1. Electric current, potential difference and resistance
AICE Chapter 10
Electric current, potential difference and resistance

2. Electric Current
Electricity – we do not speak on his name. Not a proper term, its used for electrical energy or electrical power or electric current
Conventional current – the scientific convention that the direction of the current goes from positive to negative

3. Electric current
The current occurs in a wire because the negative charged electrons are free to move about. These are called conduction or free electrons
The atoms are bound together very closely and one electron breaks free from each atom to become a conduction electron
A plain wire not connected has an equal number of free electrons and ions it has a neutral charge

4. Electric current
When the cell is connected to the wire, electrical force on the conduction electrons that makes them travel along the length of the wire.
Electrons flow toward the positive terminal, opposite of the conventional current direction.
Blame Ben Franklin for guessing wrong on the positive and negative titles.

5. Charge carriers Sometimes a current is a flow of positive charges
Beam of protons in a particle accelerator
Sometimes a current is due to both positive and negative charges
When charged particles flow through a solution
A solution that conducts energy is called an electrolyte and it contains both positive and negative ions.
Any charged particles which contributes to an electric current are known as charge carriers.

6. Current and Charge
When charged particles flow past a point in a circuit, we say that there is a current in the circuit.
Electrical current is measured in amperes (A)
Charge is measured in coulombs (C)
Current = charge/time
The Electric current is the rate of flow of electric charge past a point

7. Charge continued Manipulated the equation can be read as
Charge = current x time
One coulomb is the charge which flows past a point in a circuit in a time of 1 s when the current is 1 A
ΔQ= IΔt
Q is charge (coulomb), I is the current (ampere), t is time (seconds)

8. Charged particles Electrons have a tiny charge Approx. -1.6 x 10-19C
This charge is represented with –e
This is called the elementary charge
Protons have a positive charge, +e
This is equal and opposite to that of the electron.
Ions carry charges that are multiples of +e and -e

9. Voltage
The total voltage across the power supply is equal to the sum of the voltages across resistors
Electrical energy is transferred to the charge by the power supply
Charge flows around the circuit transferring some energy in the first, then the second resistor.

10. Voltage
A voltmeter indicates the energy transferred to the component by each unit of charge.
Placed across the power supply measure the electromotive force or e.m.f (only remember e.m.f!) long term not important
The total work done when unit charge goes around a complete circuit
Place across the resistors measure the potential difference
The energy given up by unit charge as it moves from point A to point B

11. Electrical resistance
The potential difference of voltage V – the greater the potential difference, the greater the current for a given lamp
The greater the resistance, the smaller the current for a given potential difference
Electrical resistance is the ratio of the potential difference to the current

12. Resistance
R = V/I
Resistance (ohms, Ω) = Voltage (volt V) / current (ampere A)
Copy table 10.2
One ohm is equal to 1 volt per ampere
1 Ω = VA-1

13. Electrical Power
Power is the rate at which energy is transferred (chapter 5)
P = ΔW/Δt
Power is measured in watts (don’t confuse with W, that energy, joules)
Rate at which energy is transferred in an electrical component is related to two components.
The current I in the component
The potential difference V across the component

14. Lets Derive! ΔW=VΔQ P = ΔW/t = (VΔQ)/t = V(ΔQ/t)
(ΔQ/t) is the current I, therefore:
P=VI
Power = potential difference x current
Watts = volts x amperes

15. Fuses
A fuse is a device that is fitted in an electrical circuit; it is usually there to protect wiring from excessive currents
High currents cause wires to get hot, which can lead to damaged wires and fires
Fuses are intentionally designed with a thin wire that melts and breaks if the current exceeds this value.
This is different from a circuit breaker, because a circuit breaker can be reset and reused, a fuse cannot.

16. Power and resistance
Current I in a resistor of resistance R transfers energy to it. The resistor dissipates heat.
The p.d. V across the resistor is given by V = IR
Combine this equation with the equation for power, P = VI
We get two new equations from this
P = I2R
P = V2/R

17. Calculating Energy Last combination P= current x voltage
Energy = power x time
Energy transferred = current x voltage x time
ΔW = IVΔt
Energy is always measured in joules.