Electricity

Question 1

Define current (I)

Answer 1

The rate of flow of charge

Question 2

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

Answer 2

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

Question 3

What is the difference between conventional current and electron flow?

Answer 3

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

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

Question 4

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

Answer 4

Increasing potential difference increases the current

Increasing resistance decreases the current

Question 5

What is Ohm’s law?

Answer 5

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

Question 6

When does Ohm’s law apply?

Answer 6

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

Question 7

Define potential difference

Answer 7

The work done (energy transferred) by each coulomb of charge moving between two points

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

Question 8

How does a circuit ‘short circuit’?

Answer 8

If there is an available path with 0 resistance

Current → ∞

And the circuit heats up

Question 9

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

Answer 9

Question 10

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

Answer 10

Question 11

What is the graph for a semiconductor diode?

Answer 11

Question 12

What’s wrong with this?

Answer 12

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

Question 13

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

Answer 13

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.

Question 14

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

Answer 14

The straight line passing through the origin

proves that current ∝ voltage

Question 15

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

Answer 15

• 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

Question 16

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

Answer 16

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

The diode breaks and all current flows through

Question 17

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

Answer 17

Parallel circuits have junctions (3 or more wires connect)

Question 18

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

Answer 18

Voltmeters have ~ ∞ R so no current flows through

Question 19

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

Answer 19

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

Current is constant throughout

Question 20

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

Answer 20

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

Current is constant throughout

Question 21

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

Answer 21

P.D is same for parallel branches

Current separates at junctions (according to branch resistance)

Question 22

What is Kirchoff’s 1st Law?

Answer 22

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.

Question 23

What is Kirchoff’s 2nd Law?

Answer 23

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

Question 24

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

Answer 24

Add up their resistances

Question 25

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

Answer 25

Use the following equation…

Question 26

What is the advantage of placing resistors in parallel arrangements?

Answer 26

The **total resistance is always less** than the smallest resistance

Question 27

Will the current split equally?

Answer 27

No, because the **resistance of each branch is different**

Question 28

Will each component receive the same voltage?

Answer 28

No, because the **resistance of the components are different**

Question 29

Why would you place **batteries in parallel**?

Answer 29

* The power delivered is the same * But they take longer to run flatter

Question 30

What is a potential divider circuit?

Answer 30

A circuit with **2 or more resistors connected in series** with a power supply. (usually one is a thermistor or LDR)

Question 31

How does resistance change for an **NTC Thermistor?**

Answer 31

As **temperature increases, resistance decreases**

Question 32

How does resistance change for a **Light Dependent Resistor (LDR)?**

Answer 32

As **light intensity increases, resistance decreases**

Question 33

What is the advantage of setting up a **rheostat** as a **variable resistor?**

Answer 33

* Simpler circuit * Current constant throughout * But cannot get 0V across bulb

Question 34

What is the advantage of setting up a **rheostat** as a **potential divider?**

Answer 34

* Bulb can receive full range of voltage 0V → V_source * Current through bulb can be reduced to 0A * But maximum current is lower

Question 35

How does changing the **dimensions** of a piece of metal **affect its resistance?**

Answer 35

* Increased length → increased resistance * Increases cross sectional area → decreased resistance * Increased resistivity (using different material) → increased resistance

Question 36

How do you calculate the **cross sectional area of a wire?**

Answer 36

Assume it to be a cylinder (unless told otherwise) ## Footnote **A=∏r²**

Question 37

Why do metals with a greater cross sectional area have a lower resistance?

Answer 37

There are **more paths for the electrons to propagate**

Question 38

How do you calculate the potential difference across branches?

Answer 38

* Work out the P.D of each component * Make a loop connecting the branches * Subtract the PDs of one branch from the other

Question 39

What is a superconductor?

Answer 39

A material with **0 resistance at and below the critical temperature**

Question 40

Why does a material become **superconducting at and below its critical temperature?**

Answer 40

* The **lattice ion vibrations reduce to 0** * So **electrons can pass** through **without collision**

Question 41

What is the advantage of superconductors and name a use?

Answer 41

* Transmit large currents with 0 resistance * So negligible thermal energy losses * Used to create high power magnets → MRI machines * High processing power circuits → Supercomputers

Question 42

Define **emf** of a power source

Answer 42

The **potential difference across the terminals** when **no current** is flowing through

Question 43

Define **terminal potential difference** of a circuit

Answer 43

The **potential difference** across the terminals **when a current** is flowing through

Question 44

What is the **lost voltage** in a circuit?

Answer 44

The potential difference used up pushing a current through the battery (**v_lost = emf - TPD**)

Question 45

How should you work with a circuit **involving internal resistance?**

Answer 45

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)