Give definitions of the following terms and use these terms in sentences of your own

- Resistivity

- Conductivity

- Resistance

- Conductance

7 Speaking task.

A Explain the relationship between resistance, resistivity, length and cross-section area of a conductor.

B Explain the difference between:

- Conductance and conductivity

- Resistance and resistivity

Text V

1 Work in pairs. Discuss the questions below with your partner.

- How does resistance of a conductor depend on temperature?

- What is temperature coefficient of resistance?

2 Read the text and check your ideas.

EFFECT OF TEMPERATURE CHANGE ON CONDUCTOR RESISTANCE

As the temperature of a conductor rises, the atomic nuclei gain energy and become ‘excited’. When this happens, the current in the circuit (which can be regarded as electrons in motion) experience increasing difficulty in moving through the conductor. That is, increase in temperature causes an increase in conductor resistance. Conversely, a reduction in temperature causes a decrease in the resistance of the conductor. The change in resistance is proportional to the change in temperature over the normal operating range of temperature.

This effect is shown graphically in Figure 5; the resistance of the con­ductor represented by the graph at 0°C is given the value R₀, at tem­perature q₁°С the resistance is R_1, at q₂°С it is R₂, etc.

The change in resistance of the resistor as the temperature changes from, say, 0°C to temperature q₁ expressed as a fraction of its original resistance R₀ is called the temperature coefficient of resistance referred to the original temperature (0°C in this case). This coefficient is given the Greek symbol a (alpha), where

Note: the value of ais given the value a₀in the above equation since the original temperature was 0°C.

The units of acan be determined from the above equation as follows:

That is, a is expressed in ‘per degree С’ or (°C)^-1.

Give the English equivalents for the words and word-combinations. Find the sentences in the text where they were used.

приобретать энергию, увеличение температуры, уменьшение сопротивления, пропорционально изменению температуры, температурный коэффициент, уравнение, испытывать трудность в …, величина сопротивления, единица измерения

4 Solve the problems and comment on their solution.

Problem 1. If a 75-ohm sample of copper wire at 0^oC is heated to 30^oC, what is the approximate total resistance? (the temperature coefficient of resistance of copper at 0^oC is 0,00427)

Problem 2. The resistance of a coil of copper wire at 20^oC is 25 W. What current will it draw from a 10-V supply when in a room at 0^oC?