Topic 2- Electricity:

Question 1

What is energy efficiency?

Answer 1

A higher percentage of energy input is usefully transferred.

Question 2

What is electric current?

Answer 2

The flow of electrical charge.

Question 3

What can be said aout the value of current at any point in a single closed loop?

Answer 3

Current is the same at all points in a closed loop.

Question 4

What two factors does the current in a circut depend on?

Answer 4

• Potential difference (V)
• Resistance (R)

Question 5

What is an ‘ohmic conductor’? State the condition required.

Answer 5

• A conductor for which current and potential difference are directly proportional.
• Resistance remains constant as current changes
• Temperature must be constant

Question 6

List four components for which resistance is not constant as current changes.

Answer 6

1) Lamps
2) Diodes
3) Thermistors
4) Light dependant resistors (LDR’s)

Question 7

What happens to the resistance of a filament lamp as temperature increases? Why?

Answer 7

• Resistance increases
• Ions in the metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow.

Question 8

What is different about current flow through a diode?

Answer 8

• The current only flows in one direction
• Resistance is very high on the other end, preventing current flow

Question 9

What happens to the resistance of a thermistor as temperature increases?

Answer 9

The thermistor’s resistance decreases.

Question 10

Give two examples of when a thermistor may be used.

Answer 10

• In a thermostat- to turn a heater on to a certain temperature.
• In a freezer to turn on a cooler when the temperature becomes too high.

Question 11

What happens to the resistance of the LDR as light intensity DECREASES?

Answer 11

The LDR’s resistance increases.

Question 12

Give an appliaction for an LDR.

Answer 12

• Street lights
• When ilght levels become too low, the light gains sufficient current to turn on

Question 13

What are two ways that a component can be connected in a circut?

Answer 13

• Series
• Parallel

Question 14

How does p.d. accross two components vary when connected in series and parallel?

Answer 14

• Series: total p.d. is shared between each component
• Parallel: p.d. is the same accross each component

Question 15

If 2 resistors are connected in parallel, what can be said about their combined total resistance?

Answer 15

• Their total resistance is less than the smallest of the 2 individual resistors.

Question 16

If two resistors are connected in series, what can be said about their total resistance?

Answer 16

Their total combined resistance is equal to the sum of the two individual resistances.

Question 17

Describe the current in a series cicut.

Answer 17

• The current is the same at all positions
• Because the charge has only one path to flow through

Question 18

Describe the current in a parallel circut.

Answer 18

• The current is shared between different branches
• When the charge reaches a junction, it splits.

Question 19

How is an ammeter connected to measure current?

Answer 19

In series with the component that tthey are measuring current through.

Question 20

How is a volmeter connected to measure p.d.?

Answer 20

Voltmeters should be connected parallel to the commponent they are measuring the p.d. of.

Question 21

Why is it advantageous to connect lamps in parallel?

Answer 21

• If one lamp blows
• The rest are unaffected and can still recieve the current (the circut is still complete)

Question 22

Is mains an a.c supply or a d.c supply?

Answer 22

Mains is an a.c. supply.

Question 23

Define alternating current and direct current.

Answer 23

• Alternating current: Current that continuosly changes direction at a specific frequency
• Direct current: One directional current flow

Question 24

What is the frequency and voltage of the uk mains electricity supply?

Answer 24

• Frequency: 50 Hz
• Voltage: 230 V

Question 25

What **three wires** found in **cables** connect **electricity appliances to the mains?**

Answer 25

* Live wire 🟤 * Neutral wire 🔵 * Earth wire 🟡🟢

Question 26

Explain why the **earth wire may/ may not carry a current.**

Answer 26

* Under **normal circumstances,** the earth wire does **not carry a current.** * If a **fault** occurs in the appliance, **a large current will flow to the ground,** as the wire has virtually **no resistance.** This causes the **fuse to blow**, preventing any further current flow.

Question 27

What **potential** is the **neutral wire** at?

Answer 27

0 V

Question 28

State the **potential difference** between the **live wire** and **earth wire.**

Answer 28

230 Volts

Question 29

What is the **purpose** of the **neutral wire?**

Answer 29

To **complete the circut** by **connecting the appliance** back to the **mains supply.**

Question 30

For **metal appliances,** where is the **earth wire** connected to? Why?

Answer 30

* The **earth wire is connected to the metal casing** of the appliance. * If the **live wire** becomes **loose** and **touches the casing**, the **current** will flow through the **earth wire, preventing electrocution.**

Question 31

What **two main factors** does the amount of **energy transferred by an appliance depend on?**

Answer 31

1) **How long** the applance is **being used for** 2) The **power** of the appliance

Question 32

Describe the **energy transfer** in a **battery powered torch.**

Answer 32

* The **battery** converts **chemical energy** into **electrical energy** * The **bulb** converts **electrical energy** into **light** as well as **waste energy** in the form of **heating.**

Question 33

Describe the **energy transfers** in a **battery powered motor.**

Answer 33

* **Battery** converts **chemical energy** into **electrical enrgy.** * **Motor** converts **electrical energy** into **kinetic energy** as well as **waste energy** in the form of **heating** due to** friction.**

Question 34

What **3 things** determine the **power of a circut device?**

Answer 34

1) The **p.d. accross the circut** 2) The **current through the circut** 3) The **amount of energy transferred in a given time**

Question 35

Whatis the **purpose** of the **national grid?**

Answer 35

To **link power stations to consumers**- so they have **access to a source of electricity.**

Question 36

What are the **2 types of transformers** used in the **national grid?**

Answer 36

* Step-up transformers * Step-down ransformers

Question 37

**Where** are **step-up** transformers found in the **national grid?** **What do they do?**

Answer 37

* **Step-up** transformers are used when **connecting power station**s to **transmission cables** * They **increase the p.d.**

Question 38

**Where** are **step-down** transformers found in the **national grid?** **What do they do?**

Answer 38

* **Step-down** transformers are used in **connecting transmission cables** to **domestic buildings** (like houses) * The **decrease the p.d.**

Question 39

Why do **transmission lines transfer energy** at a **high potential difference?**

Answer 39

* A **high potential difference** results in a **low current** * The lower the current, the **less energy is wasted as heat** * Therefore it is **more efficient**

Question 40

Why does the **potential difference** need to be **decreased** between the **transmission lines** and **houses?**

Answer 40

* A **lower p.d.** is **safer** for **domestic use** * So it **reduces the likelyhood of a severe electrocution** * **Appliances** are designed for **230 V**

Question 41

What happens when 2 insulating materials are rubbed together?