Energy, power and resistance

Ohm’s Law

The potential difference across a component is directly proportional to the current in the component as long as its temperature remains constant

Electrical Resistance

A property of a component calculated by dividing the potential difference across it by the current in it (ratio)

Resistance equation

R = V/I

What is the sequence of events if the EMF in a circuit is increased?

Temperature increases, the positive ions in the wire vibrate more, increasing the resistance of the wire

Resistivity

A property of a material, measured in Ωm, defined as the product of the resistance of a component made of the material and its cross-sectional area dived by its length

Relationships between resistance and length or cross-sectional area

R is directly proportional to length

R is inversely proportional to its cross-sectional area

Resistivity equation (at constant temp)

ρ = RA/L

4 factors that affect resistance of a wire

• Temperature

• Material

• Length

• Cross-sectional Area

Resistivity order of magnitude in different types of conductors

Good conductors - 10^-8 Ωm

Semiconductors

Insulators 10^16 Ωm

NTC

Negative Temperature Coefficient

resistors where the as the resistance decreases, the temperature increases

Resistance in series

R = R1 + R2 + …

Resistors in parallel

1/R = 1/R1 + 1/R2 + …

I-V Graph of a fixed (ohmic) resistor

I-V Graph of a filament lamp (non-ohmic component)

I-V Graph of a Diode (non-ohmic component)

I-V Graph of a thermistor (non-ohmic resistor)

Power Equation excluding R

P = VI

Power equation excluding V

P = I²R

Power equation excluding I

P = V²/R

Electrical power definition

the rate of energy transfer by each electrical component

Calculation for energy transferred (work done)

P = W/t → W = Pt

P = VI

∴ W = VIt

2 factors affecting energy transferred to a device

• the power of the device

• how long the device is used for

The kilowatt-hour

the energy transferred by a device with a power of 1kW operating for a time of 1hour

What is 1kWh in joules

1000 × 3600

= 3.6MJ

SI units equation for energy transferred (J)

power of device (W) x time for which device is used (s)

kWh equation for energy transferred (kWh)

power of device (kW) x time for which the device is used (h)