Flashcards in Section 4 -Electricity Exam Questions Deck (23)

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1

## State what is meant by a superconductor (2)

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Zero resistance

At critical temperature

2

## What are the required conditions for a material to become superconducting? (2)

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Resistivity decreases with temperature

Becomes superconducting when you reach the critical temperature

3

## A copper cylinder cable contains several filaments. State and explain what happens to the cable's resistance when the filaments are made superconducting (5)

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Resistance decreases to zero

Copper still has resistance

But this is in parallel with filaments

So total resistance is zero

Current goes through filaments

4

## Conducting putty is rolled into a cylinder. It is reshaped into a cylinder with half the radius and four times the length, determine the change to the resistance (2)

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Halving diameter will increase resistance by 4

Increasing length by 4, increases resistance by 4

Resistance will be 16 times greater

5

## How would a voltmeter, ammeter and standard lab equipment be used to determine a value for the resistivity of the putty? (length and radius known) (6)

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Measure length with a ruler

Measure diameter with micrometer

Use diameter to calculate cross-sectional area

Measure voltage

Measure current

Calculate resistance using V against I graph (gradient)

Flat metal electrodes at each end to improve connection

6

## Describe a procedure that could obtain an I-V curve for a semiconductor diode (6)

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Connect circuit up

Measure current and voltage

Vary resistance

Obtain a range of results

Reverse connections to power supply and repeat

Plot a graph of V against I

Disconnect between readings

7

## Explain the shape of the I against V graph of a filament lamp (4)

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At low V, I increases proportionally

As V increases, temperature of filament increases

Resistance increases

Rate of increase of I with V decreases

Same form of the curve in negative quadrant

8

## In a circuit of two resistors in series, the second is replaced by a thermistor. State and explain what happens to the reading on the voltmeter on the resistor as the thermistor's temperature increases (3)

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Reading will increase

Resistance of thermistor decreases

Current in circuit increases so pd across resistor increases

9

## Describe the procedure of obtaining accurate and reliable measurements of the resistance of a thermistor at different temperatures (6)

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Thermistor is connected in a suitable circuit with voltmeter

Thermistor is heated in a beaker of water and a thermometer measures the temperature at small regular intervals

Resistance is found at various temperatures by diving voltage by current

Water must be stirred.

Temperature in range 20-90

10

## State and explain why it is an advantage for a rechargeable battery to have a low internal resistance (2)

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Internal resistance limits current

Hence can provide higher current

11

## State what is meant by the emf of a battery (1)

### Energy changed to electrical energy per unit charge passing through

12

## Explain what is meant by the internal resistance of a battery (1)

### The resistance of the materials within the battery

13

## State and explain the effect of attempting to use a battery with a high internal resistance to start a car (2)

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Car will probably not start

Battery will not be able to provide enough current

14

## A cell is connected to a variable resistor. Why does the terminal pd (across the cell) decrease as current increases? (2)

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Energy loss in internal resistance

Pd across internal resistance increases with current

15

## Why does a voltmeter have a reading lower than the emf of a battery in a circuit? (3)

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Battery has internal resistance

Current passes through this resistance

Voltage lost, which reduces value of emf

16

## How is emf and internal resistance found from the V against I graph for a cell? (2)

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Emf is the y-intercept

Gradient is negative internal resistance

17

## State Ohm's Law (1)

### The current through a conductor between two points is directly proportional to the potential difference across the two points,provided the temperature remains constant

18

## A conducting putty of 20Ω is set up in series with a cell, ammeter and 10Ω resistor. The voltmeter is across the cell. Explain the problems for showing the putty obeys Ohm's law (4)

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Voltmeter position should not be across the cell

Voltmeter should be connected across the putty

10Ω resistor is not suitable to control the current

Because its resistance is only half of the putty

19

## With reference to two uses for superconductors, explain the advantage of their use compared with copper conductors (3)

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Electromagnet

Produces very high magnetic field strength

Transmission

Less energy wasted

20

## Explain why resistivity of wire changes with temperature (3)

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Increasing temperature, increases resistivity

Lattice ions vibrate with greater amplitude

Rate of movement of charge carriers reduced

21

## Define resistance (1)

### Ratio of voltage across component to current through component

22

## State and explain, in terms of electron flow, how the resistance of the filament lamp changes as the current in the lamp increases (3)

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Resistance increases

Temperature rise increases lattic vibration

More frequent collisions

23