Electric Current Mini Lesson

Current of Electricity

Multiple Choice

What is the definition of current?

• rate of flow of neutrons

• rate of flow of protons

• rate of flow of charges

• rate of flow of energy

Multiple Choice

What is the SI unit for current?

• Volts

• Ohm

• Ampere

Multiple Choice

Which diagram illustrates the direction of conventional current flow?

Calculation of current using charges

• Current is the rate of flow of charges.

• Q = It

• Charge = Current x Time

Fill in the Blank

A 3A current flows in a circuit for 2 minutes.

What is the total charge flowing in the circuit in Coulombs?

Electromotive force (e.m.f)

vs

Potential Difference (p.d

Electromotive force (e.m.f.)

The e.m.f of an electrical source is the work done by the source to drive a unit charge around the complete circuit.

The e.m.f of a battery can be calculated by the formula

 $E\ =\ \frac{W}{\ Q}$  
E represents e.m.f
W represents work done 
Q represents the amount of charge

Potential difference

The p.d. across a component in a circuit is the work done  to drive a unit charge through the component.

The p.d. of a component can be calculated by the formula

 $\ V=\ \frac{W}{\ Q}$  
V represents p.d.
W represents work done 
Q represents the amount of charge

Multiple Choice

The SI unit for e.m.f and p.d. is

• Volt

• Ohm

• Ampere

Multiple Choice

When 2 C of charge flows through a potential difference of 8V, how much energy is dissipated?

$$V=\frac{W}{Q}$$  

• 2 J

• 4 J

• 8 J

• 16 J

Multiple Select

The main difference(s) between the definitions of e.m.f and p.d. is/are

• one is by a power source, the other is by a component (eg. resistor)

• one deals with positive charges, the other deals with negative charges

• one is energy supplied, the other is energy dissipated

Fill in the Blank

State whether component Y is an ammeter or voltmeter.

Resistance

Resistance

Resistance is the ratio of the potential difference across a component to the current flowing through it.

 $R=\frac{V}{I}$  

Multiple Choice

What is the SI unit of Resistance?

• Volt

• Ohm

• Ampere

Fill in the Blank

A lamp draws a current of 0.25A when it is connected in series to a 240V source.

What is the resistance of the lamp in ohms?

Fill in the Blank

Calculate the current flowing through a 5 ohm resistor when a potential difference of 2V is applied across it. 

Calculation of Resistance

Fill in the Blank

The effective resistance of the following circuit is .............. ohms.

Fill in the Blank

The effective resistance of the following circuit is .............. ohms.

Fill in the Blank

The effective resistance of the following circuit is .............. ohms.

Fill in the Blank

The effective resistance of the following circuit is .............. ohms.

Factors affecting resistance

Factors affecting resistance

• length of wire (longer, more resistance)

• thickness of wire (thinner, more resistance)

•  $R=\frac{\rho l}{A}$  

•  $\rho$  is resistivity of the wire material (SI unit: $\Omega m$  ) 

Fill in the Blank

The diameter of the copper wire used in a circuit is 2.0mm. If the resistivity for copper is  $$1.7\times10^{-8}\Omega m$$  , what is the resistance for 50cm of this wire?

Fill in the Blank

A wire with length l and cross-sectional area A has resistance of 10 $$\Omega$$  . 

What is the resistance of another wire made of the same material but with length 2l and radius doubled?