Electricity

What is electric current? State its units.

The rate of flow of charge. It is measured in amperes (amps).

If two wires are the same material they’re the same

resistivity

What happens when a superconductor is at critical temperature

At the critical temperature (TcT_cTc​), the material can conduct electricity with no energy loss because there’s no resistance. The critical temperature varies for different materials.

Types of diodes

In forward bias (positive terminal to the anode), a small voltage is required to make the current flow. In reverse bias, (polarity reversed) almost no current flows (the diode resists current flow).

What is potential difference?

The work done moving a unit charge between 2 points in a circuit.

V = W / Q

What is resistance?

How difficult it is for current to flow through an appliance. A component has a resistance of 1Ω if 1A flows through it when a p.d of 1V is applied across it.

R = V / I

What is meant by an ohmic conductor?

A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.

How can you measure the current in a circuit?

You can measure the current in a circuit with an ammeter connected in series with the component.

How do you measure potential difference across a component?

Using a voltmeter, connected in parallel across the component being measured.

What does the gradient of a current - potential difference graph represent?

Rate of change of current with respect to voltage. This is not the same as 1/R

Why does the current increasing on a filament lamp cause an increase in the resistance?

As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased).

What is a diode?

A diode is a appliance that only allows current to flow in one direction.

Unless stated in the question, should you assume that voltmeters to have zero resistance or infinite resistance?

You should assume they have infinite resistance. Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance.

Why should you assume that an ammeter has zero resistance unless stated otherwise?

This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit.

What is an Light Dependent Resistor (LDR)?

A semiconductor that is sensitive to light. As the light intensity increases, its resistance decreases.

How does a thermistor work?

Similar to an LDR but, as the temperature increases, the resistance decreases (This means thermistors have a negative temperature coefficient).

What is resistivity?

The resistance of a 1m cylinder with a cross sectional area of 1 m^2. Resistivity is an inherent property of a material.

ρ = RA/L

ρ = resistivity (Ωm)

R = resistance (Ω)

A = cross-sectional area (m^2)

L = length (m)

What is a superconductor?

A material that has a resistivity of zero at or below a critical temperature. The critical temperature is an inherent property of the material.

How do you find the total resistance in a series circuit?

R_Total = R_1 + R_2 + R_3 + …

Add the individual resistances of each component.

If 6 cells, each of voltage 5V, are arranged in parallel what is the voltage in the circuit?

V = 5V

If 6 cells, each of voltage 5V, are arranged in series what voltage is provided to the circuit?

30V

(5V x 6)

How does the current vary between each component of a series circuit?

The current through all of the components is the same so the current does not vary.

Is the current in parallel components the same?

No, each branch of a parallel circuit can have different currents through them according to Kirchhoff’s first law.

What is Kirchhoff’s first law?

All of the current going into a junction is equal to the current leaving the junction.

What is Kirchhoff's second law?

For any path (loop) of a circuit, the sum of all of the potential differences must equal to total emf of the circuit.

In a series circuit, if two cells are connected negative to negative, would their emf add up or cancel out?

They will cancel out. The total emf would be equal to:

ε_total = ε_1 - ε_2

What is power?

The rate of energy transfer.

Give an equation for power in terms of current and voltage.

Power = Current x Voltage

P = I ⋅ V

What is the purpose of a potential divider?

● To provide variable potential difference, or

● To provide a constant specific potential difference

A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6Ω and 7Ω respectively, in series. What is the voltage across the 7Ω resistor?

Total resistance = 6 + 7 = 13 Ω

(7/13) x 12 = 6.5 V

What is emf?

Electromotive force: the electrical energy transferred by a power supply per unit charge.

Rearrange the equation Ɛ = I(R + r) into the form y = mx + c

y = mx + c

Ɛ = IR + Ir

Ɛ = V + Ir

So for a graph of V against I

V = -rI + Ɛ

Gradient = -r and y-intercept = Ɛ

What are two applications of superconductors?

● Power cables, which would reduce energy loss through heating to zero during transmission.

● Strong magnetic fields, which would not require a constant power source. These could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications.

What is the total resistance of a parallel arrangement of 3Ω, 7Ω and 9Ω resistors?

1 / R_T = ⅓ + 1/7 + 1/9

1 / R_T = 37 / 63

R_T = 63 / 37 = 1.7 Ω

Electric current (I)

Electric current (I) - the flow of charge per unit time, or the rate of flow of charge.

I = ΔQ/Δt

(where Q is charge)

Potential difference (V)

Potential difference (V) - the energy transferred per unit charge between two points in a circuit.

V = W/Q

(where W is energy transferred)

Resistance (R)

This is a measure of how difficult it is for charge carriers to pass through a component, and is measured by dividing the potential difference across a component by the current flowing through it.

R = V/I

Ohm’s law

Ohm’s law states that for an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions (e.g temperature) are kept constant.

Ohmic conductor

This component follows Ohm’s law therefore its current-voltage graph will look like a straight line through the origin. (This is provided physical conditions are kept constant).

Filament lamp

This component contains a length of metal wire, which heats up as current increases, therefore the resistance of this component increases as current increases.

Resistivity (ρ)

Resistivity (ρ) is a measure of how easily a material conducts electricity, it is defined as the product of resistance and cross sectional area, divided by the length of the material.

Electron flow

Electron flow is from negative to positive and is the direction that electrons actually flow in a circuit.

Conventional current flow

Conventional current flow is from positive to negative and is more commonly used to describe direction.

1 Ampere

1 Ampere is the current flow when 1 Coulomb of charge passes a point each second.

Potential difference

Potential difference is equal to the work done per coulomb of charge travelling in a circuit. As an equation this is defined as:

V = W/Q

Resistance

Resistance is a measure of how difficult it is for current to flow in a circuit. It is directly proportional to the potential difference in a circuit, and inversely proportional to the current flow. As an equation it is defined as:

R = V/I

It is measured in Ohms.

Ohmic conductor

An ohmic conductor is a conductor through which the current flow is directly proportional to potential difference, when held at a constant temperature.

Cross sectional area

Cross sectional area - the greater the area, the easier the passage of electrons.

Length

Length - the longer the length, the harder it is to send a current through it.

Resistivity

Resistivity - a property of the material that is assumed under standard conditions.

Semiconductors

Semiconductors are components for which the resistance changes depending on external conditions.

The two main types you need to know about are

1. Light Dependant Resistors

2. Thermistors

Light Dependant Resistors

Light Dependant Resistors are light sensitive components - when light intensity increases, their resistance decreases.

Thermistors

Thermistors are temperature sensitive components - when the temperature increases, their resistance decreases.

Power

Power is defined as the rate of energy transfer. It can be calculated using:

P = W/t

Series circuit

Series circuits are ones where all the components are part of the same loop.

Potential divider

A potential divider is a combination of two resistors in series, which results in the potential difference in the circuit being split into a specific ratio.

Variable resistors

Variable resistors in a potential divider are used to change the voltage supplied to something to a suitable voltage without using a transformer. However, this method wastes power.

Detecting circuits

Detecting circuits can also be constructed by adding a semiconductor, such as an LDR, as the second resistance. This means that when light intensity increases, the V-Out over the LDR will drop below a certain desired level.

Ammeter

A device that measures the current in the loop of the circuit that it is connected in series with. An ideal ammeter is modelled to have zero resistance.

Current

The rate of flow of charge in a circuit.

Electromotive Force

The amount of energy transferred by a source, to each unit of charge that passes through it.

Internal Resistance

The resistance to the flow of charge within a source. Internal resistance results in energy being dissipated within the source.

Light Dependent Resistor

A light sensitive semiconductor whose resistance increases when light intensity decreases.

Ohmic Conductor

A conductor for which the current flow is directly proportional to the potential difference across it, when under constant physical conditions.

Ohm’s Law

The current and potential difference through an ohmic conductor held under constant physical conditions are directly proportional, with the constant of proportionality being resistance.

Parallel Circuits

Components are said to be connected in parallel when they are connected across each other (separate loops).

Potential Divider

A method of splitting a potential difference, by connecting two resistors in series. The total potential difference is split in the ratio of their resistances.

Resistance

A measure of how difficult it is for current to flow through a material.

Resistivity

A quantity that is proportional to an object’s resistance and cross-sectional area, and inversely proportional to the object’s length.

Resistors in Parallel

The potential difference across resistors connected in parallel is identical for each resistor. The current is split between the resistors. The total resistance is equal to the inverse of the sum of the inverses of the resistances of the resistors.

Resistors in Series

The current through resistors connected in series is identical for each resistor. The potential difference is split in the ratio of their resistances. The total resistance is equal to the sum of the resistances of the resistors.

Series Circuits

Components are said to be connected in series when they are connected end to end (in one loop).

Superconductor

A material which has zero resistivity when the temperature is

decreased to, or below, the material’s critical temperature. Superconductors can be used to produce strong magnetic fields and reduce energy loss when transmitting electric power.

Terminal Potential Difference

The potential difference across the terminals of a power source. It is equal to the source’s emf minus any voltage drop over the source’s internal resistance.

Thermistor

A temperature sensitive semiconductor whose resistance increases when temperature decreases.

Voltmeter

A device used to measure the potential difference across components. An ideal voltmeter is modelled to have infinite resistance.