5.2 Electric Current

IB Physics Power Points

Topic 5.0

Electric Currentswww.pedagogics.ca

Electric Current

Electric current is the rate that charge flows through a conductor:

Charge Q is measured in coulombs C

Unit of electric current: the ampere, or A

1 A = 1 C s-1

Electric Current

Formal definition of the ampere:

Charge Movement in Conductors

Current is the movement of charge. A substance that allows charges to move freely is called a conductor.

Solid ConductorsWhat we are most concerned with i.e.. metal wires.

Structural model for a metal consists of positive ions surrounded by a “sea” of loosely held valence electrons. It is these electrons that transfer charge.

Charge Movement in Conductors

If an electric field is established in the conductor, the field force accelerates the electrons, but positive ions remain fixed.

Free electrons move in field (from low potential to high potential), in the process interacting with positive ions and other electrons. Movement is called “drift”.

Interactions of drift electrons increase KE of fixed ions. Current produces a heating effect in a conductor.

The “drift” velocity in an average metal conductor is approximately 10-4 ms-1. Question: if electrons move so slowly in a wire, why when you turn a switch, a light bulb lights instantly (regardless of the length of wire)?Answer: electrons are already in wire. Closing the switch establishes the electric field (instantly). All electrons in wire feel electrical force instantly.

Charge Movement in Conductors

LiquidsLiquids can also act as conductors. A liquid that conducts charge is called an electrolyte.

In electrolytic liquids, charge transfer is bi-directional. Positive ions move one way, negative ions move the opposite.

A classic example of an electrolytic solution is salt (NaCl) in water. Molten (melted) NaCl is also an electrolyte.

Charge Movement in Conductors

GasesIonized gases (plasma) are also conductors. Think charged particles that are free to move.

Current in Electrical Circuits

By convention, current is defined as flowing from high potential (positive) to low potential (negative). Electrons actually flow in the opposite direction in a typical circuit.

Current in Electrical Circuits

For charge to move through a circuit, a continuous path must be supplied from the point of high potential to the point of low potential (i.e. the terminals on this battery)

Note the schematic drawing that represents the circuit in the photo.

Resistance and Resistors

Experiments indicate that the current in a wire is proportional to the potential difference between its ends:

The ratio of voltage to current is called the resistance:

V

RI

Resistance and Resistors

Resistance can be thought of as opposition to current flow. Resistance is a property of all devices requiring electrical energy ex. toasters, light bulbs etc.

Resistance is measured in ohms (Ω).

What does 1 Ω equal in fundamental units?

2 2

22

2 2

JV J s N m sCCA C Cs

kg m m s kg msC C s

When charge encounters resistance there is a decrease of electrical potential energy. This decrease is measured by a voltmeter as the potential difference across the resistor.

A

battery

resistor

voltmeterV

ammeter

The EPE decrease per C of charge across the bulb is measured by the voltmeter.

The current through the circuit (and the bulb) is measured by the ammeter.

Understanding the energy transfer in resistors is critical to an understanding of electric circuits.

The graph shows the EPE changes as a coulomb of charge moves through the circuit.

What happens to the EPE in the bulb?

How does the EPE get restored in the battery?

Demonstration – potential difference in a simple circuit

Ohm’s LawThe electrical resistance of a material depends on its internal structure.

Under normal operating voltages, the resistance of many conductors is constant.

These conductors are said to obey Ohm’s Law i.e. resistance is independent of the normal operating voltage

Materials that do not follow Ohm’s law are called non-ohmic.

Resistivity and Other Factors Affecting Resistance

The resistance of a wire is

• directly proportional to its length

• inversely proportional to its cross-sectional area

• related to the type of material

Mathematically

The constant ρ, the resistivity, is characteristic of the material.

Resistivity and other factors affecting Resistance

The resistance of a wire also depends on temperature:

• resistivity of conductors increases with temperature

• some semi-conductors exhibit the opposite effect

Resistivity of common conductors at 20oC. (values would increase at higher temperatures)

Electric Power and Energy Dissipation

Watt does this mean?

Electric Power and Energy Dissipation

Power, as in kinematics, is the energy transformed by a device per unit time:

energy transfertime

q VP I V

t

Power as before is measured in WATTS (W)

C J JWatt s sC

Express power in terms of current and resistance, and voltage and resistance. USEFUL equations

KEY CONCEPT: when applying these equations, the values used are specific to a particular device.

Demonstration

What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time. Instead of measuring energy in joules, the electric company measures it in kilowatt-hours (kWh).

Refrigerator problem