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

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

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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

Calculation of Resistance

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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: $\Omega m$  )