Electric Current & Charge (OCR A Level Physics)

Electric Current

• Electric current is defined as the rate of flow of positive charge carriers
  • It is measured in units of amperes (A) or amps
  • The symbol for current is I

• The charge, current and time are related by the equation:

• Where:
  • I = current (A)
  • ΔQ = change in charge (Q)
  • Δt = time interval (s)

• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current

Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative

• There are several examples of electric currents, including in household wiring and electrical appliances
• Current is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

An ammeter can be used to measure the current around a circuit and always connected in series

ANSWER: B

Step 1: Write out the equation relating current, charge and time, rearranging for charge Q

Q = It

Step 2: Rule out any obviously incorrect options

• • Option A does not contain charge or time, so can be ruled out

Step 3: Try the rest of the options to determine the correct answer

• • Consider option B:

I = 4 / 500 = 8 × 10^–3 = 8 mA

• • Consider option C:

I = 8 / 100 = 80 × 10^–3 = 80 mA

• • Consider option D:

I = 1 / 8 = 125 × 10^–3 = 125 mA

• • Therefore, the correct answer is B

• Charge is a property certain particles have. It can either be:
  • A positive charge (+) (eg. proton)
  • A negative charge (–) (eg. electron)
  • A neutral (no) charge (eg. neutron)

• An atom is neutral. This is because it has an equal number of protons (positive charge) and electrons (negative) charge
  • However, just the nucleus which is made up of protons and neutrons is positively charged

• In physics, the charge is represented by the symbol Q or q

The Coulomb

• The unit of charge is the Coulomb (C)
  • This is defined as the quantity of charge that passes a fixed point per second when a current of 1 A is flowing

• The coulomb (C), in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. 1 C = 1 A s

Definition of the Coulomb

Quantisation of Charge

• The charge on charge carriers is quantised
  • This means the charge comes in definite, finite quantities

• In this way, the quantity of charge can be quantised depending on how many protons or electrons are present
  • Positive and negative charge has a definite minimum magnitude and comes in multiples of that magnitude

• This magnitude is the elementary charge, e = 1.60 × 10^-19 C
  • The magnitude of the charge just refers to its value, rather than whether it is positive or negative

• The net charge on a particle can be quantised, meaning it is always a multiple of the charge of an electron by convention
  • The charge of an electron, e is -1.60 × 10^-19 C
  • The charge of a proton, +e is +1.60 × 10^-19 C

Step 1: Calculate the charge

1e = –1.60 × 10^–19

3e = 3 × (–1.60 × 10^–19) = –4.8 × 10^–19

Step 2: Include the unit for charge

The units of charge is coulombs (C)

Therefore 3e = –4.8 × 10^–19 C