Peoples Physics Concepts

http://www.ck12.org Chapter 13. Electric Circuits: Batteries and Resistors

13.1 Voltage and Current

• Describe voltage, current and the relationship between current and charge.

Students will learn the concepts of voltage and current and the relationship between current and charge.

Key Equations

I=∆∆qt; current is the rate at which charge passes by; the units of current are Amperes( 1 A= 1 C/s)

Guidance

Conductors have an effectively infinite supply of charge, so when they are placed in an electric field, aseparation

of chargeoccurs. A battery with a potential drop across the ends creates such an electric field; when the ends are

connected with a wire, charge will flow across it. The term given to the flow of charge iselectric current, and it is

measured in Amperes (A) — Coulombs per second. Current is analogous to a river of water, but instead of water

flowing, charge does.

Currentis the number of Coulomb’s that flow by per second. Thus 1 Amp of current is equivalent to saying that 1

C of electric charge is passing every second (i.e. the rate of change of charge is 1 C/s).

Voltageis the electrical energy density (energy divided by charge) and differences in this density (voltage) cause

electric current in the circuit.Batteriesandpower suppliesoften provide a voltage difference across the ends of a

circuit, but othervoltage sourcesexist. Using the water analogy that current is a river, then differences in voltage

can be thought of as pipes coming out of a water dam at different heights. The lower the pipe along the dam wall,

the larger the water pressure, thus the higher the voltage. If that pipe is connected then current will flow. Current

will be larger for the pipe with the greatest pressure (i.e. the lowest pipe on the dam wall).

Example 1

Somehow, you are able to see the electrons passing through a wire as current flows. Over the course of 5 seconds,

you count 1. 5 ∗ 1020 electrons pass a single point in the wire. How much charge passed you and the current in the

wire?

Solution

We can use the known charge of an electron to answer the first part of this problem.

q= 1. 5 ∗ 1020 electrons∗ 1. 6 ∗ 10 −^19 C/electron

q=24 C

Now we use the equation above to determine the current