5.2 Electric Circuits

Question 1

Law of Conservation representted by Kirchoff’s Laws

Answer 1

• Conservation of CHARGE: total current into a junction = total current out of a junction
• Conservation of ENERGY: loss in electric potential = gain in electric potential in a closed loop

Question 2

Diode

Answer 2

allows current to flow in one direction only; used to convert AC into DC

Question 3

Fuse

Answer 3

melts and breaks the flow of current;
used to prevent overheating/fire

Question 4

When electrons flow through a component in the circuit (e.g. a resistor)…

Answer 4

1. electrons transfer electric potential energy to the component
2. component transforms electric potential energy into other forms of energy (eg. thermal, light)

Question 5

measuing the potential difference across a component =

Answer 5

measuring the difference in the electric potential energy of electrons before they enter the component and after they leave it

Question 6

Ohm’s Law

Answer 6

V = IR
For a conductor at a constant temperature, the current through it is proportional to the potential difference across it

Question 7

the gradient of an ohmic resistor’s I-V graph =

Answer 7

gradient = I/V = 1/R

Question 8

What happens to the I/V graph of an ohmic resistor?

Answer 8

straight line through origin (since I is directly proportional to V)

Question 9

What happens to the I/V graph of an filament bulb?

Answer 9

• Middle section is straight and passes through origin (ohmic)
• Gradient decreases as V increases → more I flows → temp increase → R increase → I increases at a slower rate (non-ohmic)

Question 10

What happens to the I/V graph of an diode?

Answer 10

• increasing gradient on RHS because diode is forward biased
• zero current on LHS because diode doen’t conduct

Question 11

resistivity

Answer 11

ρ = RA/L
resistance per unit length of a material with unit cross-sectional area

Question 12

Wires are made from what material? Why?

Answer 12

copper: good conductor, low resistivity at room temperature

Question 13

In a series circuit:

Answer 13

• same current throughout
• split p.d. depending on each component’s resistance

Question 14

In a parallel circuit:

Answer 14

• same p.d. across all loops
• total current is sum of the currents in each parallel branch

Question 15

advantage of parallel circuit

Answer 15

• single power source supplies all with same p.d.
• if one breaks, current can still flow through the rest

Question 16

explain the heating effect of current

Answer 16

electrons flow through the conductor
–> collide with ions within the metal conductor, making them vibrate more –> heat up

Question 17

for a given resistor, if the current or voltage doubles, the power dissiplated will

Answer 17

x 4
(because P = IV = I^2R=V^2/R)

Question 18

The main purposes of a potential divider are:

Answer 18

• To provide a variable potential difference
• To provide a specific potential difference
• To split the potential difference of a power source between two or more components

Question 19

potential divider equation:

Answer 19

V out = (R2/R1+R2) (V in)

Question 20

If the resistance of one of the resistors is increased

Answer 20

it will get a greater share of the potential difference, whilst the other resistor will get a smaller share (V=IR)

Question 21

primary vs secondary cell

Answer 21

primary: non-rechargable, chemical used up, disposed after use; electrons flow from - to +

secondary: rechargable (chemical reaction reversible); during recharge, electrons are forced from + to - by an external current

Question 22

capacity of a cell

Answer 22

the amount of charge that it contains AND is able to discharge

Question 23

Lamp A is connected to a 240 V supply and lamp B is connected to a 12 V battery. Both lamps have the same current, yet 240 V lamp glows more brightly. Explain in terms of energy transfer

Answer 23

Voltage is energy transferred per coulomb charge.
Since the lamps have the same rate of flow of charge (current), the energy transferred to each coulomb of charge in the 240 V lamp is 20 times greater than for the 12 V lamp.

Question 24

explain the difference between p.d. and e.m.f

Answer 24

both are energy transfered per unit charge, but the type of energy transfer is different

• p.d. is the energy transfered from electrical to other forms per unit charge
• (eg. when charges pass through a resistor, their electrical energy is converted to heat, creating a potential different across the resistor)
• e.m.f is the energy transfered from other forms to electrical per unit charge
• (eg. when charges pass a power supply, the chemical energy in power supply is converted to electric energy and transfered to the charges)

Question 25

electromotive force (emf)

Answer 25

power supplied per unit current / energy supplied per unit charge; in moving charge completely around the circuit

Question 26

Explain why the headlights of a car may go slightly dimmer when the engine is started.

Answer 26

the starter motor needs more I → increased I increases lost volts (Ir, where r is internal resistance)

Question 27

Explain why a high-voltage supply for use in school demonstrations has a very large internal resistance included inside it.

Answer 27

if someone touches and current flows, the p.d. will immediately drop to a very low value as Ir is so large

Question 28

electronvolt (eV)

Answer 28

work done to move 1 electron through a p.d. of 1V

Question 29

electric potential difference

Answer 29

energy per unit charge; when a positive test charge is moved between two points

Question 30

electric current

Answer 30

the rate of flow of charge past a given cross-section

Question 31

how do you determine the energy stored/transferred by a constant current from a discharging graph (V-t)

Answer 31

area under graph = Vt
energy = VIt = current x area

Question 32

The current needed to recharge a secondary cell must

Answer 32

flow in the opposite direction to the natural current flow