THE RELATIONSHIP THE RELATIONSHIP BETWEEN ELECTRIC BETWEEN ELECTRIC

CURRENT AND POTENTIAL CURRENT AND POTENTIAL DIFFERENCEDIFFERENCE

Potential difference, V

• A gravitational analogy can be used to explain electric potential difference.

• The movements that require work done by an external force would turn the this work into potential energy.

• For example, the potential energy of water at top of a waterfall is greater than at the bottom.

Potential difference

• The potential difference, V, between two points in a circuit is defined as the amount of work done, W, when one coulomb of charge passes from one point to the other point.

• The SI unit for potential difference is Volt, V

Example:

If a charge of 5.0 C flows through a wire and the amount of electrical energy converted into heat is 2.5 J, calculate the potential difference across the ends of the wire.

Solution:

Charge, Q = 5.0 C Energy, E = 2.5 J

Potential difference, V = E/Q

V = 2.5J/5.0 V

V = 0.5 V

Current and potential difference

• Example of electric circuit.

• In the electric circuit, the greater the potential difference or voltage, the greater the current flow

• In other words, when the potential difference between two points in circuit increases, the current flowing through it increases and vice versa

Ohm’s Law

• The potential difference across a conductor is directly proportional to the current flowing through it when the temperature of the conductor is kept constant

V α I

or V/I = R

• This relationship is known as Ohm’s Law

• The constant R is the resistance of the conductor.

Ohm’s Law

Ohm’s law states that the current that passes through an ohmic conductor is directly proportional to the potential

difference applied across it if the temperature and other physical

conditions are constant

An ohmic conductor is one which obey Ohm’s Law, while conductor which does not obey

Ohm’s law is known as a non-ohmic conductor

Resistance, R

• What is the resistance? Resistance is a term that describes

the opposition experienced by the electrons as they flow in a conductor.

Measured in the SI unit called ohm, Ω. The resistance, of an ohmic conductor

is the ratio of the potential difference, V to the current, I

Example:

• A potential difference of 3.0 V applied across a resistor of resistance R drives current of 2.0 A through it. Calculate R

• Solution:

V = 3.0 V , I = 2.0 A

R = V/I

R = 3.0/2.0

R = 1.5Ω

Resistance, R

• Resistance can be good or bad

Good Bad •Resistance allow us to use electricity for heat and light.

•The heating coil of an electrics kettle produces heat because it has resistance

•In the light bulb, the current flowing through a resistance filament causes it to become hot and then glow

•Resistance causes some of the electrical energy to turn into heat, so some electrical energy is lost along the way

Factors that affect the resistance

• Length

• Cross-sectional area

• Types of material

• Temperature of a conductor

Superconductors

• The resistance in many electrical machines such as electric motors, generators and transformers is large, producing vast amounts of heat during its operation

• The resistance of conductors will decrease when the temperature of the metal drops.

• There are some metals and compounds whose resistances fall to zero below a certain critical temperature, Tc.

• When their resistance become zero, these materials are called superconductors.

• The graph shows the abrupt disappearance of resistance at critical temperature, Tc.

• Many more superconductors have been found but all these materials still require cooling by expensive cooling agents such as liquid helium or liquid nitrogen.

Application of superconductors

• Superconductors are very useful because they can make electric cars more feasible and computers much faster.

• Superconducting magnets which would make things like magnetic levitation (MAGLEV) trains and medical imaging machines such as the magnetic imaging machines such as the magnetic resonance imaging (MRI) scanner more economical

Question

1. The work done to move 4 C of charge from a point to another point in an electrical field is 100 J. What is the potential difference between the two points?

2. Diagram shows an incomplete circuit used to investigate how the resistance of a bulb varies with the current.

Question

a) What is meant by resistance

b) Add a voltmeter and ammeter to the circuit in diagram to measure the potential difference across the bulb and the current flowing through it

c) How do you measure the resistance of the bulb

d) If the potential difference across the bulb is 2.5 V and the current flowing through it is 0.25A, what is the resistance of the bulb?

e) The resistance of the bulb is 50Ω when its filament is cold. Sketch a graph of resistance against the current flowing through the bulb.