Physics B.5 terminology

Define electric current + what is it measured in

The rate of flow of charge measure in amperes (A)

Define charge + what is it measured in

Can be either positive or negative measured in coulombs (C)

Define conventional current?

The flow of positive charge from the positive terminal to the negative terminal

What is the direction of conventional current?

From positive to negative

What is the direction of electrons in a current?

From negative to positive

Define potential difference (p.d.)

The work done per unit charge on moving a positive charge between two points along the path of the current

Define electronvolt (eV)

The amount of energy needed to move an electron through a potential difference of one volt

What are electronvolts measured in?

Joules (J)

What are conductors made up of? (on the atomic scale)

They are made up of positively charged metal ions within a sea of delocalised electrons

What are insulators?

It is a material that has no free charges, hence it does not allow the flow of charge through them very easily

What type of electricity can insulators conduct?

static electricity

When does electric resistance happen?

When free electrons collide with metal ions which resist their flow

Define resistance

The ratio of the potential difference across the component to the current flowing through it

Name three things resistance of a sample depends on

• the material it is

• the length of the sample

• the cross-sectional area of the sample

What is the relationship between resistance and length of a wire/cross-sectional area?

Resistance of a wire is

• Directly proportional to its length

• Inversely proportional to its cross-sectional area

What is the constant of proportionality for resistance?

resistivity

Define resistivity

The resistivity of a material is equal to the resistance per unit length of a material with unit cross-sectional area

State Ohm’s law

For a component at constant temperature, the current through it is proportional to the potential difference across it

Give an example of an ohmic and non-ohmic component

• fixed resistor - ohmic component

• filament lamp - non-ohmic component

Give three examples of non-ohmic devices

• lamps

• LEDs

• thermistors

When making a current-voltage graph, what would be on the y and x axis?

x-axis —>potential difference

y-axis —> current

What is a light-dependent resistor?

An LDR is a resistor whose resistance depends on the light intensity

What happens to the resistance of an LDR when light intensity increases?

The resistance decreases

What happens to the resistance of a thermistor when the temperature it receives increases?

The resistance decreases

What is a potentiometer?

A resistor with a sliding contact to form an adjustable voltage divider

What is a diode?

A diode is a two-terminal electronic component that conducts electricity primarily in one direction . It has high resistance on one end and low resistance on the other end

Which graph (blue or red) represents the resistance in either a semiconductor diode or a resistor?

blue → resistor

red → semiconductor diode

What happens to the current if a diode is reverse biased?

The current through the diode is zero

What component has these I-V characteristics and produces this graph?

A filament lamp

Under what circumstances can a filament lamp behave as an ohmic component and why?

When it has very small voltages going through it (it doesn’t heat up)

Define direct current

Direct current is current that only flows in one direction and has one value

Explain resistance on an atomic level

1. Free electrons collide with metal ions which resist their flow

2. This transfer of energy results in an increase in the kinetic energy of the atoms in the lattice

3. This raises the overall internal energy of the metal

4. The macroscopic result of this transfer is the heating up of the wire which causes resistance

What is the resistance of an ideal voltmeter?

It is infinite and no current passes through it

What is the resistance of an ideal ammeter?

It is zero and all the current passes through it

The combined resistance of resistors connected in parallel is (more/less) than the resistance of any of the individual components

less

Define electrical power

Electrical power is the rate of change of work done

Name 6 main sources of electrical energy

1. Electric cells

2. Chemical cells / batteries

3. Solar cells

4. Mains electricity

5. Wind generators

What do chemical cells (batteries) do?

They utilise chemical reactions to provide a potential difference

(can be rechargeable or non-rechargeable)

What do photovoltaic (solar) cells in solar panels do?

They convert electromagnetic radiation (photons) from the Sun into electrical energy

What happens when photons from the Sun are incident on the solar heating panels?

1. The light energy is transferred into thermal energy

2. The solar photons are absorbed by electrons on the surface of the photovoltaic cells, giving them enough energy to move and be released from the surface

3. These electrons transfer the thermal energy into electric energy which is then transferred to the external circuit

Name 3 advantages and 3 disadvantages of single-use batteries

Advantages:

1. High energy density

2. Portable source of electrical energy

3. Potential to join many is series to increase p.d.

Disadvantages:

1. Non-rechargeable/limited power supply

2. High internal resistance

3. Made from non-renewable materials

Name 3 advantages and 2 disadvantages of mobile phone batteries

Advantages:

1. High electrical efficiency

2. Rechargeable, long lifetime

3. Fast charging time

Disadvantages:

1. Capacity of the cell degrades over time

2. Internal resistance increases over time

Name 2 advantages and 3 disadvantages of car batteries

Advantages:

1. Rechargeable

2. Delivers high currents in a short time

Disadvantages:

1. Low energy density - very heavy compared to power output

2. Internal resistance increases as battery degrades

3. Made from non-renewable materials

Name 3 advantages and 3 disadvantages of a solar cell

Advantages:

1. Unlimited supply of energy - renewable

2. Cheap maintenance

3. No fuel required

Disadvantages:

1. Variable output - highly weather dependent

2. Only available during the day

3. Requires large areas

Name 2 advantages and 3 disadvantages of a wind generator

Advantages:

1. Unlimited supply of energy - renewable

2. High set-up cost but becomes economical

Disadvantages:

1. Variable output - highly weather dependent

2. Requires favourable local conditions to be placed in windy locations

3. Noise/visual pollution

Name 3 advantages and 2 disadvantages of a mains electricity

Advantages:

1. High energy density of fuel

2. Reliable/available energy at any time

3. Well-known and developed technology

Disadvantages:

1. Produces greenhouse gases

2. Unsustainable (non-renewable)

Define electromotive force (e.m.f.)

EMF is the amount of chemical energy converted to electrical energy per coulomb of charge (C) when charge passes through a power supply

Name two sources of EMF

Cells and batteries