section 5 - electricity and circuits

Question 1

what is potential difference

Answer 1

the energy transferred per coulomb of charge that passes between two points in an electrical circuit

Question 2

how can the current of a supply be altered

Answer 2

• variable supply

* variable resistor (bc current is affected by resistance)

Question 3

how do current and resistance change as temperature increases

Answer 3

• work is done against resistance when charge flows through a component which causes an electrical transfer of energy (work done = energy transferred)

so, when a current flows through a resistor, the resistor heats up
• the electrons collide with the ions in the lattice that make up the lattice as they flow through

• this gives the ions energy, making them vibrate and heat up, making it harder for electrons to get through bc there are more collisions
• so, current decreases as the resistor heats up, if it gets too hot no current will be able to flow

Question 4

using the standard test circuit to investigate a component

Answer 4

• the component and ammeter are in series, so they can be anywhere, but the voltmeter has to be parallel across the component
• change the output p.d. of the power supply, this alters the current in the circuit and the p.d. across the component
• take several pairs of reading to see how the current through the component varies as the potential difference across it changes (do repeats and the mean as well)
• plot current against potential difference
• use this to calculate resistance and see if it changes with current and voltage
• if your circuit gets too hot, results can be altered: disconnect it between readings to allow it to cool if it gets too hot

Question 5

diodes

Answer 5

only allow current to flow in one direction

Question 6

resistance of thermistors and when are they used

Answer 6

as the temperature increases, the resistance decreases, so the current flowing increases

used in: car engine temperature sensors and electronic thermostats

Question 7

resistance of LDR’s

Answer 7

as light intensity increases, resistance decreases and the current flowing through LDR increases

Question 8

current, resistance and potential difference in a series circuit

Answer 8

• current is equal everywhere
• the supply’s total pd is shared between components, dependant on their resistance
• total resistance of the circuit increases as resistors are added

Question 9

current, resistance and potential difference in a parallel circuit

Answer 9

• potential difference is equal across components
• current splits at junctions depending on the total resistance in that branch
• the total resistance decreases if resistors are added in parallel

Question 10

why does total resistance increase when adding resistors in series

Answer 10

the two resistors share the total p.d., making it lower in each resistor which also makes the current lower (which is equal everywhere in series)

Question 11

when is energy transferred in a circuit

Answer 11

when an electrical charge goes through a change in potential difference, energy is transferred because work is done against resistance

Question 12

when can the heating effect of a circuit be an advantage

Answer 12

• if you want to heat something !
• toasters - have a coil of wire with very high resistance. when current flows, the temperature increases so much that it glows, giving off infrared radiation which transfers energy to the bread an cooks it
• filament bulbs and electric heaters work similarly