IGCSE physics notes | Circuits to nuclear physics | brief notes | grade 9 - 10

CURRENT:
Current is the amount of charge passing a point in a circuit every second
Charge = current x time
Q=Ixt

 Q stands for charge (measured in coulombs, C)
 I stands for current (measured in amps, A)

Direct Current (dc)

 Direct current is produced when using dry cells and batteries (and sometimes
generators, although these are usually ac)
o The electrons flow in one direction only, from the negative terminal to the
positive terminal

Alternating Current (ac)

 Alternating current typically comes from mains electricity and generators
 It is needed for use in transformers in the National Grid (covered later in this
topic)
o The direction of
electron flow
changes direction
regularly
o A typical
frequency for the
reversal of ac
current in mains
electricity is 50 Hz

Current
- is measured using an ammeter
- Ammeters should always be connected in series with the part of the circuit you
wish to measure
- Ammeters measure the amount of charge passing through them per unit time,
so the ammeter has to be in series so that all the charge flows through it.

CONVENTIONAL CURRENT:

o Electrons are negatively charged
o This means that the electrons flow from negative to positive.

ELECTROMOTIVE FORCE:

,The electromotive force (e.m.f) is the name given to the potential difference of the
power source in a ciruit
𝑊
E=
𝑄

 E = electromotive force (e.m.f.) (V)
 W = energy supplied to the charges from the
power source (J)
 Q = charge on each charge carrier (C)

Potential difference:

As charge flows around a circuit energy is transferred from the power source to the
charge carriers and then to the components

o This is what makes the bulb light up
o Potential difference is the work done by a unit charge passing through a
component
o Its measured in volts (V)
𝑊
V=
𝑄

o V = potential difference (p.d.) (V)
o W = energy transferred to the components from the charge carriers (J)
o Q = charge on each charge carrier (C)



RESISTANCE:
It’s the opposition to current, for a given potential difference, the higher the
resistance, the lower the current. Therefore resistirs are used in circuits to control
the current.
- The unit of resistance is ohm
𝑉
- R=
𝐼

o R = resistance (ohms, Ω)
o V = potential difference (volts, V)
o I = current (amperes, A)



Consequences of Ohm's Law

,  Resistors are used in circuits to control either
o The current in branches of the circuit (through certain components)
o The potential difference across certain components
 This is due to the consequences of Ohm's Law
o The current in an electrical conductor decreases as its resistance
increases (for a constant p.d.)
o The p.d. across an electrical conductor increases as its resistance
increases (for a constant current)




 As the potential difference (voltage) across a component is increased,
the current in the component also increases
 The precise relationship between voltage and current can be different for
different types of components and is shown by an IV graph:

 The IV graph for a resistor is very simple:
The current is proportional to the potential difference



 This is because the resistor has a constant resistance

 For a lamp the relationship is more complicated:
The current increases at a proportionally slower rate than the potential difference

 This is because:
o The current causes the
filament in the lamp to heat
up
o As the filament gets hot,
its resistance increases
o This opposes the current,
causing it to increase at
a slower rate

RESISTANCE OF A WIRE:
The longer a wire, the greater its resistance
The thicker the wire, the smaller its resistance
- Resistance is directly proportional to length
- Resistance is inversely proportional to cross – sectional area