9.9 The LDR | 9.10 Electrical energy and power | 9.11 Paying for electricity

What is an LDR?

• It’s made from a semi-conductor, in which the number density if charge carriers changes depending on the intensity of the incident light.

• In dark conditions, the LDR has high resistance, the number density of the free electrons inside the semiconductor is very low so the resistance is very high

• When the light shines onto an LDR, the number density of the charge carriers increases dramatically, leading to a rapid decrease in the resistance of this component.

How can you investigate an LDR?

Carry out an experiment of how resistance of an LDR varies with distance from a constant light source. Make sure to reduce background sources of light.

Describe energy transfer:

• Electric circuits are often used to transfer energy from one place to another, eg power source to a lamp.

• A filament lamp transfers electric energy to heat and light, whereas a loudspeaker transfers electrical to sound and heat.

• When a circuit is broken or current is zero, no energy is transferred and the circuit is off.

What is electrical power?

The rate of energy transfer by each electric component.

P= VI

Electrical power (watts) = p.d (V) × I (A)

What are some other equations for power?

P = I²R

P = V²/R

How do you derive P = VI?

P = W/t

Using defined equation for p.d, make W subject:

V = W/Q

W = VQ

Substitute work done into generic equation for power:

P = VQ/t

Q/t = I

P = VI

How do you calculate energy transferred?

P = W/t is rearranged to W = Pt

Substituting in P = VI gives W = VIt

Describe paying for energy:

• All electricity passing supplied to the house passes through meter.

• The energy transferred to to each individual electrical device and its cost to run depends on the power of the device and how long it's used for.

What’s the defining equation for power?

P = W/t

Power = energy transferred/ time taken

W = Pt

What is the kilowatt hour?

• The SI unit for energy is the joule but this is too tiny to measure energy transferred to homes.

• It is the energy transferred by a device with a power of 1kW operating for a time of 1 hour.

What is 1kWh in MJ?

3.6

What’s the equation for kWh?

• SI units: energy transferred (J) = power of device (W) × time for which device is used (s)

• kWh: energy transferred (kWh) = power of device (kW) × time for which device is used (h)

What's the cost of each kWh?

It varies depending on the company and their tariffs and even sometimes the time of day.

Usually 6-15p per kWh