Electricity

Define current

The rate of flow of charge

Define conventional current

Conventional current flows from positive to negative but the net movement of electrons is actually from negative to positive

State the equation for current

Current = Charge / Time

Define Voltage

The amount of work done per unit charge

State the equation for voltage

Voltage = work done/charge

What does a multimeter measure

Voltage, current and resistance

Describe current and p.d in series and parallel circuits and the reason

Series

• The current remains constant

• The potential difference is different across different components, resistance and potential difference are directly proportional

Parallel

• The current is split between each branch, resistance and current are inversely proportional

• The potential difference is equal between each branch

Define resistance and why electrical components have a resistance

• Resistance is how difficult it is for the charge carriers/electrons to flow through the component

• They have a resistance due to charge carriers colliding with positive ions of the material

State the equation of resistance

Resistance = voltage/current

State Kirchhoff’s second law

The sum of the voltages of each component must equal the voltage across the power supply

State the resistance equations in series and parallel

Series

Rt = R1 + R2 + …

Parallel

$$\frac{1}{Rt}=\frac{1}{R1}+\frac{1}{R2}+...$$

State Kirchhoff’s first law

The total current entering a junction = total current leaving a junction

Define a rheostat

A device to vary the resistance in a circuit (variable resistor)

Describe the shape of an IV graph for an ohmic conductor

• Gradient = $\frac{1}{R}$

• Steep gradient = low resistance

• Shallow gradient = high resistance

Describe the shape of an IV graph for a (filament) bulb

Initially steep from 0 and then plateaus, in the positive and negative V

Describe the shape of an IV graph for a diode

Almost 0 current below 0V, then almost 0 current above 0V until the threshold voltage, and then a sharp rise

Why do semiconductors require energy

Because they have few free charges so need energy to release more free charges, so have less resistance and can conduct better.

Describe how temperature affects resistance in LDRs and Thermistors

As temperature/light intensity increases, the resistance decreases

Explain how the resistance of a wire changes

• Temperature: a higher temperature = lattice vibrates more = more electrons collide = higher resistance

• Length: Longer wire = more atoms = higher chance of collision = greater resistance

• Cross sectional area: wider wire = more paths = higher chance the charge can pass through = lower resistance

• Material: …

Does a thick or thin wire have more resistance

A thin wire has more resistance

State the equation for resistivity

$$\rho=\frac{RA}{L}$$

Define superconductors and its resistivity temperature graph

• Materials with no resistivity below a critical temperature

• No resistance then suddenly increases vertically at the critical temperature, then a linear graph

What are the advantages of superconductors

• No resistance below critical temperature = less energy lost + high currents

State some uses of superconductors

• Really strong electromagnets

• Power cables (no energy loss)

• Fast electronic circuits (no resistance which slows current)

Explain what the resistance of a wire depends on

• Higher temperature = lattice vibrates more = more electrons collide = higher resistance

• Longer Wire = More atoms = higher chance of collision = greater resistance

• Wider wire = Greater charge can pass through = more paths = lower resistance

• Material with higher resistivity = higher resistance

Define power and its units

The rate of transfer of energy, Unit = Watts

State the proportionality between the power and brightness of a bulb

The power of the bulb is (directly) proportional to the brightness of the bulb

How is electrical energy usually dissipated

Electrical energy is usually dissipated as heat/thermal energy

Define power rating of bulbs

It is the power consumption/brightness of bulbs at a given voltage

Define Internal Resistance

The source of lost volts per unit current, when current passes through a cell

Define EMF

The energy transferred from chemical to electrical energy per unit charge passing through the source

Explain how to find EMF of a cell experimentally

When a voltmeter is attached to an unconnected cell, no current flows due to the high resistance of the voltmeter so minimal energy dissipation, so minimal work done. Therefore, no lost volts are generated. So the reading on the voltmeter is the EMF

Define terminal potential difference

The amount of energy per unit charge provided to the external circuit

Define lost volts

• The difference between the EMF and the terminal p.d

• The p.d lost across the internal resistance of the cell

State what each letter represents: EMF = $\frac{E}{Q}$

• E → Energy supplied by the battery (J)

• Q → Charge

State the equation that can be derived from $\char"0190 =I\left(R+r\right)$

$$\char"0190 = V + Ir$$

State how to calculate the total EMF of cells in series and identical cells in parallel

• Series: Add the EMFs together

• Parallel = The EMF of one cell

Define Potential Divider

A circuit consisting of two resistors in series to split the total supplied potential difference

State an equation for potential dividers

$V_{out}=\frac{R_2}{R_1+R_2}\times V_{in}$ , where $V_{out}$ is across $R_2$

State another name for variable resistor

Potentiometer

State the differences between using a variable resistor and using a potential divider

Variable Resistor

• Current can vary, but not from 0A, the maximum current will be lower

• PD can vary up to the EMF but cannot achieve 0V

• No power wasted as current only flows through connected resistor

Potential Divider

• Current can vary from 0A and has a greater maximum current

• PD can vary from 0V to the EMF

• Some power wasted as some current will always flows through the external end of variable resistor coil

What can potential dividers and semiconductors be used to create

Sensor circuits

Describe the thermistor potential divider circuit as temperature increases

As temperature increases

1. The thermistor resistance decreases

2. Thermistors resistance is a smaller proportion of the circuits total resistance

3. Thermistor takes smaller portion of the total potential difference

4. Voltmeter reading decreases

5. Current increases as total resistance decreases (V=IR)

Describe the LDR potential divider circuit as light intensity increases

As light intensity increases,

1. LDR resistance decreases

2. LDRs resistance is a smaller proportion of the circuits total resistance

3. LDR takes smaller portion of the total potential difference

4. Voltmeter reading decreases

5. Current increases as total resistance decreases (V=IR)