Current & Circuits (DP IB Physics)

A thermistor is a resistor whose resistance changes depending on its temperature.

This circuit symbol represents a light dependent resistor (LDR).

A potentiometer is a resistor with a sliding contact that forms an adjustable voltage divider.

Electric current is the rate of flow of charge carriers.

The charged particles that flow in electrical circuits are electrons.

Conventional current is the flow of positive charge from the positive terminal of a cell to the negative terminal.

Direct current (d.c.) flows around a circuit in one direction.

The equation for current as the flow of charge is:

Where:

• = current, measured in amps (A)

• = change in charge, measured in coulombs (C)

• = change in time, measured in seconds (s)

True.

Electrons do flow around a circuit towards the positive terminal, while conventional current flows towards the negative terminal. It is electrons that actually form the current, but the idea of conventional current was in use before the electron was discovered.

The current in the circuit is 10 A.

The amount of charge that has passed through the ammeter in this time is 3 C.

Electric potential difference is the work done per unit charge on moving a positive charge between two points along the path of the current.

The equation linking potential difference, work done and charge is

Where:

• = potential difference, measured in volts (V)

• = work done, measured in joules (J)

• = charge, measured in coulombs (C)

True.

One volt is equal to one joule per coulomb, 1 V = 1 J C^–1.

False.

A cell or battery provides the source of potential difference in a d.c. circuit.

False.

The electrons in the circuit are the delocalised electrons in the metal of the wires and components. They flow around the closed circuit when a potential difference is applied.

The electronvolt is the amount of energy needed to move an electron through a potential difference of one volt.

One electronvolt is equal to .

12 eV in J is

An electrical conductor is a material that allows charge (usually electrons) to flow through it easily.

Examples of electrical conductors are:

• copper

• silver

• aluminium

• steel

• gold

Metals are good electrical conductors because metals have delocalised electrons that flow when a potential difference is applied.

An electrical insulator is a material that has no delocalised electrons, so it does not allow charge to flow easily.

Examples of electrical insulators are:

• rubber

• plastic

• glass

• wood

Static electricity occurs when charge builds up on the surface of an insulator and is discharged when brought into contact with a conductor.

An ideal ammeter has zero resistance, such that all the current passes through it.

An ideal voltmeter has infinite resistance, such that no current passes through it.

The resistance of a component is the ratio of the potential difference across the component to the current flowing through it.

The equation for resistance known as Ohm's law is

Where:

• = resistance of component, measured in ohms (Ω)

• = potential difference, measured in volts (V)

• = current, measured in (A)

False.

The greater the resistance of a component, the lower the current flowing through it.

The SI base units for 1 Ω are kg m^–2 s^–3 A^–2.

True.

For resistance calculations, mA and kV must be converted into A and V.

True.

Resistance is caused by collisions between flowing electrons and the metal ions in the conductor.

The factors affecting resistance are:

• material

• length

• cross-sectional area

• temperature

True.

The resistance of a wire is directly proportional to its length.

False.

The resistance of a wire is inversely proportional to its cross-sectional area.

Resistivity is the electrical resistance of a conductor of unit cross-sectional area and unit length.

The equation for resistivity is

Where:

• = resistivity, measured in ohm metres (Ω m)

• = resistance, measured in ohms (Ω)

• = cross-sectional area, measured in metres squared (m²)

• = length, measured in metres (m)

Metals have low values of resistivity, and insulators have high values.

The equation for the cross-sectional area of a circle is

Where:

• = cross-sectional area, measured in metres squared (m²)

• = radius of circle, measured in metres (m)

The equation for the cross-sectional area of a circle in terms of diameter is

Where:

• = cross-sectional area, measured in metres squared (m²)

• = diameter of circle, measured in metres (m)

The gradient of an I-V graph represents the resistance of a component.

A linear I-V (current-voltage) graph shows

• the relationship between current and potential difference is directly proportional

• the resistance of the component is constant

This is the I-V graph for a fixed resistor or a wire (at a constant temperature).

This is the I-V graph for a filament lamp.

This is the I-V graph for a diode.

False.

As the current through a filament lamp increases, the resistance increases.

The resistance of a filament lamp increases as the voltage is increased because

• as the voltage increases, current increases, and the lamp heats up

• as the temperature increases, metal ions in the filament wire vibrate more

• the number of collisions between the ions and free electrons increases

The I-V graph for a diode shows they have

• a high resistance when the current flows in one direction

• a low resistance when the current flows in the opposite direction

False.

Current is the same everywhere in a series circuit.

True.

In a parallel circuit, the current from the power source is larger than the current in each branch.

True.

The amount of current flowing in any circuit depends on the potential difference and resistance in the circuit.

True.

Current is conserved at a junction in a parallel circuit because charge is always conserved.

False.

Current will only split equally at a junction if the resistance in each branch is equal.

The current in a series circuit is the same through all the components.

This can be expressed mathematically as:

The total current through the components in a parallel circuit is equal to the sum of the individual currents through each branch.

This can be expressed mathematically as:

False.

When several cells are connected in series, the potential difference across them all is the sum of the separate potential differences.

The total potential difference across the components in a series circuit is equal to the sum of the individual potential differences across each component.

This can be expressed mathematically as:

The total potential difference across an arrangement of parallel resistances is the same as the potential difference across one branch in the arrangement of the parallel resistances.

This can be expressed mathematically as:

True.

The combined resistance of two resistors in parallel is less than that of either resistor by itself.

When two or more resistors are connected in series, the combined resistance is the sum of the individual resistances.

This can be expressed mathematically as:

When two or more resistors are connected in parallel, the combined resistance is the sum of the reciprocals of the individual resistances.

This can be expressed mathematically as:

Adding components to a circuit increases the total resistance of the circuit, so the current would decrease (for a constant potential difference).

If the potential difference of the power source is increased, but everything else in the circuit remains constant, the current will increase.

Electrical power is the rate at which energy is transferred or dissipated by a resistor.

True.

Work is done when energy is transferred by moving charges in a circuit.

The equation for electrical power is:

Where:

• = electrical power, measured in watts (W)

• = potential difference, measured in volts (V)

• = current, measured in amps (A)

The equation for electrical power is:

Where:

• = electrical power, measured in watts (W)

• = current, measured in amps (A)

• = resistance, measured in ohms (Ω)

The equation for electrical power is:

Where:

• = electrical power, measured in watts (W)

• = potential difference, measured in volts (V)

• = resistance, measured in ohms (Ω)

The equation for energy transferred is:

Where:

• = electrical power, measured in watts (W)

• = potential difference, measured in volts (V)

• = current, measured in amps (A)

• = time, measured in seconds (s)

Chemical cells use the energy stored in chemicals to arrange charges in a way that produces a potential difference.

Solar cells use the radiant energy from the Sun to produce a potential difference.

Electrical generators use the energy from burning fuels to convert kinetic energy into electric potential energy to produce a potential difference.

Wind generators use the energy from moving air to convert the kinetic energy of wind into electric potential energy to produce a potential difference.

Advantages of single-use alkaline batteries are:

• the chemicals used have a high energy density

• they are a convenient source of energy used in everyday appliances

• they are a portable source of electrical energy

• several batteries can be combined in series to increase p.d.

• they are very inexpensive

Disadvantages of single-use alkaline batteries are:

• they are non-rechargeable and need to be replaced often

• they have a high internal resistance

• they contribute to pollution when disposed

• they are made from non-renewable materials

Advantages of lithium-ion batteries are:

• they have a very high energy density

• they have a high electrical efficiency

• they are a convenient source of energy used in everyday appliances (e.g. laptops, phones)

• they are rechargeable and have a long lifetime

• they have a fast charging time

• they are a portable source of electrical energy

• they have a low internal resistance

Disadvantages of lithium-ion batteries are:

• the charge capacity of the cell degrades over time

• the internal resistance increases over time

• they are expensive compared to other types of battery

Advantages of lead-acid batteries are:

• they are relatively cheap

• they are rechargeable

• they are able to deliver very high currents in a short time which is useful for starting up vehicles

Disadvantages of lead-acid batteries are:

• they have a low energy density

• the internal resistance increases as the battery degrades over time

• they have a limited number of full discharge cycles

• they contribute to pollution when disposed as they contain toxic and corrosive materials

• they are made from non-renewable materials

Advantages of photovoltaic cells are:

• they provide a virtually unlimited supply of energy

• they are a clean energy supply and do not contribute to pollution

• the energy source (the Sun) is freely available everywhere

• they are cheap to maintain

• no input of fuel is required

Disadvantages of photovoltaic cells are:

• the output can be variable due to dependence on weather conditions

• energy output can be significantly impacted by poor weather

• they have a low efficiency

• they can only generate energy during daylight hours

• each cell only produces a small amount of electricity

• solar farms are required for large scale electricity production

Advantages of wind generators are:

• they provide a virtually unlimited supply of energy

• they are a clean energy supply and do not contribute to pollution

• the energy source (the wind) is freely available everywhere

• they are cheap to maintain

• no input of fuel is required

Disadvantages of wind generators are:

• the output can be variable due to dependence on weather conditions

• energy output can be significantly impacted by lack of wind

• they have a low efficiency

• they contribute to noise and visual pollution

Advantages of fossil fuel generators are:

• there is already an extensive infrastructure in place

• the fuels used have a high energy density

• they are highly reliable and provide energy consistently

• they are a well-known and developed technology

Disadvantages of fossil fuel generators are:

• they produce significant greenhouse gas emissions which contribute to global warming

• they produce other polluting gases which contribute to acid rain and other environmental issues

• fossil fuels are a non-renewable energy source

Electromotive force (emf) is the total energy transferred in a source of electrical energy per unit charge passing through it.

Internal resistance is the resistance of a source of electrical energy, such as batteries, as they are not perfect conductors.

Terminal potential difference is the potential difference across the terminals of a source of electrical energy when it is supplying a current to a circuit.

'Lost volts' refer to the voltage drop, or decrease in potential difference, that occurs when a source of electrical energy supplies a current to a circuit.

The equation linking emf and internal resistance is:

Where:

• = electromotive force (emf), measured in volts (V)

• = current, measured in amps (A)

• = resistance available to the rest of the circuit, measured in ohms (Ω)

• = internal resistance, measured in ohms (Ω)

False.

When the internal resistance is zero, the terminal p.d. of a cell is equal to its emf. This is when no current flows.

True.

When a current flows, the terminal p.d. of a cell will be less than its emf. This is because there will be a potential drop across the internal resistance.

A thermistor is a temperature-dependent resistor.

This means its resistance changes depending on its temperature.

When the temperature of a thermistor is increased, its resistance decreases.

LDR stands for light-dependent resistor.

This means its resistance changes depending on the intensity of light on it.

When light intensity on an LDR is increased, its resistance decreases.

(Remember: Light Decreases Resistance - LDR)

For a thermistor, the relationship between resistance and temperature is

For an LDR, the relationship between resistance and light intensity is

A variable resistor is a resistor that can change the resistance by changing the length of the wire that makes up the circuit.

This is because a longer length of wire has more resistance than a shorter length of wire.

A potentiometer is a type of variable resistor that uses a sliding contact to vary the potential difference in a circuit.