Electricity

Question 1

What is electric current?

Answer 1

Electric current is the flow of electrical charge.

Question 2

What is potential difference?

Answer 2

The driving force that pushes charge round a circuit. Potential difference is energy transferred per charge passed.

Question 3

What is the general relationship between resistance and current?

Answer 3

The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component).

Question 4

What is charge?

Answer 4

Basically, charge is like the electrons in the circuit.

Question 5

What is the relationship between resistance and potential difference in ohmic conductors?

Answer 5

At a constant temperature, the current flowing though an ohmic conductor is directly proportional to the potential difference across it.

Question 6

How does the resistance change in a filament lamp?

Answer 6

When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament, which is designed to heat up. Resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases.

Question 7

How does an LDR work and what are they used for?

Answer 7

In bright light, the resistance is low. In darkness, the resistance is highest. They are used for automatic night lights, outdoor lighting and burglar detectors.

Question 8

How does a thermistor work and what are they used for?

Answer 8

In hot conditions, the resistance is low. In cold conditions, the resistance goes up. They are used in temperature detectors.

Question 9

What are the rules of series circuits?

Answer 9

Potential difference is shared (Vtotal = V1 + V2 + …).
Current is the same everywhere.
Resistance adds up (Rtotal = R1 + R2 + …).
More cells means a bigger potential difference.
Voltmeters are always connected in parallel.

Question 10

Why does resistance add up in a series circuit?

Answer 10

Because by adding a resistor in series, the two resistors have to share the total pd. The potential difference across each resistor is lower, so the current through each resistor is also lower. In a series circuit, the current is the same everywhere so the total current in the circuit is reduced when a resistor is added. This means the total resistance of the circuit increases. The bigger a component’s resistance, the bigger its share of the total potential difference.

Question 11

What are the rules of parallel circuits?

Answer 11

Potential difference is the same across all components.
Current is shared between branches. The total current going into a junction has to equal the total current leaving.
Adding a resistor in parallel reduces the total resistance.

Question 12

Why does adding a resistor in parallel reduce the total resistance?

Answer 12

In parallel, both resistors have the same potential difference cross them as the source. This means the ‘pushing force’ making the current flow is the same as the source pd for each resistor that you add. But by adding another loop, the current has more than one direction to go in. This increases the total current that can flow around the circuit. Using V = IR, an increase in current means a decrease in the total resistance of the circuit.

Question 13

What is alternating current (ac)?

Answer 13

Current that is constantly changing directiob.

Question 14

What is the potential difference and frequency of the UK mains supply (the electricity in your home)?

Answer 14

Potential difference of 230V, Frequency of 50Hz.

Question 15

What is the colour and function of a neutral wire?

Answer 15

A neutral wire is blue. The neutral wire completes the circuit and carries away current. It is around 0V.

Question 16

What is the colour and function of a live wire?

Answer 16

A live wire is brown. The live wire provides the alternating potential difference from the mains supply. It is about 230V.

Question 17

What is the colour and function of a earth wire?

Answer 17

A earth wire is green and yellow. The earth wire is for protecting the wiring, and for safety - it stops the appliance casing from becoming live. It is at 0V.

Question 18

How does touching a live wire give you an electric shock?

Answer 18

Your body (just like an earth wire) is at 0V. This means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you.

Question 19

Why does the national grid use a high potential difference and a low current?

Answer 19

To transmit the huge amount of power needed, you need either a high potential difference or a high current. The problem with a high current is that you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings. It’s much cheaper to boost the pd up really high and keep the current as low as possible. For a given power, increasing the pd decreases the current, which decreases the energy lost by heating to the wires and surroundings. This makes the national grid an efficient way of transferring energy.

Question 20

What is the function of transformers?

Answer 20

Step-up transformers have to step the potential difference up at one end, for efficient transmission, and then bring it back down to safe, usable levels at the other end using step-down transformers.

Question 21

How is build up of static caused by friction?

Answer 21

When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other. This will leave the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other.

Question 22

How does too much static cause sparks?

Answer 22

As electric charge builds up on an object, the potential difference between the object and the earth increases. If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth - this is the spark. They can also jump to any earthed conductor that is nearby (e.g. a human).

Question 23

What are the rules of electric fields?

Answer 23

Electric fields are created around any electrically charged object. The closer to the object you get, the stronger the field is. You can show an electric field around an object using field lines.

Question 24

Explain how a charged object feels a force in an electric field.

Answer 24

When a charged object is placed in the electric field of another object, it feel a force. This force causes a attraction or repulsion. The force is caused by the electric fields of each charged object interacting with each other. The force on an object is linked to the strength of the electric field it is in. As you increase the distance between the charged objects, the strength of the field decreases and the force between them gets smaller.