Current, potential difference and resistance

What is electric current

the flow of electrical charge

State the equation linking charge, current and time, give units

Q = I t

charge (coulombs) = current (amps) x time (secs)

What can be said about the value of current at any point in a single loop?

Current is the same at all points in a points in a closed loop

What two factors does the current in a circuit depend on?

1. potential difference

2. resistance

Potential difference

a measure of energy, per unit of charge, transferred between two points in a circuit

What equation should be used to calculate potential difference if current and resistance are known? with units

V = I R

potential difference (V) = current (A) x resistance (Ω)

What is an ‘Ohmic Conductor’? state condition required

• a conductor for which current and potential difference are directly proportional

• resistance remains constant as current changes

• temperature must be constant

List 4 components for which resistance is not constant as current changes

1. lamps

2. diodes

3. thermistors

4. light dependant resistors (LDRs)

What happens to the resistance of a filament lamp as the temperature increases? why?

• resistance increases

• ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow

What is different about current flow through a diode?

• current only flows on one direction

• resistance is very high in the other side direction, preventing current flow

State what happens to the resistance of a thermistor as temperature increases

the thermistors resistance decreases

Give two examples of when a thermistor may be used

1. in a thermostat to turn a heater on below a certain temperature

2. in a freezer to turn on a cooler when the temperature becomes too high

State what happens to the resistance to the resistance of a LDR as light intensity decreases

the LDRs resistance increases

Give an application for a LDR

• street lights often use LDRs

• when light levels become to low, the light gains sufficient current to turn on