Electric Circuits

Chapter 20

Electric Current

• Electric = charges• Current = flow

• Flow of electric charges (electrons in solids, ions in electrolytes)

• What is required for EC to occur?• 1) Source of potential difference• 2) Charges free to move• 3) Complete circuit

Conventional current is the hypothetical flow of positive charges that wouldhave the same effect in the circuit as the movement of negative charges thatactually does occur.

If the charges move around the circuit in the same direction at all times,the current is said to be direct current (dc).

If the charges move first one way and then the opposite way, the current is said to be alternating current (ac).

Source of DC – battery / capacitor / hand generator

Source of AC – power lines / (hand) generator

The electric current is the amount of charge per unit time that passesthrough a surface that is perpendicular to the motion of the charges.

t

qI

One coulomb per second equals one ampere (A).

Elements of circuits

Quick lab

Construct a circuit that has a light bulb connected to a switch and a battery. Draw a circuit diagram, show direction of the conventional current.

Does the current change its direction?Draw a graph I(t).

Quick lab

Use a hand generator to charge a capacitor. Draw a circuit diagram which would include: a capacitor, a switch, a light bulb and an Am-meter.

Construct the circuit and observe what happens with the current. Draw a graph I (t)

Quick lab

Construct a circuit with: a generator, a light bulb, a switch, and an Am-meter.

1)Keeping the switch open, spin the handle of the generator

2)Complete the switch. Does the effort you need to apply to spin the handle change? Explain.

3)Observe how the speed at which you are spinning the handle changes the brightness / current.

4)Spin the handle back and forth and observed what happens with the current / Am-meter reading.

Within a battery, a chemical reaction occurs that transfers electrons fromone terminal to another terminal.

The maximum potential difference across the terminals is called the electromotive force (emf).

The current in a 3.0 V battery of a pocket calculator is 0.17 mA. In one hourof operation, (a) how much charge flows in the circuit and (b) how much energydoes the battery deliver to the calculator circuit?

Short Circuits

Circuits where there is a path of least resistance avoiding loads (appliances, resistors, etc.) is called a short circuit (hazardous!)

Use 2 light bulbs, a battery and 2 switches to construct a circuit in which one of the switches will dim one of the light bulbs

OHM’S LAW

The ratio V/I is a constant, where V is thevoltage applied across a piece of materialand I is the current through the material:

SI Unit of Resistance: volt/ampere (V/A) = ohm (Ω)

The resistance (R) is defined as the ratio of the voltage V applied across a piece of material to the current I throughthe material.

The filament in a light bulb is a resistor in the formof a thin piece of wire. The wire becomes hot enoughto emit light because of the current in it. The flashlightuses two 1.5-V batteries to provide a current of0.40 A in the filament. Determine the resistance ofthe glowing filament.

For a wide range of materials, the resistance of a piece of material of length L and cross-sectional area A is

A

LR

oo TT 1

Longer Extension Cords

The instructions for an electric lawn mower suggest that a 20-gauge extensioncord can be used for distances up to 35 m, but a thicker 16-gauge cord shouldbe used for longer distances. The cross sectional area of a 20-gauge wire is5.2x10-7Ω·m, while that of a 16-gauge wire is 13x10-7Ω·m. Determine the resistance of (a) 35 m of 20-gauge copper wire and (b) 75 m of 16-gauge copper wire.

(a)

(b)

There are many circuits in which more than one device is connected toa voltage source.

Series wiring means that the devices are connected in such a waythat there is the same electric current through each device.

Construct a circuit with 1 light bulb. Add another. Observe the change in brightness.Make a conclusion about the current. Connect the A-meter to the left of the first resistor (LB). Write down the reading. Repeat your measurement putting the Am-meter to the right of the second resistor (LB). Compare the reading. Make a conclusion.

Turn the multi-meter into a V-meter. Measure potential drop across the battery and across each resistor. Measure potential drop on the battery. Make a conclusion

Rules for Connection in Series