GCSE - Physics - Electricity HT

What is electric current?

The flow of electrcial charge

What is current like in series?

the same everywhere

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

Potential Difference (V)

Resistance (R)

What is an 'Ohmic Conductor'? State the condition required.

● A conductor for which current and potential difference are directly proportional

● Resistance remains constant as current changes

● Temperature must be constant

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

- lamps

- diodes

- thermistors

- light dependant resistors (LDRs)

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

Resistance increases

Ions in the metal have more energy, so vibrate more, causing collisions with electrons as they flow through the metal

Creating a greater resistance to current flow

What is different about current flow through a diode?

- The current only flows in one direction

- Resistance is very high in the other direction, preventing current flow

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

resistance decreases

When may a thermistor be use?

To turn heating on when a temp is reached

To cool down a freezer when temp gets too high

What happens to the resistance of a LDR as light intensity decreases?

The LDR's resistance increases.

When may a LDR be used?

Street lights -> light levels too low the light gains sufficient current to turn on

How can complements be connected into a circuit

Series

Parallel

What is potential difference like in series and parallel?

Series -> shared between components

Parallel -> same across each component

Resistors in parallel

The total resistance is less than the smallest of the two individual resistances

Resistance in series

sum of all resistances

Current is series

The same everywhere

As charge has only one path to flow

Current in parallel

Shared between branches

How do you connect an ammeter in a circuit?

series to the component measuring

How to connect a voltmeter in a circuit

parallel to the component measuring

Why is it good to connect lamps in parallel?

The other lamps won't be affected if one lamp goes

What is mains electricity?

an ac supply

what does ac mean?

alternating current

What does dc mean?

direct current

What is an alternating current?

current that continuously changes direction

What is direct current?

Current that flows in only one direction

What is the frequency and voltage if the U.K. mains electricity supply?

Frequency - 50Hz

Voltage - 230V

How many wires are their in cables?

Three

Names the wires in cables

Live wire, neutral wires, earth wire

What is the colour if the earth wire?

Yellow and green

What is the colour of the live wire?

brown

What is the colour of the neutral wire?

blue

How does the earth wire work?

Usually it has no current

When a fault occurs is will allow the current to flow to the ground

What potential is the neutral wire at?

0 volts

What is the potential difference of the live wire?

230V

What is the purpose of the neutral wire?

To complete the circuit by connecting the appliance back to the mains supply.

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

● Earth wire is connected to the metal casing of the appliance

● If live wire becomes loose and touches the casing, the current will flow through the Earth wire, preventing electrocution

What are the two main factors that the amount of energy transferred by an appliance depends on?

How long the appliance is being used for

The power of the appliance

Describe the energy transfers in a battery powered torch

● Battery converts chemical energy into electrical energy

● Bulb converts electrical energy into light as well as waste energy in the form of heating

Describe the energy transfers in a battery powered motor

● Battery converts chemical energy into electrical energy

● Motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction

What three things determine the power of a circuit device

1. The potential difference across the circuit

2. The current through the circuit

3. The amount of energy transferred in a

given time

What is the purpose of the national grid?

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

What are the two types of transformers used in the national grid?

1. Step-Up Transformers

2. Step-Down Transformers

Where are step-up transformers found in the National Grid? What do they do?

● Step-Up Transformers are used when connecting power stations to transmission cables

● They increase the potential difference

Where are step-down transformers found in the National Grid? What do they do?

● Step-Down Transformers are used in connecting transmission cables to domestic buildings (like houses)

● They decrease the potential difference

Why do transmission lines transfer electricity at high potentials?

● A high potential, results in a low current

● The lower the current, the less energy that is wasted as heat

● Therefore it is more efficient

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

● Lower potentials are safer for domestic use and reduces the likelihood of

severe electrocution

● Appliances are designed for 230V

What can happen when insulating materials are rubbed together?

They can become (statically) electrically charged.

Why can insulators become electrically charged when rubbed together?

● Electrons are rubbed from one material onto the

other

● The material gaining electrons becomes

negatively charged

● The material losing electrons becomes equally

positively charged

What happens when two electrically changed objects are brought close together?

They exert a force on eachother

What happens when two identically charged objects are brought close together?

They exert a repulsive force on each other and repel.

What happens when two oppositely charged objects are brought close together?

They exert an attractive force on each other and attract.

Give an example of a non-contact force

The repulsive or attractive force acting between two electrically charged objects.

What is an electric field?

A region in which a charged particle will experience a non-contact force

Where can electric fields be found?

Surrounding any charged object.

Describe the electric field around a charged particle

● Strongest closest to the object

● Decreases in strength as you move

away from the object

What happens to the force between two charged objects when they are moved closer together?

The force between them becomes stronger as the separation reduces.

What precautions can be taken to prevent the build up of static charge?

Any surfaces rubbing against each-other should be earthed to allow the charge to flow off the materials