Chapter 5 - Electricity and Magnetism Flashcards

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1
Q

What is conventional current?

A

It’s a current that flows from positive to negative.

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2
Q

What are conduction electrons?

A

They are negative electrons that flow the opposite way than conventional current does in a circuit.

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3
Q

Why does the metal wires heat up in a circuit?

A

As the conduction electrons move the interaction between them and the lattice ions means work is done, so when the current flows the metal heats up.

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4
Q

What is drift velocity?

A

The speed of the electrons due to the current.

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5
Q

What’s the equation to calculate the electrical conduction?

A
I = nAvq
n = number of density of the charge-carries (the number per unit volume that are available to move) 
q = the charge on each carrier 
v = average speed
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6
Q

What is current and its units?

A

Rate of flow of electrical charge
1A = 1Cs^-1
1As = 1C

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7
Q

What is an ohmic material?

A

If current and potential difference are proportional

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8
Q

What’s Ohm’s law?

A

The current flowing through a piece of metal is proportional to the potential difference across is providing that temperature remains constant.

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9
Q

What’s a resistor?

A

A device with constant resistance. (Aka an ohmic device)

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10
Q

What’s resistance?

A

The mathematical ratio between potential difference and current:

Potential difference/ current
1 ohm = 1VA ^ -1

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11
Q

What would you need for a current to flow if something had a high resistance?

A

A large potential difference

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12
Q

What are the three equations for power?

A
p = V x I 
p = I^2 x R 
p = V^2 / R
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13
Q

What’s the first conversation law?

A

The conversation of charge (it’s never created or destroyed only transformed)
The sum of I = 0 (junction)
Current flowing in a junction is the same as the current flowing out.

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14
Q

What’s the second conservation law?

A

The conservation of energy.
Sum of voltage = zero (loop)
Around any loop total energy per unit charge must sum zero.

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15
Q

What happens if the chosen loop direction is from the negative side of the battery to its positive side?

A

It’s an increase in pd and the value is positive when calculating the sum

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16
Q

What happens of ten direction around the loop is in the same direction as the current flowing through the component?

A

This is a potential drop and the value is negative when calculating the sum.

17
Q

What’s the relationship between pd, energy and charge?

A

V = E/Q

18
Q

Series circuits

A

Current is the same everywhere due to it being one loop so charge is conserved. pd is shared among the components.

19
Q

Features of an ammeter

A

Connected in series

A perfect ammeter would have zero resistance

20
Q

Features of a voltmeter

A

Placed in parallel with the component

A perfect voltmeter would have infinite resistance

21
Q

Parallel circuits

A

Each component has the same pd across it

Total current is the addition of currents in each branch

22
Q

What does a potential divider do?

A

It’s 2 resistors that divide up the potential difference of the battery.

23
Q

What’s an potentiometer?

A

A variable potential divider that is often the best way to produce a variable power supply.

24
Q

What is a light-dependant resistor?

A

A device whose resistance depends on the amount of light shining on the surface.

25
Q

What effect does a change of light shining on the LDR cause?

A

An increase of light causes a decrease in resistance

When light is shining on the LDR: decrease in pd across it, and a increase in pd across the fixed resistor.

26
Q

What’s a thermistor?

A

It’s a resistor whose value of resistance depends on its temperature.
An increase in temperature causes a decrease in resistance, as more current can flow through.
As it heats up the lamp gets brighter.

27
Q

What does the output of pd of a sensor circuit depend on?

A

On an external force

28
Q

How does a change of temperature effect the of pd the fixed resistor?

A

As temperature of the thermistor increases, the pd of the fixed resistor will increase.

29
Q

What is resistivity?

A

The resistivity, p, of a material is defined in terms of its resistance R, its length l, and it’s cross-sectional area A.
Unit: Ohm metres

30
Q

What happens to the resistivity if ‘l’ increases.

A

Increasing l is like putting another resistor in series. Doubling l is like putting an identical resistor in series

Doubling l means doubling R.
A graph of l vs R is a straight line through the origin

31
Q

What happens to the resistivity if you increase A?

A

Doubling A means halving R.

So the graph of R vs 1/A will be a straight line going through the origin.

32
Q

What’s the rule for applying Kirchoff’s laws?

A

With chosen direction around the loop in the direction shown, E and IR are both positive in the kirchoff equation.

Sum of emf = sum of IR

If chosen the direction opposite to that shown, values are negative.

33
Q

Equation for Emf, internal resistance.

A
V = IR = E - Ir 
E = V + Ir = IR + Ir
34
Q

Equation for potential dividers

A

V(1):V(2) = R(1):R(2)

V(out) = (R out /R total) x V total

35
Q

What’s the relationship between voltage and work done?

A

Voltage = work done / charge

36
Q

What’s the ratio between resistance and potential difference?

A

High resistance = large pd across it in order to get a current to flow.

37
Q

What material in particular obeys ohms law?

A

Metals

38
Q

How does connecting a filament lamp in series with thermistor protect components from an initial surge current?

A

Thermistor has a high resistance when it’s cold, so there isn’t an overflow in current

39
Q

What’s the short-cut for 2 resistors in parallel?

A

Divide on the the resistors resistance by 2