Flashcards in Electricity Deck (26)

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1

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

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Zero resistance

At or below 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

## 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

4

## 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 with voltmeter in parallel

Measure current with an ammeter in series

Calculate resistance using V against I graph (gradient)

Flat metal electrodes at each end to improve connection

5

## 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

6

## 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 with temperature due to atoms/ions vibrating and taking up more space.

Rate of increase of I with V decreases

Same form of the curve in negative quadrant

7

## 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

8

## 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

9

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

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

10

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

### The resistance of the materials within the battery

11

## 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

12

## 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

13

## Why does a voltmeter across a battery 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

14

## 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

15

## 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

16

## 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 (electrons) reduced

17

## Define resistance (1)

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

18

## Explain in terms of electron motion, why the I-V characteristic for the filament lamp is a curve (4)

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Increase in voltage causes temp increases

This causes an increase in the movement of the atoms

Increase in rate of collisions with electrons

So resistance increases as shown by V/I changing

19

## Define current

### Rate of flow of charge

20

## Define potential difference

### Work done per unit charge

21

## What is the resistance of an ideal ammeter?

### zero

22

## What is the resistance of an ideal voltmeter

### infinity

23

## Describe and explain how the resistance of a NTC thermistor varies with temperature

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As temperature increases, resistance decreases.

The increase in temperature causes atoms to vibrate and take up more space. This also causes more electrons to be released for conduction. the extra electrons released over compensates for the atoms vibrating more and taking up more space.

24

## What are the rules for current, potential difference and resistance for series circuits

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Current - same at all points

p.d. from battery shared amongst components

Resistance increases as more components are added.

25

## What are the rules for current, potential difference and resistance for parallel circuits

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Current - splits up but total current must stay the same

p.d. from battery same across each branch

Resistance decreases as electrons have more branches/paths to take

26