal.

How to Work Through It

1. Start by recalling the units coulomb and ampere, then connect current to charge passing a point each second.
2. Practise direct Q = It calculations, including unit conversions for minutes, hours, and milliampere currents.
3. Use a wire model to identify A, n, v, and q in I = Anvq.
4. Explain why changing the wire area or number density changes the current for the same drift speed.

Check Your Understanding

• What is meant by a charge carrier?
• How much charge passes a point in 30 s when the current is 0.40 A?
• Why is electron flow not drawn in the same direction as conventional current?
• In I = Anvq, what happens to the drift speed if the cross-sectional area is doubled but the current stays the same?

Common Mistakes

• Describing current as a store of charge rather than a rate of flow of charge.
• Forgetting to convert time into seconds before using Q = It.
• Treating charge as continuous instead of quantised.
• Assuming electrons move quickly along the whole wire because the circuit responds quickly.

Next Steps

• Practise rearranging Q = It and I = Anvq without losing units.
• Carry the charge and energy language into the next lesson on potential difference and power.