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Henry Physics RSFC > Electricity > Flashcards

Electricity Flashcards

1
Q

Define current (I)

A

The rate of flow of charge

(A) Coulombs per second

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2
Q

How do you work out the number of electrons carrying a charge (eg 10C)?

A

Divide charge by the charge of each electron (1.6x10-19)

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3
Q

What is the difference between conventional current and electron flow?

A

Conventional current flows from the +ve terminal to the -ve terminal

Electron flow shows the direction the electrons flow, from -ve to +ve

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4
Q

How is the current in a circuit related to potential difference and resistance?

A

Increasing potential difference increases the current

Increasing resistance decreases the current

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5
Q

What is Ohm’s law?

A

The current flowing through a metallic conductor is proportional to the potential difference applied across it at constant temperature

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6
Q

When does Ohm’s law apply?

A

When the component has a fixed resistance (eg a fixed resistor at a constant temperature, or a filament at a low current)

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7
Q

Define potential difference

A

The work done (energy transferred) by each coulomb of charge moving between two points (measured in JC-1)

(Eg a 12V battery adds 12J of energy to each coulomb of charge passing through)

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8
Q

How does a circuit ‘short circuit’?

A

If there is an available path with 0 resistance

Current → ∞

And the circuit heats up

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9
Q

What is the I-V graph for a fixed resistor?

A
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10
Q

What is the I-V graph for a filament bulb?

A
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11
Q

What is the graph for a semiconductor diode?

A
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12
Q

What’s wrong with this?

A

Resistance is not calculated using the gradient (of a tangent) of an I-V graph!!!

Instead just use the voltage and current at that point

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13
Q

Explain the shape of the I-V graph for a filament

A

As current increases, temperature of filament increases

This increases lattice ion vibrations.

Which increases the number of collisions per second with electrons.

So resistance increases.

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14
Q

How does the I-V graph for a fixed resistor prove it is ohmic?

A

The straight line passing through the origin

proves that current ∝ voltage

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15
Q

Explain the shape of the semiconductor diode (in positive bias)

A
  • As the potential difference increases weakly bound electrons in the conductor gain energy
  • After the threshold pd, some electrons become free to carry a current
  • The lattice vibrations still increase but this is less significant
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16
Q

What happens if a semiconductor diode is connected in reverse bias?

A

No current flows until the breakdown voltage is reached (~50V)

The diode breaks and all current flows through

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17
Q

What is the difference between a series and a parallel circuit?

A

Parallel circuits have junctions (3 or more wires connect)

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18
Q

Why doesn’t adding voltmeters in parallel affect the circuit? (it is still series)

A

Voltmeters have ~ ∞ R so no current flows through

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19
Q

What are the p.d and current rules for a series circuit?

A

P.D is shared across the components (by resistance)

Current is the same for every component

20
Q

What are the p.d and current rules for a parallel circuit?

A

P.D is same for parallel branches

Current separates at junctions (according to branch resistance)

(current for each component on the same branch is equal!!)

21
Q

What is Kirchoff’s 1st Law?

A

At any junction in a circuit the sum of the current flowing into the junction is equal to the sum of the current flowing away from it.

22
Q

What is Kirchoff’s 2nd Law?

A

In any complete “loop” of a circuit the sum of p.d’s equals the source p.d.

23
Q

How do you combine series resistors in the same branch? (no junction between them)

A

Add up their resistances

24
Q

How do you combine resistors in parallel branches? (one junction between them)

A

Use the following equation…

25
What is the advantage of placing resistors in parallel arrangements?
The **total resistance is always less** than the smallest resistance
26
Will the current split equally?
No, because the **resistance of each branch is different**
27
Will each component receive the same voltage?
No, because the **resistance of the components are different**
28
Why would you place **batteries in parallel**?
* The power delivered is the same * But they take longer to run flatter
29
What is a potential divider circuit?
A circuit with **2 or more resistors connected in series** with a power supply. (usually one is a thermistor or LDR)
30
How does resistance change for an **NTC Thermistor?**
As **temperature increases, resistance decreases**
31
How does resistance change for a **Light Dependent Resistor (LDR)?**
As **light intensity increases, resistance decreases**
32
What is the advantage of setting up a **rheostat** as a **variable resistor?**
* Simpler circuit * Current constant throughout * But cannot get 0V across bulb
33
What is the advantage of setting up a **rheostat** as a **potential divider?**
* Bulb can receive full range of voltage 0V → Vsource * Current through bulb can be reduced to 0A * But maximum current is lower
34
How does changing the **dimensions** of a piece of metal **affect its resistance?**
* Increased length → increased resistance * Increases cross sectional area → decreased resistance * Increased resistivity (using different material) → increased resistance
35
How do you calculate the **cross sectional area of a wire?**
Assume it to be a cylinder (unless told otherwise) ## Footnote **A=∏r2**
36
Why do metals with a greater cross sectional area have a lower resistance?
There are **more paths for the electrons to propagate**
37
How do you calculate the potential difference across branches?
* Work out the P.D of each component (from V = IR) * Make a loop connecting the branches (with opposite components) * Subtract the PDs of one branch from the other
38
What is a superconductor?
A material with **0 resistance at and below the critical temperature**
39
Why does a material become **superconducting at and below its critical temperature?**
* The **lattice ion vibrations reduce to 0** * So **electrons can pass** through **without collision**
40
What is the advantage of superconductors and name a use?
* Transmit large currents with 0 resistance * So negligible thermal energy losses * Used to create high power magnets → MRI machines * High processing power circuits → Supercomputers
41
Define **emf** of a power source
The **potential difference across the terminals** when **no current** is flowing through
42
Define **terminal potential difference** of a circuit
The **potential difference** across the terminals **when a current** is flowing through
43
What is the **lost voltage** in a circuit?
The potential difference used up pushing a current through the battery (**vlost = emf - TPD**)
44
How should you work with a circuit **involving internal resistance?**
Treat the internal resistance as another resistor in series with the components Then solve as a regular circuit (using ohm's law, kirchoff's laws, P=IV etc)
45
What can be said of batteries placed in series?
Batteries will deliver double the power but will lose charge at the same rate as a single battery
46
What is a sensor circuit?
A circuit where external factors affect the voltage distribution An example would be a potential divider circuit with a LDR attached

Henry Physics RSFC (27 decks)

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