ELECTRIC CURRENT

Chapter 22

What is an electric current?An electric current is a flow of microscopic particles called electrons flowing through wires and components.

+ -

In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell.

For current to flow you must have a complete

circuit

A power source is needed to create a

POTENTIAL DIFERENCE

When these conditions are met free electrons move about inside the metal.

Potential Difference• Charges will continue to move as long as

the potential difference (voltage) is maintained.

Chargetime

Measuring Current

A Charge pump

How do we maintain a charge?

The CELLThe CELLThe cell stores chemical energy and transfers it to electrical energy when a circuit is connected.

When two or more cells are connected together we call this a Battery.

The cells chemical energy is used up pushing a current round a circuit.

Animation on page 599

simple circuitssimple circuitsHere is a simple electric circuit. It has a cell, a lamp and a switch.

To make the circuit, these components are connected together with metal connecting wires.

cell

lamp

switch

wires

simple circuitssimple circuitsWhen the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around.

If there were any breaks in the circuit, the current could not flow.

POWERPower is the rate at which energy is transfer or transformed. It is measured in Watts (W).Example: Most of our appliances at home are rated in watts.

Measuring Power

If the current through the motor is 3.0and the potential difference is 120V. What is the

motor’s power?

(3.0 C/s) (120 j/C) = 360 J/s (W)

Electrical Resistance• Electrical resistance – resistance to the flow

of electrical charges

Ohm’s Law

• For many conductors, current depends on:• Voltage - more voltage, more current– Current is proportional to voltage

• Resistance - more resistance, less current– Current is inversely proportional to resistance

Adding resistances in series

• Each resistance in a series circuit adds to the total resistance of the circuit.

Rtotal = R1 + R2 + R3...Total

resistance(ohms)

Individual resistances ()

Total resistance in a series circuit• Light bulbs, resistors, motors, and heaters usually have

much greater resistance than wires and batteries.

Voltage in a series circuit• Each separate resistance creates

a voltage drop as the current passes through.

• As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy

• Ohm’s law is used to calculate the voltage drop across each resistor.

Measuring resistance / Ohm’s Law

Simple problem solving exercises1. A 30 V attery is connected to a 10 Ω resistor.

What is the current of the circuit? 2. A lamp draw a current of 0.50 A when it is

connected to a 120 V source. a. What is the resistance of the lamp? b. What is the power consumption of the

lamp?3. A 75 W lamp is connected to 125 V. a. What is the current through the lamp? b. What is the resistance through the lamp?

Simple problem solving exercises

4. The current in a certain wire is 0.35A. Calculate the charge passing a point in the wire a)In 10 s b) in 10 minute5. Calculate the average current in a wire through which a charge of 15C passes in a) In 10 s b) in 10 minute

SERIES AND PARALLEL CIRCUITS

Chapter 23

Series Circuits• In series circuits, current can only take one path.• The amount of current is the same at all points in a

series circuit.

Parallel Circuits• the current can take more than one path.• Because there are multiple branches, the current is not the same at all points

in a parallel circuit.• If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it

stays alight.

circuit diagramscircuit diagrams

cell battery

switch

lamp

motorammeter

voltmeter

buzzer

resistor

variable resistor

Diode

LED

Measuring currentElectric current is measured in amps (A) using an ammeter connected in series in the circuit.

A

Diagraming a circuit

A A

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

Measuring currentSERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same at all points in the circuit.

2A 2A

2A

• current is shared between the components

2A2A

1A

1A

measuring voltageThe ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter

V

Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current.

measuring voltage

Unlike an ammeter a voltmeter is connected across the components

Scientist usually use the term Potential Difference (pd) when they talk about voltage.

measuring voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUITV

V

measuring voltage

VV

V

series circuit

1.5V

• voltage is shared between the components

1.5V

3V

• voltage is the same in all parts of the circuit.

3V

parallel circuit

3V

3V

Three circuit laws

Direct Current VS Alternating current

Direct current (DC) - If the voltage is maintained between two points in a circuit, charge will flow in one direction - from high to low potential. (Ex. Battery-powered circuits, electrified railway lines)

Direct Current VS Alternating currentAlternating current (AC) - If the high & low voltage terminals switch locations periodically, the current will flow “back and forth” in the circuit. (Ex. Circuits powered by electrical outlets, domestic electricity )

Converting AC to DC• AC is converted to DC using devices called

diodes, which allow charges to move in only 1 direction.

Exercise on Drawing electric currents

Try Drawing the following circuits:1. Series circuit: 1 (3 cell) battery, 1 switch, 3 light bulbs, 1

ammeter and connecting wires

2. Series circuit: 1 (4 cell) battery, 1 resistor, 1 light bulb, 1 switch and connecting wires

3. Paralle circuit: 1 battery, 1 switch, 2 light bulbs connected in parallel and connecting wires.

4. Parallel Circuit: 1 battery, light bulbs connected in parallel, a voltmeter to measure the potential difference of the batteryand connected wires.

complete the missing current and voltage readings.

answers

3V 3V

6V

4A 4A6V

6V

6V4A 4A

2A

2A

4A

a)a) b)b)

Helpful simulation sites

• http://www.docircuits.com/#home• http://phet.colorado.edu/en/simulations/cate

gory/physics/electricity-magnets-and-circuits• https://www.youtube.com/watch?v=x2EuYqj_

0Uk• http://www.sengpielaudio.com/calculator-oh

m.htm