Chapter 17Current and Resistance

Hw# 1, 4, 7, 10, 11, 13, 14, 15, 17, 23, 27, 33, 37, 43, 45

Static electricity devices, such as a Van de Graaf generator, produced small flow of charge for short time intervals.

Not very useful…

When it was learned how to produce a continuous flow for longer times, electricity became an important part of human technology.

For example: run a light bulb, use to power a refrigerator.

The continuous flow of charge leads up to electric current.

Current: I = ∆Q/∆T

SI unit of current is the Ampere (A)

1 Ampere = 1 Coulomb/second 1 A = 1 C/1 s

Direction of current: The direction of the current is the direction in which positivecharges flow.

Remember that in a conductor, the charges that move are the negative electrons. So in a wire, the current is in the opposite direction of the electron’s motion.

(Figure 17.4)

Work example 17.1, 17.2.

As charges flow through a conductor (wire), they are constantly colliding with each other and the walls of the conductor. It is the net movement of charge that produces the current. fig. 17.4

Think of water through a pipe. If a drop of water is forced into an already filled pipe, a drop will be pushed out the other end.

A current flow at one end produces a similar flow at the other end very quickly.

Current and voltage measurement in circuits.

circuit – closed loop of some sort around which current circulates

Basic circuit – battery hooked to a light bulb. The battery pushes the charges through the light bulb and around the loop.

Circuits need closed loops for current to flow.Current won’t flow through a ‘dead end’.

The battery increases the potential energy of the charges.

Then the charges lose the potential energy as they go around through other elements (devices) in the circuit.

Think of the circuit as a roller coaster that the charges ride. The initial ‘lift’ that the charges need to get over the hill is provided by the battery.

Resistance has units of Volts per AmpsCalled Ohm’s (Ω) 1Ω = 1V/1A

Material is Ohmic, if the resistance is constant over a large range of voltages.

Nonohmic – resistance changes with changes in voltage.

See graphs in figure 17.8Table 17.1

Switching to a high resistivity material will increase resistance.

Good conductors have small resistivity.

Notice the large range of resistivity values!

see table 17.1

example 17.3

Second column in table 17.1

Since resistivity changes with temperature,so does resistance.

Figure 17.11, table 17.2

Remember that there are collisions between the moving charges and the atoms in the resistor.

This causes the resistor to gain thermal energy.(Resistor heats up)

The potential energy is turned into thermal energy.

Connect a wire or paperclip across a battery. It will heat up.