Electricity

Question 1

Define current (I)

Answer 1

The rate of flow of charge

Question 2

What is the difference between conventional current and electron flow?

Answer 2

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

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

Question 3

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

Answer 3

Increasing potential difference increases the current

Increasing resistance decreases the current

Question 4

What is Ohm’s law?

Answer 4

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

Question 5

When does Ohm’s law apply?

Answer 5

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

Question 6

Define potential difference

Answer 6

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 7

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

Answer 7

Question 8

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

Answer 8

Question 9

What is the graph for a semiconductor diode?

Answer 9

Question 10

What’s wrong with this?

Answer 10

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 11

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

Answer 11

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 12

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

Answer 12

The straight line passing through the origin

proves that current ∝ voltage

Question 13

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

Answer 13

• 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 14

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

Answer 14

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

The diode breaks and all current flows through

Question 15

Why doesn’t adding voltmeters in parallel affect the circuit?

Answer 15

Voltmeters have ~ ∞ R so no current flows through

Question 16

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

Answer 16

P.D is divided across the components (proportional to resistance)

Current is constant throughout

Question 17

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

Answer 17

P.D is same for parallel branches

Current separates at junctions (according to branch resistance)

Question 18

What is Kirchoff’s 1st Law?

Answer 18

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 19

What is Kirchoff’s 2nd Law?

Answer 19

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

Question 20

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

Answer 20

Add up their resistances

Question 21

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

Answer 21

Use the following equation…

Question 22

What is the advantage of placing resistors in parallel arrangements?

Answer 22

The total resistance is always less than the smallest resistance

Question 23

Will the current split equally?

Answer 23

No, because the resistance of each branch is different

Question 24

Will each component receive the same voltage?

Answer 24

No, because the resistance of the components are different

Question 25

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

Answer 25

* The same potential difference delivered to the same resistance * Less current needs to flow through each cell * But they take longer to run flatter ## Footnote (when internal resistance = 0)

Question 26

What is a potential divider circuit?

Answer 26

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

Question 27

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

Answer 27

As **temperature increases, resistance decreases**

Question 28

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

Answer 28

As **light intensity increases, resistance decreases**

Question 29

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

Answer 29

* Simpler circuit * But cannot get 0V across bulb

Question 30

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

Answer 30

* Bulb can receive full range of voltage 0V → V_source * Current through bulb can be reduced to 0A

Question 31

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

Answer 31

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

Question 32

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

Answer 32

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

Question 33

What is a superconductor?

Answer 33

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

Question 34

What is the advantage of superconductors and name a use?

Answer 34

* 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 35

Define **emf** of a power source

Answer 35

The amount of energy per coulomb of charge transferred from chemical energy to electrical energy inside a cell

Question 36

Define **terminal potential difference** of a circuit

Answer 36

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

Question 37

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

Answer 37

The potential difference used up inside the battery (**v_lost = emf - TPD**)

Question 38

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

Answer 38

Treat the internal resistance as another resistor in series with the cell Then solve as a regular circuit (using ohm's law, kirchoff's laws, P=IV etc)

Question 39

When reading an **electricity question** what should you first be looking out for?

Answer 39

The phrase 'negligible internal resistance'

Question 40

What should you remember about an **ideal voltmeter**?

Answer 40

* Resistance of voltmeter effectively infinite * No current goes through the voltmeter * Has no effect on circuit

Question 41

What should you remember about a **non-ideal voltmeter**?

Answer 41

* Has a large resistance (but not infinite) * Small current passes through * **Treat as a resistor in parallel**

Question 42

What should you remember about an **ideal ammeter**?

Answer 42

* Resistance of ammeter effectively 0 * No potential drop across ammeter * Has no effect on circuit

Question 43

What should you remember about a **non-ideal ammeter**?

Answer 43

* Has a small resistance (but not zero) * Small potential drop across * **Treat as a resistor in series**

Question 44

What is a **kilowatt hour**?

Answer 44

How many hours a power source could deliver a power of 1kW for (Measure of total energy stored in power source)

Question 45

What is an **Amp-hour**?

Answer 45

How many hours a power source could deliver a current of 1A for (Measure of total charge stored in power source)